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MG MGB Technical - Alternate Alternators

Since I put A/C, relay-controlled electric cooling fans, and relay-controlled halogen headlights on my '73 B/GT, I went ahead with a 105 amp Saturn (GM) alternator as per

A number of others have swapped for higher-output alternators for similar reasons. Nothing against the Lucas units that have served me so well for so many years, but due to higher electrical demands of my added conveniences.

Makes good sense so far, BUT I installed a voltmeter on this set-up, and I noticed that with the twin Hayden fans and the A/C system running that my voltage dropped to below ten volts with engine running 1500-2000 rpm or more. The battery is the largest group 26 I could find: 450 cold cranking amps.

So, I reasoned that the single ten gauge wire from the alternator to the solonoid was insufficient and I added another #10. Under similar operating circumstances, that brought the voltage up to a little over ten. So I added a third #10 and that brought the voltage up to just about twelve. At that point I turned on everything electrical on the entire car and the voltage still held at twelve or maybe half a needle width under.

It would make a neater job to replace the two #10 added leads with one #8, but the principle is the same. If you want to get all that current to the battery under these electrical load conditions, it takes the electrical equivalent of a six-lane interstate highway to handle the traffic. That's the moral of the story.
If you're planning on a system to cope with higher demands, keep this in mind.

BTW: I've thought of two group 26 twelve-volt batteries
in the old twin battery compartments, but wired in parallel rather than series in order to increase the amperage rather than the voltage. Has anybody done that? Any observations?


Allen Bachelder

I installed one group 34 Optima battery - 750CCA I believe. It fit fine in the (modified) battery box. Knowing the Lucas state of affairs, I figured every any advantage was worth taking...
Jeff Schlemmer

You know, Allen, adding extra 10 ga. wiring to handle higher alternator output was part of Munch's Ford Fiesta wiring plan too and it is indeed a good idea. Worked for my car, and perhaps your single 8 ga. wire plan would be better and simpler than 3 # 10s. Certainly cleaner. Seems like there really is some wiggle room to improve on our cars if we take a chance. If there weren't, I suppose we'd all be driving bullnosed Morris's still.

In today's world, two 12Vs in parallel would be overkill, considering the technical differences between the two 6Vs of yore and one decent 12V such as Jeff mentioned. If cranking amps were your only parameter for judging battery need, why not change the parameter by installing a lower amperage gear reduction starter? You would gain two things weightwise - 1) lighter weight from a smaller lighter starter motor, and 2) only one battery necessary, even in Jeff's Minnesota in winter.
Somewhere WestofLaramie

Allen - With 105 amps at 12 volts (1.26KW) and with doubled #10 wires, have you considered getting an inverter to plug the MG into so it can be used as an emergency power source for your house when the power goes out in the winter? If you put an extra battery in the car on top of all that, the Army Corp of Engineers would require you to license your car as a power generating plant ;) Cheers - Dave
David DuBois

I'm confused here as well unless of course this poor MG is going to be used to power a carnival. I have a single battery in place of the two 6v ones in both my cars I also have a standard Lucas 17ACR alternator which is capable of charging the battery (just) with everything switched on. Now I do agree that with modern accessories etc the alternator must be capable of supporting these when all switched on plus a few amps spare to keep the battery topped up but to fit another 12 battery in parallel is completely unnecessary. You'vc got 105A output from your alternator, wow, I can't see you every reaching that capacity but a single heavy connecting cable will be far more effective as the current in three seperate ones will not be equal as it will travel down the path of least resistance - and there will be variations.
Iain MacKintosh

Allen, I wonder about the 12 volt reading you are getting. If the meter is correct you are probably using the battery to help supply the load. I just read the voltage on my almost 8 year old battery, engine hasn't been started in several days, it reads 12.50 volts at the brown terminals on the fuse box. If your system isn't putting out enough to at least equal battery voltage you will end up with a dead battery.

