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MG MGB Technical - Temporary wiring setup

Finally, after two years, I am almost ready to try firing my completely rebuilt engine. I just need to get a new battery, put in place the fuel lines and get a new seal for the radiator fan switch.

I haven't done the wiring loom yet though as I have two old US looms I need to splice into one simplified one. I am wondering if I can wire up a temporary mini loom and a couple of instruments (tach, oil pressure and water temp) and make a sort of test dashboard.

What do I need to wire up? I guess I need to hook up the battery, fuel pump, starter and associated relay, the radiator fan, the alternator and then the coil and distributor.

Fuel pump and radiator fan and so on can just be on manual switches for the test setup.

I think it is all straight forward except for the ballast on the coil. My car has the resistance built into the loom. Can I just wire up an ordinary ballast resistor rigged so it is out of circuit for starting and in for running? I believe that's how it works on the car?

Simon
Simon Jansen

I should add I would run fuses to everything too.
Simon Jansen

Hi Simon.

For test firing up purposes you don't really need to worry too much about the ballast resistor, especially in your climate.

I would either connect a ballast resistor in series with the coil and start the engine without bothering to bypass it OR temporarily use a 12V coil.

The only permutation that I would avoid is running the engine for more than a few minutes with a ballasted coil but no ballast resistor, as the coil would tend to overheat.

Don
Don

Simon, If you have a remote starter switch with clips you will not need to bother with wiring the ignition switch or relay. You can run a temporary wire from the starter solenoid with a toggle switch, a fuse or circuit breaker in series is a good ides. Run wiring to the coil, fuel pump, electric fan and heat gauge if you want to observe the engine temperature. If you are going to run only a few minutes you don't need to wire the temperature gauge. I wouldn't wire the alternator for a short test. Be sure you have installed the ground strap from chassis to the engine/transmission. I believe yours goes to the tranny.

Clifton
Clifton Gordon

Wow Simon, you are really keen to get that motor fired up but I cant help thinking that you would be as well to make up the final loom as there are quite a number of things to connect. I assume that you are going to strip the insulation from both looms, make the alterations and then recover with self amalgamating tape. This does't take all that long and I've done it a couple of times. I think that to prepare the final loom will save you time in the long run.
Iain MacKintosh

Iain, I think you might be right. I am going to take a look at the looms I have this weekend sometime. I am putting in a more modern fuse block and relays and losing a lot of the original extra US car wiring.
Simon Jansen

Hi all.

I am not sure that it is safe to run an alternator with the wiring disconnected.
Might there be enough residual magnetism to allow it to start generating without the 'ign' light current ?, I believe that they can be damaged if run in that condition.. does anyone know for sure ?.

Don
Don

Don, I think you are right. I was assuming the alternator wouldn't have any residual magnetism after sitting a few years and would not charge, but we cannot be sure.

Clifton
Clifton Gordon

Simon:

Keep it simple. You did not mention the year of the car, but the radiator switch puts it in the 77-80 range.

One wire from one of the big terminals on the alt. to the battery connection at the starter will prevent any damage to the unit.

One wire from the battery connection to a momentary starter switch such as Don described, then down to the small connection on the side of the starter will get the starter to crank up without a relay.

One wire from the battery connection to a switch, then to both the fuel pump and the coil +. Either use a 12volt coil, or go through an external ballast to the +V connection.

Hook up your ignition correctly (points or electronic?)

Hook up the mechanical oil pressure gauge.

Use a hand held infrared thermometer to monitor coolant and any other hot spots, forget about the gauge for now. These units are getting cheap and are a very good diagnostic tool.


I think that is all you are going to need to get running. Someone please chip in if I missed anything.

I ran my race car with this minimal loom for the first two races till I had a chance to rework the original loom and reinstall it.

You should not need the radiator fans, but hooking up the thermal unit is just as easy as putting in a manual switch. It is a good idea to run the fans through a relay, so if the car was not originally wired this way I would suggest adding it. The factory made this change late on after tired motors started to blow the switches.

good luck

Kelvin Dodd
KJ Dodd

Kelvin, it's a 78 ex US car. When I rebuild the loom properly I am going to use relays for the headlights, horn, heater fan and radiator fan. I have a non standard shrouded radiator fan instead of the two open yellow types. The alternator does worry me a little. Isn't one of the reasons fo rthe label near the battery box about not running the car with the battery disconnected something to do with damaging the alternator?

Simon
Simon Jansen

You don't need the alternator unless you plan on running the engine for a very long time, assuming the battery is good. Just to be on the safe side, I think I would just disconnect it.
Dan Masters

How about using a short fan belt just on the water pump and missing the alternator out entirely? If the only thing that is taking current is the distributor, you will be able to run the engine for ages before the battery dies.

You will burn out the diodes in your alternator if you run it disconnected.

Mike
Mike Standring

From a one-off personal experience of leaving the alternator unplugged for about 40 miles (until the battery went flat!) it caused no lasting damage to the alternator. However there are two ways of connecting the voltage regulator - one is internally the other is externally via the starter solenoid, these may have different effects. But if you are talking about loom ballast then it refers to a rubber bumper car, which originally had alternators with the internal connection.

What you must not do with any alternator is run the engine with the *battery* disconnected, but even then alternators for some time have been equipped with protection against that. I wouldn't expect an unused alternator to lose its residual magnetism unless subjected to very many heat/cool cycles in various orientations.
Paul Hunt 2

This thread was discussed between 11/01/2006 and 12/01/2006

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