Alternator wiring for track car?

[n0m4d]

I'm just finishing up an m42 based track car. I have no gauge cluster in the car (using all aftermarket gauges) and am having issues getting the alternator to charge. My m20 car would kick in and charge after a bit of revving but the m42 alternator doesn't seem to do this. I understand there is an exciter circuit that runs through the OE cluster but I'm not quite sure how to bypass this when there is no cluster.

Is it as simple as adding a jumper between the charge and exciter lugs on the back of the alternator?

I was worried that doing so would bypass the voltage regulator and cause bad things to happen.

[JMPAuto]

You need to connect the 12v signal wire straight to the ignition. You can run the small wire from the alternator directly to the ignition switch. Connect it to one of the 2 green ignition wire, I forgot which one but if you connect it to the wrong one, car will not turn off even if you turn the ignition off. In that case, just connect it to the other one. I run this setup on my race car. I dont have the cluster too and I gutted the whole factory chassis harness.

[n0m4d]

Very cool, thanks for this answer! If I use a 4 post kill switch, one set killing the battery and one set breaking the green ignition wire do you think I can connect the exciter circuit to the switched side of the ignition circuit on the kill switch?

[jlevie]

That will work, but the smart person will run exciter power through a light, like in the stock configuration. Not so much for telling that the alternator is charging as to provide notice that the fan/alternator belt has failed and that an overheat and engine damage is about to happen.

The right way to implement a kill switch is to use a three-pole switch. The heavy contacts interrupt the main battery cable, the small normally open contacts interrupt the DME power lead, and the small normally closed contacts are used to switch a resistor in to provide a drain for the alternator when the switch is turned off while the engine is running to prevent a voltage spike and damage to to DME. Turn the ignition on/off with a toggle switch.

[JMPAuto]

That will work, but the smart person will run exciter power through a light, like in the stock configuration. Not so much for telling that the alternator is charging as to provide notice that the fan/alternator belt has failed and that an overheat and engine damage is about to happen.

The right way to implement a kill switch is to use a three-pole switch. The heavy contacts interrupt the main battery cable, the small normally open contacts interrupt the DME power lead, and the small normally closed contacts are used to switch a resistor in to provide a drain for the alternator when the switch is turned off while the engine is running to prevent a voltage spike and damage to to DME. Turn the ignition on/off with a toggle switch.

Do what he said. I just started rewiring my car today for next week's Redline time attack. I will definitely put a light for the alternator. I'm running my kill switch just like he described. You can get one cheap for like $40 with resistor included.

[n0m4d]

Sorry for the late reply, got side track with other issues. Thanks for the great info, I will apply it all. A couple more quick questions before I commit all this to memory.

On the alternator, I understand the large lug is the output to operate and charge the vehicle. Is the small lug an input?, when getting 12v it lets the alternator charge, when off the alternator stops charging?

Also, I have 2 alternators, one has a small black box and a one wire lead on the back of it, the other does not (but has the place for it). I tried looking up the p/n on it but didn't really find much. I think its a voltage filter and isn't necessary per say, is this correct?

[jlevie]

The large lug is the output and the small lug is for power to the exciter coil.

[M-technik-3]

If it's lemons a four pole is required for tech inspection and you will not race without it. Fail inspection and it will be tough to get a 4 pole in the Greater Manchester area. Happened to us last year we went with a two pole.

Some one in the infield had a spare and hooked us UP.

[n0m4d]

Been in that boat at Stafford about 4 years ago but did manage either to get the 2 pole to work or we wrecked the alternator trying, not really sure which. We lasted 60 laps before getting t-boned by a civic.

We then fed-ex a proper kill switch and have been fine for the last few years. I don't think we had it setup best for the car, but it did kill the engine.

Some of the details of the charging system is a little bit of a mystery to em so I appreciate the lesson here. Thanks everyone for the info.