So in my quest to discover what the cause of my electrical gremlins are I've narrowed it down to the alternator. (I've replaced the battery) My question is, is it possible for an alternator to generate fine when its cold, or at high RPM's but not when its warm or at lower RPM's? This seems to be my predicament, Ive been told these are a two stage alternator with diodes for both low and high RPM's is this true? Is there anything I should check on before caving in and replacing it?

 

Yes. My GF had a truck with an alternator that would sometimes not work. After replacement I took it apart and found that one of the brushes was worn down till it was barely making contact with the commutator. This caused it to sometimes work and sometimes not.

 

Here's a thread about fixing your alternator by Dano, one of the forum geniuses.

If he shows up here, you might want to ask him about your gremlins. Maybe it isn't your alternator.

 

Yep, start with my write up on the alternator to know that its clean and the connections are greased and clean/tight as well.

As for once you get the car warmed up, let it idle and take some readings with a multi meter with the A/C off.

Body of the alternator as the ground, and the terminal on the back under the rubber boot as your positive source. You should come up with 14.6+ volts.

Next, battery terminals with the car still idling. Should only have a .1V drop. So if 14.6 at the alternator, then should be 14.5 at the battery. If you have a major drop in voltage, then do the next test and see if you still have a major drop in voltage.

Next, pop the hood on the engine fuse block and the center of the positive terminal (not the wire connector) as your positive source for the probe, and the alternator body as your negative source for the negative probe.

If direct to the alternator body has you back up at 14.5, but when you use the battery negative terminal as your ground and the voltage drops back down, then the problem is our negative wire connection point to the engine block just above the starter. If both test end up with way lower voltage then back at the alternator first test of 14.6, then the problem is the positive terminal connection ports for the battery to starter, alternator to starter wires isntead.

Now the fun one, and that is to check the DIC for it voltage. with 14.6 at the alternator, 14.5 at the battery, you should be reading 14.3'ish at the DIC. The DIC is telling the voltage at the BCM/ECM ,and the .3V drop in voltage from the alternator to them is normal from the engine fuse block (connectors and smaller wire).

If you have a much lower voltage at the DIC, then double check the battery terminal wire loom where it passed back behind the passenger head to make sure that the wire loom as now worn and you have a slight short to the head on the postive battery wire for starters, but the solution is you will need to clean up the connectors from the engine bay fuse block, to the ECM and BCM connector points.

Get me this far with your voltage readings and we can take it from there on the solutions.


As for the 170A billet alternator, it put out more voltage at idle due to its designs (stock Valeo 140 amp puts out less than 10% amps at idle), so they are a plus for guys that will be sitting in traffic at stop lights with the A/C blasting, since the OEM alternator will not keep up with needed amperage at idle instead. What they will not solve, a problem with the wiring in the charging system from the start instead.

 

I am currently wondering about this same issue as well, cold startup i read 14 volts, and anything above idle the same. But on really hot days with a/c on and fans running i can get down into the 11.7v range.

I've researched this alot and it appears that ALOT of people complain about this exact same thing. I've also read people have replaced alternator and battery and made no difference.
What doesn't make sense is that if the voltage should be around 14v and the ecm is monitoring it then why is it not throwing battery/voltage lights on the dash? My 89 crx would put up a battery light if the alternator was weak.
It almost seems to me that this is how it was designed to be.

 

My 05 goes down to 11.6v with a/c on at idle. Like yours, the cold startup is around 14v. I'm only on my 2nd battery after 11 years of ownership. So I'm just going to leave it, and not replace anything yet. I'm guessing it's normal.

 

Found this in another thread, appears to be GM troubleshooting manual:

1. Engine running, accessories OFF, measure and record the battery voltage at the battery terminals. The voltage should be between 12.6 and 15.5 V.

· If not within the specified range, refer to Charging System Test

So voltage can be as low as 12.6v with everything off, So if you've got a/c on, sound system and headlights on I can see high 11's being normal.

 

Dano's last paragraph, from above: Just some random thoughts from an old shade-tree mechanic:

This has been said here before, but I do agree that maybe we have too much information for our own good sometimes.

Back when cars had generators, the "GEN" light would usually flicker at an idle (depending if you had the lights on, heater fan on, etc.) showing that it was producing less than the car needed to run on (I have a '51 Fergie tractor with a gen, but no gen light or gauge, so I never know what's going on).

But under normal driving for any length of time the batteries on the old cars would charge back up, even if they'd been drained a bit while idling (assuming the battery was in decent shape). I'm guessing the same thing is happening today but it's not an issue for most of us (back to Dano's quoted paragraph) nor was running generators a problem in the old days.

The use of alternators (mid-'60s?) solved the discharge problem at idle, but today we have climate control, stereos, and whatever else drawing juice all the time so maybe the factory charging system isn't quite what it should be -- especially for you guys in the south, driving in stop and go traffic with the air running full blast. I will gladly stand corrected, but I don't think it hurts anything except that it might shorten your battery and alternator life by some amount.

But we have all this information in front of us today that maybe isn't really necessary and makes worry-warts out of all of us. I find it hard myself to resist checking the DIC all the time for this and that. But I don't let it worry me too much.

Aside from all that, where's all the chicks?

 

Again, the Valeo 140amp alternator will only put out about 14amp when the car is idling.

So normal voltage on the DIC should be around 14.3V, and with the AC on and just sitting at a stop light at idle, will drop around to 13.7 volts instead. Hence at this point, the draw from the system is more than the alternator can keep up with, and it now working off the power of the battery instead. Hence a fully charge battery should be around 13.7 volts (where the alternator should be keeping it), and if the voltage is dropping much more than that, then the battery is not being kept in a charge state when the car is running and being used when the alternator can not keep up with the demand isntead (read the battery for the most part, is just a energy storage area when the motor is not running isntead).

So in the above case, would start with a good alternator cleaning, and then pull/clean/re-attach the starter and motor ground wire connector points, as well as the battery terminals as well.

Now with the charging system test for voltage points and checking out fine, you can figure out if the problem has been resolved with the charging wires and alternator, or if the problem is deeper with the fuse box to BCM and ECM connectors isntead.

The engine fuse box is a snap to pull apart to clean it contact parts,


As well as the ECM and BCM connectors and grounds as well.

Simply put, GM did not use dielectric grease on the connector points, and it just a matter of time before the connector points start to corrode to cause problems isntead (read worst the humidity in your area, the faster the problem). And the more that the battery is pulled down from it normal 13.7 charged voltage state, the faster is going to wear out as well.

With voltage drops down in the 11.7v on the DIC (if no problems with contact problems between the BCM and ECM to the fuse box/battery), that puts the battery at around 12 volts (.3 voltage drop as the norm), and your flirting with a battery drained down enough to start killing cells isntead.


So to bottom line it, spend the 10 mins to check your voltages, and if needed, then spend the hour to correct the excess voltage drop between points problems if needed. The reason, it will solve a lot of problems (electrical gremlins), including not having to replace the battery so often as well.