Strange Alternator Behavior

[3krazygt]

I'm having a weird issue with my alternator on my 98 SL.

My setups is an alternator that appears to be good and an Optima red top. The car is essentially stock other than I did a Digital Climate Control conversion. I have my degree in electrical engineering and work for a company that does a lot of electrical assembly so I feel I did the conversion properly with quality splices and wire routing. I'm not sure if this issue occurred prior to doing the conversion, since I did the conversion right after buying the car with a blown motor, and hadn't started it until after the motor was swapped.

OK. So here is the behavior that is occurring. With a good battery in, the car starts up great. The car will idle with the voltmeter reading right around 14VDC. Right away this indicates the alternator is working properly, but just to make sure I removed (+) terminal from the battery to ensure the alternator is still working. When I removed it, the car ran normally verifying the alternator was good at idle.

When driving a different behavior occurs. Again when idle the car behaves normally. Then when ever I physically hit the gas, the car drops in voltage. It is apparent on both the voltmeter and visually as lights dim. It drops to approximately the voltage from the battery... so if fully charged (which the optima is brand new and is a fairly beastly battery) 12VDC.

There seems to be some issue with how the system is auctioneering power. Any out there with similar experiences? Any culprits to suggest?

Appreciate the help

[BlackStealth]

Originally Posted by 3krazygt

... just to make sure I removed (+) terminal from the battery to ensure the alternator is still working.

Bad idea, and is harmful to the voltage regulator. That technique you described only works on old school vehicles.

Also, temperature has an influence to the voltage regulator, higher voltage when cold.

Since you're good at electronics, you might consider rebuilding your alternator with new parts.
http://www.3si.org/forum/f4/alternat...t-75-a-542750/

[3krazygt]

Thanks for the reply. Just out of curiousity, could you elaborate on why that method would be harmful to the voltage regulator?

I see that the voltage regulator would see a difference of 2V, but i would imagine that it would be part of the voltage regulators duty to handle differences of 2V. Possibly a transient spike?

I'm a little familiar with working with voltage regulators, but I'm not an expert. So more info would be great. Do you you think the behavior I described would be a result of a bad voltage regulator? I kind of thought the gas-voltage relation was suspicous, and I thought maybe there was some sort of auctioneering circuit... something that switches between alternator voltage vs. battery voltage.

[BlackStealth]

It's a well known fact that removing battery with engine running on a modern vehicle is a big no no. You can research it on google. My dad already learned this lesson, fried his voltage regulator, old school mechanic working on a newer vehicle.

A couple things that can go wrong on the alternator:
1. Worn brushes... when more load is demanded, it fails to deliver
2. Voltage regulator... incorrect charging voltages
3. Diodes...a failure can lead to battery premature draining when not in use, or the charging pulses are irregular and not providing full current.

[twinblown]

When you remove the battery + in a running system It's called "load dump" ; in modern vehicles it can be very harmfull to the extent that it will damage the regulator due to the transient voltage spike created. The Alternators in these cars operate on the highside ("A" type) and removing the B+ is more detrimental to causing damage. You do not mention where are you taking the voltage readings; if you are going by the gauge you should know they are not accurate ; take more accurate readings at the battery post and even better ones at the Alternator B+ (be carefull). If you are not having crank issues then your system is charging properly. good luck

[Steve '93ES]

Have you checked the obvious? If the belt is loose it could be slipping under higher RPMs.

I thought maybe there was some sort of auctioneering circuit... something that switches between alternator voltage vs. battery voltage.

Nope.

Removing the battery with the engine running can do more than damage the regulator. It can also damage the ECU and any other electronics in the car. It's not just the transient spike (which is the major problem) but also that the battery works as a filter cleaning up the normally noisy output from the alternator. People argue back and forth on this but I'm in the "don't do it" camp.

[Escape-Performance]

I have seen bad alternators do this. Work at idle, stop working as the rpm goes up.