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MG TD TF 1500 - Ammeter/Warning Light Operation

More 50 TD idiosyncrasies…

Can anyone explain the “proper” functions and interrelations between the ammeter and the amber discharge light on the instrument panel? One would guess that they both indicate if the battery is discharging, with the light illuminating at some specific amperage as a warning, but I’m not so sure. Here are the symptoms. With the engine off and headlights and fog lights turned on, the ammeter will peg on the negative side but the light does not illuminate. With the engine running and all exterior lights on, there is only a slight discharge on the gauge (this may be another problem, but not the subject of today’s question), but still no amber light. On the other hand, the amber light will come on after starting the engine, but when I push the choke in way too soon, so that the idle is well below 1000 RPM. When this happens, the ammeter will show only a slight discharge. Could the amber warning light and ammeter be wired to different parts of the charging system, so that they are showing me status of different components? Or, are they supposed to be wired essentially in parallel, so the warning lights acts as an attention-getter to draw my attention to the ammeter?

Thanks in advance for your comments.

Steve Markman
Steve Markman

Steve, there are others much more knowlegable than I, but here is what I think without having the wiring diagram in front of me. The ammeter measures total current flow ("volume" or amps) going to (from the generator) and from (all lights, wipers, etc.) the battery, excluding the starter circuit-regardless of running or not. The warning light is connected to the generator output on one side, and battery voltage the other. Therefore, if the generator is only putting out 9 volts (at slow idle), current flows and lights the bulb, in this case caused by a slight current flow due to different voltage. This circuit is only active with the key on. The regulator cut-out prevents "reverse flow" from the bat. to the gen. in this situation, but when volts equal or higher, the cutout closes, so off goes the light. I hope this makes sense, and anyone else please correct whatever is not correct.
George Butz

Steve - From your description, everything on your car is operating just fine. The ignition warning light is a voltage driven device that tells when the generator is putting out a minimum voltage (somewhere between 12 - 14 volts. The ammeter is current driven and indicates the relative charge/discharge rate of the battery. Keep in mind that on the T series car, the ammeter is a relative premitive insturment that really gives only gross indications. Ammeters at best are a poor indication of the situation with the battery. A better monitor of the health of the electrical system is a voltmeter across the main 12V feed, but those did not come along until many years after the T series went out of production.
Good luck - Dave
David DuBois

Steve
I had a similar situation on my car when I finished my restoration. Try switching / reversing the two wires comming into the back of the ammeter. Off the top of my head I remember them to be large and brown. When the fuel pump worked(ticked) the ammeter read charge instead of discharge.Hope this helps. P.S. I'm still waiting for the low fuel light to come on. For saftey sake I carry a quater inch dowl about 3 foot long.
John
John Hambleton

Steve: Might want to upgrade your bezel. It should be red and not amber :-)

Probably faded.

Chris
Chris Couper

Guys,

Thanks for your advice. I think the system on my 50 TD is working properly. As you stated, it appears that the red warning light (it is red, not amber), indicates that the generator is producing less that some specified voltage and has nothing to do with the load on the electrical system. Haven’t yet figured out what the voltage is, but it starts to flicker at about 750 RPM and is fully illuminated at 500 RPM (of course…I don’t know how accurate my tach is). Above 800 or so RPM, the light stayed off, regardless of how many electrical items I turned on.

I did a quick check of the load that each electrical device produces and came up with the following:

Ignition: 5 amps
Running lights: 3 amps
Head lights: 9 amps
Fog lights: 10 amps
Panel lights: 1 amp

Given that the generator only produces an anemic 15 amps, just running the car with the running and head lights on results in a load of 17 amps. Thus, the 2 amp discharge that my ammeter showed is correct. The batter probably can provide that 2 amps for quite a few hours, so running at night shouldn’t be a problem. However, I probably would want to run the car for a while the next morning to get the battery charged back up.

Thanks again,

Steve
Steve Markman

I think everybody explained it well. Suffice it to say that with the ammeter you see the current flow one way or the other, while with the light you see whether the generator gives the same output voltage as the battery, which is not the case with the engine at, say, 600 rpm.

