Engine smooths out when positive cable removed

[gysgtc]

Trying to help my brother out with his engine problem. He has a mild cam in it. When it is at idle it lopes like it has a cam in it but if you pull of the positive battery cable it smooths right out. Discovered this because when you turn the headlights on the engine dies. Trying to figure out what is going on. He replaced the points in the distributor with a module when he rebuilt the engine so no points anymore. Any suggestions? Thanks

[HavinSomeFun]

Off the top of my head. Alternator is bad or wired wrong.
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[Eddie H.]

It sounds like the diodes in the alternator are shorted causing RFI interference with the ignition module.

[MagmaJct]

While I can't solve your problem due to insufficient data, I'm amazed you pulled the battery cable at all. If nothing electronic was fried, consider yourself lucky.

The voltage regulator senses whether the battery needs to be charged. If the battery needs a charge the regulator will increase alternator output to deliver the charge. The "deader" the battery to greater the alternator's output.

Disconnecting the battery on a running engine tricks the regulator into thinking the battery is totally dead, causing it to increase alternator output to the max. If the battery was still in place, and actually dead, it would pull a large enough load to keep the voltage from jumping too high.

Back to your original problem: I would do some testing, what is the system voltage with everything connected at idle? (System voltage can be checked at the battery). What is the system voltage with the lights one, just before it dies?

What is the voltage at the coil, from positive to ground? What type of ignition coil is on the truck? Are the points set right? I'm just trying to think how a voltage change would affect idle quality. Something is affecting the system voltage.

[Shortnwide4x4]

An alternator when charging creates a load on the engine. I imagine when you disconnected the pos terminal the alt stopped charging and then there was no more load on the engine. Could be anything from a overcharging problem to a bad spark plug or wire.
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[davepl]

If you have shorted out diodes in the alternator (ie: a bad alternator) it will cause the alternator to charge itself at maximum load (just creating heat across the short) and put a significant load on the engine.

In a modern car the IAC will open up and increase the idle speed, but on an older vehicle it will pull the engine down.

I can't really connect that to disconnecting the positive cable, although that might have the effect of disengaging the field circuit and causing the alternator to stop trying to output. If you get the same effect by disconnecting the signal (small) wires to the alternator with the cable still connected, I'd wager that's the problem.

[68gmsee]

Scratching my head on this one... Need a lot more info like year and engine type and what sort of ignition module installed.

On older non computer vehicles, I have disconnected the positive side for an instant to see if engine keeps running with no ill effects.

Heres my WAG.

If the engine dies when you turn the lights on, I suspect both the battery and alternator are defective. A weak battery and a shorted alternator may cause a drastic drop in voltage to the coil/distributor to cause the engine to die.

On the other hand, disconnecting the shorted alternator allows enough voltage from the battery to go to ignition smoothing the engine out.

Test it as per above posts to see what you find and let us know what you find.

[koda360]

Check you grounds. Engine to chassis, chassis to body, body to engine. You need all three and sometimes several of each. Bad ground problems will drive you nuts...