Have you checked the accuracy of the voltmeter? New or rebuilt alternator? What is the voltage reading with ignition on, engine not running? Reading with with engine running without air? Is the ignition warning light on when the voltage reading is low?

I agree with the others, an extra battery isn't needed if your system is working as it should.

Regards, Clifton
Clifton Gordon

R = V/I so you only need a resistance of 0.019 ohms to drop 2v at 105A.

Even if you only(!) drew 50A, just 0.04 ohms would drop that 2V, so yes you need a low R connection, including the terminations.

AH is the rating to look for regarding battery capacity. That may, or not, go in hand with CCA but, as others say, shouldn't be an issue so long as the alternator will balance the load.

I would use a known good digital meter to check voltage as you are looking to read fractions of volts. Check for 12.6V from your fully charged battery with the engine off. Then drain the battery a little by turning on the headlights for 2 mins. Then run the engine, switch on all loads and check for charging voltage at the battery of around 13.8V, indicating it can balance the load and still have spare supply to charge.


Many thanks, all, for your many responses. First of all, let me assure you that I have run MGBs for 21 years with original Lucas electrics, save for the single group 26 battery, and given the demands for which they were designed, the original components work fine. My concern here was to accommodate my new electrical demands. The combination of two 10" Hayden radiator fans, the A/C clutch and A/C circulating fan however, was reducing the output from my new 105-amp alternator to less than ten volts in the system.

If I understand things correctly, this would indicate a constant state of gradual discharge at highway speeds with the A/C on. My initial point was only to point out that a higher-capacity alternator by itself will not necessarily solve these problems induced by those of us who insist on loading our cars up with modern comforts and conveniences.

Jeff, thanks for the tip regarding the 750 CCA Optima battery. I didn't know that would fit. Were modifications to the battery support needed, or only to an aftermarket battery box? I lived in Minnesota for 34 years and know that cold-weather starting is a vital source of water-cooler conversation in offices throughout the state. Here in Virginia, we have no water-cooler conversation in the winter because you don't ask "Did your car start this morning?" when the temperature is 30 degrees outside.

Iain, I was wondering about the effectiveness of my multiple #10s vs a single #8. Your point is well-taken. I would have run a #8 at first, had #8 wire been readily available, but #10 was the heaviest I could find. Fortunately, it is a short distance from the alternator to the solenoid.

And to SomewherewestofLaramie - I'm only using CCA as an index of battery capacity. I have used a gear-reduction starter on an MG Magnette and I loved it. Indeed, when this starter needs replacing, I'll buy another gear-reduction unit. However, in this instance, starter demands are not part of my concern.

I agree that 105 amps is serious overkill, but it's a very easy and inexpensive conversion, and it's nice to know that alternator capacity will never again be a concern. On the aforementioned Magnette (with 1800 engine and A/C) I used a 63-amp Delco alternator ($36 US). I had no voltmeter on that car and never thought about the capacity of the alternator output wire. The ammeter indicated that at least I wasn't losing ground (maybe!), and this system, with a 1000 CCA battery, did work fine - probably because I didn't know it shouldn't, and partly because my attempt at A/C didn't work well and it was therefore shut off a lot of the time!

The idea of two 12-volt batteries in parallel was idle speculation. I wasn't about to do it unless, to my surprise, somebody was about to seriously recommend it.

And now, the city of Roanoke Virginia is suffering a brown-out and they're asking me to give them a jump from my GT. 8^) 8^) 8^)

Thanks again,
Allen Bachelder

And two more responses came in while I was writing the last.

Clifton & Rich, given the superficial calibrations on this voltmeter dial, specific readings are difficult, but here's what I get. Note that the readings without the A/C are close enough in the ballpark to lead me to believe this rebuilt Saturn alternator and aftermarket NAPA voltmeter are working as they should. My local mechanic-friend tells me that my current (no pun intended) reading of 12- volts is OK, but since it's still lower than battery output, it concerns me.