So far I have not added nothing new, but here comes something else. It had happened to me, at night, with headlights on and at full speed, to see a faint glow in the ignition light (red of course). Since the generator was charging fully, I could not understand what that was. Took me a while to see that it was the reverse: if at idle the battery has a higher voltage than the generator and so the light glows somewhat, at full speed, with a lot of accessories fed by the battery, the generator voltage is higher than that of the loaded battery! Since that bothered me, I put a diode in series with the light, under the appropriate screw of the generator.

Denis
Denis L. Baggi

Last sentence corrected:

... under the appropriate screw of the regulator.

(apologies)
Denis L. Baggi

Denis - I believe that I would check that the regulator is holding the generator to the correct voltage. I believe that the shop manual calls for somewhere around 15 volts. It should never be greater than that and can run down around 14 volts for modern batteries. Cheers - Dave
David DuBois

I've been following this thread to try to diagnose my electrical difficulties. Full restoration, and have just started to drive the car. The red ignition light always stays on, the ammeter remains neutral, never showing a positive charge, and will peg negative as soon as lights are turned on. Until recently the battery didn't lose its charge, so I believe the generator is producing. Several weeks ago, after driving about 1/2 hour on a hot day the car began to skip badly (I've also been following the "Bad Miss" thread) Couldn't figure out if the problem was fuel or electrical. When driving home after dark one night, I quickly concluded that it was electrical when my smooth running engine began to cough as soon as the headlights were turned on. Drove home 5 miles in the dark! Replaced the condensor, and I have a happy engine again. Decided to go for a drive last night at dusk. With the headlights on, the engine began to miss again after driving a couple of miles. Turning the lights off didn't make a difference this time. The battery discharged, engine quit, and there I sat. After 25 minutes recovery, there was enough charge to start the engine, and drove home in the dark again. (at least I'm getting good at it) I don't have any electrical test equipment, but feel that the generator is putting out since the battery remains charged after many start-ups (starter spins with gusto). Do I have enough information to conclude that a new regulator is in order? The workshop manual makes regulator adjustment sound like I'd be tampering with nature. Thanks for any suggestions!
William Metevier

As Question, Is the Ammeter Guage a fault, My Ammeter never reads but volts on the rear pins.
Thanks Do I need a new Ammeter
S.
Smitt

William, your problem could range from a bad ground to improper wiring. If you do not have the factory shop manual, get one. Then go to RadioShack and buy a Volt-Ohm meter. A decent one will be 20-30 bucks. Set it on the scale to read 12 volts. They can instruct you in use of it at the store. Then go to work. The manual explains how to check the generator output- do this first. You must have around the specified voltage here to start with. If that is ok, then start checking wiring- ie are all the wires tight, on the correct terminal, etc.- at the regulator and under the dash. If all that is fine, then clean the contacts (there are two sets of "points") inside the regulator. If that fails, maybe then try a new reg. The manual also has plain wiring diagrams with color coding- study and you can figure them out. Modern batteries have a huge reserve capacity, so you are evidently charging not at all or very slightly.
George Butz

I would like to add one thing to George's recommendations. When you get the factory shop manual, remove the page with the wiring diagram that pertains to your specific car, take it to Kinko's and have an enlarged copy made (about 2 - 3X) and then have them laminate the diagram. You now hav a large, easy to read diagram that can be marked up with a grease pen to make tracing circuits much easier.
Cheers - Dave
David DuBois

William, one thing comes to mind that has not been mentioned in previous advice. With a new restoration, sometimes the generator loses its polarity when it sits for a long time. Have you repolarized your generator? It wont charge without this procedure and you probably have a brand new battery that recovers rather quickly on its own after a few starts.
Jim Merz

George/David & Jim are 100% correct. My 2cents are that the T series workshop manual does not give enough information. I had to use a Jag. wsm to give me a more complete picture. I would recomend the W.E. Blower T series WSM to every MG owner from '29 to '54, to give a more detailed information on the veh. from bumper to bumper. Len
Len Fanelli

Hey Len, I need some clarification on the better information source. I don't have any electrial problems now but I want to be a boy scout. Thanks, Tom
Tom Mcnamara

