Alternator causing rough running?

[justinbrooks]

My 93 500 has been running very rough (throughout the rev range) for the last few weeks and I think I have now isolated it to be related to the alternator charging.

If I start it with no throttle the dash voltmeter reads only 11v but the engine appears to run very smoothly, if I then blip the throttle the voltmeter jumps to approx 13v indicating that the alternator is then charging but the engine then starts to run very rough. I am not sure whether this indicates that the alternator is faulty or whether some other component is affected when the alternator is increasing the voltage. I have looked at the usual suspects and replaced leads/plugs/rotor arm etc. Any suggestions would be appreciated.

[blitzracing]

The voltages look about right. The rough running may be simply showing up with the extra load on the engine as the alternator starts to charge. The only way I could see of the alternator voltages causing this if it was producing some sort of spike(s) on the 12 volt rail, but it seems unlikely.

[BIG DUNC]

Thats the kind of reading you would expect from the alt if nothing was wrong.
If you want to check the volt meter reading, just but the probes from a multi meter onto the battery.
I susspect something else is wrong. The problem starts when you open the throttle. Air flow meter or ECU? or any other ignition or fueling problem?
Sorry to be of not much help, but I am not convinced its the alt ( or for that matter how it would effect the engine running )

[justinbrooks]

Thanks for the suggestions. I checked it over again last night with a multi meter and noticed that the TPS voltage is only about 0.27V which is a bit lower than it should be according to the bible. I will adjust this and see if fixes the problem.

[350Matt]

It may be that the extra load of the alternator is dragging it down into a poitn where its not happy, so its most likely that its not running right at idle, check for air leaks and clean out the stepper I'd say

Matt

[BliarOut]

1/6th of the voltage could also indicate a duff battery cell. Modern ones seem to fail differently to the old ones.

[spend]

The early alt doesn't start charging until you first hit @1000-1500rpm IIRC, I suppose it needs to spin up to excite the field coils. So the ign light and voltmeter seem normal in that respect.

My voltmeter only drops to 11v when the battery is lowish & cranking the starter. It shows around 12v without charging.

I would first check the main cables connections on the battery, chassis and starter motor. Then make sure the engine has a good grounding (should be near the drivers side engine mount)

Dave

[That Daddy]

350Matt said:

It may be that the extra load of the alternator is dragging it down into a poitn where its not happy, so its most likely that its not running right at idle, check for air leaks and clean out the stepper I'd say

Matt