1. Ignition on, engine off: 12+ volts (could be 12.5 but meter cannot be read that accurately)

2. Ignition on, engine running, no other load: 14+ volts (again, could be 14.6; it looks appropriate)

3. Ignition on, engine running, A/C on, single original output wire: 9+ volts

4. Same as 3, with additional #10 wire: 10+ volts

5. Same as 3, with 2 additional #10 wires: 12- volts, but still slightly less than condition 1.

6. Same as 5, but with everything (halogen headlamps, wipers, heater motor, brake lights, etc.) turned on: voltage of 12- unaffected vis a vis condition 5.

The ignition warning light is on under condition 1 (which proves it is working) but does not glow under any conditions with the engine running - so the voltage between brown/yellow and white wires appears balanced. But then, if we're talking about an imbalance of less than one volt, I wouldn't expect to see any indication from the light.

More thanks!


Allen, I installed the red top Optima that they carry at Advance Auto Parts. It's the smallest one they make. After my battery boxes rusted away, I made enclosed (and vented) sheetmetal boxes. The battery is in the RH side, the aftermarket fuel pump in the LH side. I honestly never knew what the original batteery box size was, but I'd bet this battery would fit. It's something like 6x10x8"s.
Jeff Schlemmer


I recently upgraded to an 85 Amp Bosch alternator, and I get a voltage reading of just over 14 volts at the battery terminals, regardless of what's turned on (including 4*50 watt CD player). 105 Amps and a single, garden gnome 12 V should be MORE than enough to current to maintain adequate charge.

In your case (assuming your voltmeter is accurate), you'll soon see if the battery will go flat after a long run with everything on. You should be running at over 13 volts at the battery terminals with the charging circuit working properly and a new alternator.


1) Check all connections, including major earth straps and connections between battery, starter and alternator.
2) Check the status of your peripheral loom and connections...a high output alternator will rapidly burn through tired, high resistance connections, and can also add up to a voltage drop across the car.
3) By my calculation - it seems that A/C unit is sucking an inordinate amount of juice - is there a bad ground or high resistance across it? What's the voltage at the terminals with everything turned on EXCEPT the A/C?
Curtis Walker

Rich, your post reminded me of my basic electronics training I received 50 years ago. THey tried to convince us that ohms law is important. It's something we tend to forget and then you mention how a very small resistance can cause a 2 volt drop when you try pushing a lot of current through the circuit. In my

I found some wire size information in an Automotive Electric bokk I have. This information is about battery cables. The table gives ohms per 1000 feet of different size wires. The MGB battery cable is about 7 ft. long. I computed the drop at 105 amps. For AWG gage wire size 6, some cables are that size. .4 ohms/1000 ft. will drop .294 volts. Gage size 4 is .3 ohms/1000 ft. will drop .2105 volts, gage size 1 .14 ohms/1000 ft. will drop .1029 volts. THis is only the battery positive cable. The alternator to solenoid cable and the battery ground cable would also add some additional drop.

Allen, I understand the problem with the voltmeter they all seem to be marked that way. I had my son's Jensen Interceptor for about three years. 440 Mopar engine with 70 amp Mopar rebuilt alternator. The fans were 11" heavy duty Bosch with lots of current draw. Driving the car around town required three eyes, one for the voltmeter, one for the temperature gauge and onee for the road. The Jensen generates enough heat to heat a rather big house and has lots of electrical components.

Sorry about the rambling. I agree with Rich's suggestion to check the balance of your system by testing voltage at the battery at the battery.

Clifton Gordon

Jeff, Someone recently mentioned in a thread they installed an Optima by bending the front lip of the battery tray and installing it front to back in the tray. It will not go left to right without some major alterations. I didn't mean to use double words in my last post. Too bad we can't make changes.

Clifton Gordon

Allen, All,

The battery to solenoid cable is very heavy and will drop very little voltage in itself.