Moss Pn# 210-100, claims to less comprehensive than the TD-TF manual, but i think it is more comprehensive
on a few areas. You should have both. Any old wsm from a Jag./MG/Healy, etc. is a valuable reference book, look for one @ a flea market/[autojumble]. Len
Len Fanelli

Steve,
One other little point! Was the car ever switched to "neg ground" system? I have helped out 3 people in my little neighborhood here after completting a long rebuild....2 of them (like my TF) had been "somewhat" converted to a neg ground system! Seems to have been a very popular thing to do in the 60's. Don't forget you will need to do a little "left brain/right brain" thinking if this is the case when using the manual!
The 3rd one I helped out had to do with "repolarize your generator" as Jim said! My TF needed this after only being torn down for 3 months.
Cheers,
David
David Sheward

Sorry Gentleman,

Do you not have ideas about Ammeter guage problem I have, who to ask.
Smitt

Smitt - It would appear that your ammeter is stuck. Someone with some experience with electronics could probably free it up for you and make it work.
Good luck - Dave
David DuBois

Smitt,
It's Lucas ...try a little "impact matainance"!
Possibly stuck from being parked for an extended period of tyme!? Gently "rap' the back of it with a screw driver handle....do you see any movement?
Carefull disasembly and oiling might bring it around.
At this point does not sound like you have much to lose if it does not move at all. If you can get your hands on external amp meter test your wiring to that point with that. (disconect meter in car in case it is shorted!)
Only consume "warm beer" when working with Lucas...least you anger the gawds!
Cheers,
David
David Sheward

Jim nailed it! I re-polarized my generator with his help, and found out that the needle can actually move to the right! Still drops to negative with the lights on but not all the way. As George has suggested, the huge capacity of the battery may just need to be restored by more than a few minutes running in the garage. I'll hook up the charger tonight and see what happens. Thanks everyone!
Bill Metevier

Smitt, since you dont give any details about the problem, I think I would disconnect the battery and remove the ammeter from the car. Have it checked by an instrument repair person like John Marks at Vintage Restorations in England. He does fine work.
Jim Merz

Steve:

If you used the ammeter to determine the current load, the ammeter is off.

<<<snip>>>I did a quick check of the load that each electrical device produces and came up with the following:

Ignition: 5 amps
Running lights: 3 amps
Head lights: 9 amps
Fog lights: 10 amps
Panel lights: 1 amp
<<<Snip>>>

Tungsten headlamps are 45 watts times two equals 90 watts total. 90 watts divided by a regulator setting of 14 volts gives the current required as 6.42 amps for both lamps. If you're using halogen 55 watts then the current is 7.86 amps.

Heads 90 watts total
sidelamps 12
tail 12
stop 18
number plate 6
panel 13
Fog 48
warning lamps 1
-----
200 watts/14 volts=14.29 amps

If the regulator is set at about 15.5 volts as per the shop manual then the amps are 12.9. However a 15.5 volt setting for the regulator is too high for many modern batteries and the battery will boil way too much.

<<<snip>>>
Given that the generator only produces an anemic 15 amps,
<<<snip>>>

Early TD C39PV dynamos are rated at 17 amps and the later TD C39PV2 are rated at 19 amps when the regulator is set according to the workshop manual.
Blake J.

I believe the high [voltage] regulator setting was because of the performance of all generators on all cars w/ generators.The batteries on cars w/ generators would boil the batteries [early type batteries]. When a car went on a long trip [daytime] drivers would leave the headlights on to prevent this from happening.
The higher output @ lower speed of an alternator has negated the requirement of a high voltage setting. I.E. voltage setting a matter of generator vs alternator; not early type battery vs late type. After 3 min on a battery charger @ 40 amps you will see 18.5 volts on a maintanince free & 16.5 on a conventional battery, this is normal. Open circuit voltage should be 12.2 -12.8 volts , if not recharge/retest. Len
Len Fanelli

Thanks to you, I taken Ammeter out. Clean, Maybe the magnet broken from the needle.

Smitt

This thread was discussed between 13/08/2002 and 30/08/2002

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