The wire Allen uses between alternator and solenoid may be around 6ft long and will be of more concern. 6ft of 10G wire will drop around 0.7V at 105A.

What may be more significant though is the resistance of the connections. It is a circuit and ground connections and battery terminals are easily overlooked. Clean to bright metal, all live and ground connections and, perhaps, run a separate heavy wire from a nut on the alternator body to chassis. This is a messy route of paint, bolts and rust and it would be easy to find a fraction of an ohm here. Crimp, then solder, lucars and rings to wires.

You should see =/>13.8V at the battery terminals into a slightly discharged battery, engine running, with all loads on or off. Allen's tests 1 and 2 are fine but the rest indicate loss from resistance, be it wire or connection.


Thanks again, guys. Don't forget that it's not just the A/C load, but the two 10" Hayden radiator/condenser fans that also switch on with the A/C.

Rich, actually my run from the alternator to the solenoid is less than two feet long. I have freshly-soldered ring connectors to clean terminals on both ends. However, you remind me of another important point: the ground. I had forgotten that when I first fired this engine up, I got an overheated throttle cable. I tightened up the ground strap enough to resolve that problem, but I could still be losing some amps here. I'll revisit both ends of the ground strap and the ground connection on the battery cable and see where that gets me. Also, I have yet to take readings at the battery. ' Will do so and report back.


If you measure between the battery 12v post (not the connector) and the solenoid stud, and the battery ground post and the starter body, you ideally will have only a tenth or two of a volt in each 'leg' lost during cranking. I've seen up to 3v being lost just in one leg, which results in slow cranking and low voltage to the ignition, which together result in difficult starting. If you have more than half a volt in either leg it will be worth investigating each connection.
Paul Hunt

Allen, You've probably answered the question then. A poor ground from chassis to alternator casing would give exactly the problem you describe. It would not matter (much) if the starter lost a couple of volts, it would just turn more slowly, but if you loose a couple of volts on charging it's a different matter. Regards, Rich.


Allen, Paul and Rich have good points; connections and grounds. The (3) 10 ga wires should carry a right good bit of current, 10 ga is routinely used to carry 12vdc loads in excess of 30 amps, depending on the length of the wire, of course. A single 8 ga would probably be better but it's all for naught if the voltage is dropping across connections and marginal grounds.
Wray Lemke


I played with it a bit tonight and believe I've figured it out. It looks like my grounds are good, but I did some checking with my utility voltmeter and came up with some interesting readings, leading me to believe I haven't wired things in the best way.

First of all, refering back to my earlier report on voltages under various conditions, I added condition No.7: Engine running and everything BUT the A/C and cooling fans switched on. Under this "normal full load" condition, the voltmeter did not flinch. It held at an indicated 14 volts ± .5 volts - unchanged from engine running, everything turned off.

Next, with the utility voltmeter, I checked brown wires at fuse box to ground on either alternator case or body grounds (made no difference). With everything, including A/C and Hayden fans, sh*tched on I found that voltage to the browns stayed more or less constant. I watched the meter as the compressor cycled a few times. The needle would flinch a bit as the compressor kicked in but would recover and voltage to the browns held at about 14 volts.

But my aftermarket voltmeter is connected to the white circuit, downstream from the ignition switch - of course. So, when I compared the brown at the fusebox with the white at the fuse box, when the A/C is on, I consistantly got in excess of two volts difference between the two, the white, of course, being lower.

At this moment came ENLIGHTENMENT, and I saw the folly of my ways. While I have a gaggle of relays on this car, my fan and A/C relays are now wired to draw their secondary power from the white circuit! It's a wonder I haven't fried something on the white circuit already. Certainly the primary or control side of these relays needs to be on the white circuit, and their draw will be negligible. But the secondary power source should go through a fuse directly to the browns - maybe even directly to the solenoid.

In this sense, the low reading I was getting was artificial. The good news is there's still plenty of voltage going to the browns. The bad news is that I'm bordering on toasting the white circuit. It will be easy to change that tomorrow morning.

Another solution, and probably a good idea, might be to protect the ignition switch with another relay, but as long as all these heavy-draw motors are already on relays, an ignition switch relay is not absolutely necessary to solve the instant problem.

Is there anything I'm overlooking here?

Allen Bachelder

Bordering on toasting the white circuit? You are hanging by one fingernail!

In as much as the AC, fans, and headlamp loads are predominately operating off the alternator direct, and not the battery, Run a new "brown" 8 or 10 gauge, directly from the Alt + to a fuseblock. It will only be about 18" long. Feed the relays from this, separate fuses for each relay. This will remove several feet of wire and multiple connections from the heavy current path. If the presumably uprated A/C in car fan is also fed from this point , that will take another big bite out of the original wiring load. The original N to the solenoid will suffice for all the original loads, esp. now that you've got the headlamps off that feed. The N/IGNswitch/W/fuse/G circuit will be much happier, as will all the things it still feeds - wipers might actually wipe!

Not withstanding that the grounds are alleged "Good" (at the moment), permanently fix/improve the other side of the circuits by running a 8 or 10 ga ground direct to the rear case of the Alt - most (except small Lucas!) have a threaded hole or stud for this purpose. At some convenient close point, maybe a fender mounting bolt, join this to the AC compressor, fan motors, headlamps, AND the original ground wires (RH inner fender and front of car), with ground jumpers as needed. The sheetmetal sucks as a heavy current ground return, especially if spotwelded or cage nuts are involved.

Use an electrical anticorrosive grease such as Ox-Gard on all connections including bullets, make sure that all paint and other crap is scraped off any ground points. Any ground point going to sheetmetal should have a shakeproof washer between the wire(s) and the sheetmetal, Ox-gard between all surfaces.

A particular point on the MGB is the main underdash ground cluster - directly above the wiper motor - it is all of the dash and in-car grounds. Your voltmeter is NOT an analytical instrument, it can at best only give the voltage between its connect points (as you have found). This dash ground circuit could be influencing it as well.

Everybody should read Bob Muenchausen's links on grounding in the MGB General Board. I will post some further troubleshooting instructions there, but not now as I'm tard!
FR Millmore

Allen, That's good news and I'm sure you'll sort it now. My posts, and others I think, were understanding you had the meter on the battery terminals. That's something to check when you've sorted the AC wiring. Rich.


Just a repeat of the links FR Millmore mentions:
Bob Muenchausen

Rich, Fletcher,

Many thanks for your unflagging help and follow-up. What I had already done is to wire in a second Lucas fuse box where the starter relay used to be. The bolt centers fit an additional fuse box. Then I replaced the starter relay with a modern 30-amp unit which mounts nicely next to the fuse boxes. I had wired the second fuse box with one brown and three white circuits. Today I changed it to one brown, one new (red) with a #10 feed directly from the alternator, and two white (the top two fuses are already bridged together).

The headlight secondary goes to the brown, the A/C fan, clutch and radiator fan secondaries go to the new "red". The only thing on the two extra white fuses are the fan and A/C switches - leading to the relay primaries. That load is negligible, but it keeps these switches within control of the ignition switch. The next step will be to add an ignition switch relay. I don't even want to keep loading it with the usual stuff.

But now, with the A/C, radiator fans, and everything else all switched on, I'm still getting 14 volts on the aftermarket meter perhaps 0.5 volts - no more than .5 volts less than with everything off. Again, I get a knee-jerk from the meter when the A/C and fans come on all at once, but it immediately stabilizes at 14 volts and stays there unless idle under load drops below about 700 rpm. Now I should rig a solenoid that would reset the idle when the A/C is on. Fletcher, I'll bet you have that figured out already.

BTW, my A/C fan now runs noticeably faster!

Life is good.



I would have acknowledged you too except your post went up while I was writing mine. Thanks for the articles!

And I forgot to mention that I also ran another ground strap from the alternator case to a fender bolt. ' Don't know if it made a measurable difference now, but the security is worth it. I also cleaned up the connection where the battery ground cable bolts to the battery box, and cleaned/tightened the engine ground strap. I'll have to get some Ox-gard and go over these connections again (I live 30 miles from the nearest source of such things). Regrettably, access to the under-dash grounds is impaired by the evaporator. But it's nice to know where to look when failure occurs... ' Think I can get at these by removing the glovebox. Whenever I connect ground wires to the body, I use tooth washers - on both sides of the connector rings. All of my ring connectors are soldered as well as crimped. I have little faith in solderless crimp connectors.

The instructions for my voltmeter simply said to connect it to any ignition-switch-controlled circuit. I still have it connected to the same place in the white circuit, but with the other stuff moved off that circuit, everything works fine.

Thanks to all, once again.


This is additive and not new. In rewiring my 67 GT headlamps with relays etc., I noted that there is a little black wire going out to the headlights. I cut the wire and put a lug on each end of the cut and connected it to a ground screw just behind the radiator. My theory was that it was a long run and I could shorten it ( and the voltage loss) by grounding it closer to the load. While at it, I also rewired with 10 guage ( ok it's overkill, but the headlights are higher amperage than stock) ground and hot side wiring to the headlamps. All the ground wires connect at the same screw into the inner fender next to the alternator.
The key not to forget in upgrading the wiring system is to remember that the ground wire has the same amount of current as the hot side and it's "connection" to the body for ground is just as important as the hot side.
Barry Parkinson

Allen and all,
Just posted a rather lengthy piece on Bob's Ground thread on General BBS.
Glad you're getting somewhere!
I am unclear on how your headlamps are wired. You should have a relay for High and one for Low beam, controlled by original wires from dipswitch. Secondary or load power should come from two separate fuses. If one side blows, you still have lights, and all heavy current loads are removed from both main and dip $witches.
Rad cooling fans should get load power from one or better two fuses (and relays), but definitely separate from AC functions. Little point in cooking the engine if the AC dies.
Not much point in using two fuses for relay control power; they would be fine on straight white circuit, better use one for fuel pump/OD (if any). Those wires are vulnerable!
Idle solenoid? Sure!

Barry, if you read my post on Bob's thread, you will see that you gain about 8-10% light with your headlamp pigtail fix - maybe a bit more if you have overwattage bulbs. 14ga would be fine and a lot easier to deal with. Change the plugs too, they frequently are a significant drop. Really killer plugs available from Daniel Stern/Candlepower.
FR Millmore

Thanks Fletcher,

My headlights are wired as you described. Actually, I "cheated" and bought a made-up headlight relay wiring harness from Vicky Brit and that's the way it's made up. Existing wiring is only used to trigger the two relays. Only difference is I have both headlights on the same fuse. You have a good idea there - I can and will separate them on two differenct fuses. Cooling fans are on separate fuses but same relay. Actually, I just realized the fans are fused twice. I can wire them on the upstream side of my "red" circuit (powered directly from the alternator) and use the downstream side for the second headlight fuse. The primary side of the fan relay is wired to be triggered in three ways: the thermostat switch; through a diode to the primary side of the compressor relay (which is separate and separately fused on primary as well as secondary sides), and through a manual override switch (actually a heater fan switch) on the console. The diode prevents the fan switches from triggering the compressor.

Granted I have overkill on fused circuits for the relay primaries - I just found myself with an extra circuit. I already put the overdrive on a pigtail fuse, and I'll be adding a relay to this also. I hadn't thought of fusing the fuel pump, but since I have a spare white fused circuit, that makes good sense. If the lead to the infamous double bullet connector reaches, all I have to do is remove the bullet, replace it with a female spade, and plug it in.


This thread was discussed between 06/07/2005 and 10/07/2005

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