Ok I have posted about this before and have gotten no where. I recently got the car back from the dealership and here is the scoop. The computer is supposed to send a 5v. reference to the alternator and then it will charge at idle. Problem is that when I tap the wire to the alternator, it is only getting like 1.3-1.4v so at idle it won't charge. The dealer says they checked every wire and replaced every ground but still no charging. They also replaced the computer. So? Any ideas on what the problem might be?

 

Common problem, and sorry, we don't have an answer yet....

 

Sounds like you have a bad wiring harness. To find out which one, they will have to check every connection point from the computer to the alternator. Depending on which harness is bad, it could be a very expensive and time consuming repair. I would suggest that you ask the dealership to call GM and get some advice from the engineers.

 

If they've already replaced the computer, then the problem must be the path between the 5V output terminal of the computer and the input to the alternator. Check for 5V on the terminal of the computer, if it is there, then it's in the wiring along that path to the alternator (loose connection, bad connector, cut or scraped wire, etc). If it's not there, then the computer is not sending a 5V signal, something for the technicians to figure out why (bad computer, bad programming, etc), a problem that is internal to the computer.

 

No bad harness in any we or the local dealer have tried to trouble shoot......we are still searching for a solution.

 

1.3 - 1.4v at terminal "L" on the generator??

If so, check the voltage on the same circuit on the pin at the ECM. If the ECM puts out a 5v signal, (at idle) the ECM is OK, and there is a wiring harness issue, or a generator issue. If the dealer did not resolve the problem, they should call technical assistance.

Also see DTB #02-06-03-008C.

 

Issue is that all of these things HAVE been checked!!! My question is can I jump off of another 5v reference such as the MAP? And believe me, GM engineers have been asked from 4 different techs. This is not as simple as it seems. But if I can bypass the problem by using a different source, who cares for now?

 

That may cause additional problems as the draw & resistance should fall into a certain value....you may try off a non-critical source (can't think of one right now...dead tired) and put a 1 way diode inline to prevent any unwanted reverse feed or a voltage spike...just a thought.

 

Do lemon laws exist in Texas? Time for a refund.

 

No, you can not. The circuit in question is ckt 225. This is the circuit from the C2 connector, pin 61, on the ECM to the "L" terminal on the generator. The ECM switches the 5v signal on and off to control the generator output. No other circuit can control the generator.

You stated the voltage is low at the generator, but what about at pin 61 on the ECM C2 connector? If the voltage is low at the BCM pin C2, 61 with ckt 225 disconnected at the ECM, you still have a ECM problem. If you do have 5v at C2, pin 61, with ckt 225 disconnected, run a new by pass for ckt 225 from the ECM C2 pin 61 to the "L" terminal on the generator and see what happens. NOTE: The old circuit 225 must be disconnected at both ends.

I saw where you have a C5 generator and a "tune". If the above does not work, I would go back to the stock tune, correct generator and pulley and start from there. Also, are you sure the generator drive belt is not slipping? Hopefully, you will not have to do this.

 

Worth trying....let's put our heads together & figure this out for GM.....we have several customers w/this issue & it needs to be solved.

 

A couple of things you might try. Unhook both ends and measure to ground. Should be open. Measure end to end, should be low resistance. Also, since this is a gated signal (on and off for control) what else is taped up "next" to this wire in the harness? It may be getting 'noise' from some other signal and confusing the alt. Does it run near the plug wires for instance? I would even hang a scope on there and look at the signal. Just throwing things out there......

 

If the connector to the ECM was removed while you measured 5V at the pin, then either the wiring to the alternator or the alternator itself is pulling too much current (looking like a short) to bring the voltage down when the connector is hooked back up. I cannot see it being more complicated than that.

 

I'm leaning toward too much draw from the alternator that's pulling the voltage down when it tries to fire off the charging circuit.

 

Some good advice there. But may I say that the car has been through 3 ECM's all with factory tunes; 6 alternators, and ALL grounds have been replaced. I am starting to lean towards outside interference. The wiring harness for the generator runs parallel to the intake next to all driver side injector and coil wires. Any chance that could be it? I will pass this info onto my tech and make sure that everything is checked. Fortunately right now the lowest voltage we are seeing is 11.3 at idle so we went ahead and retuned. Just for those wondering,

748 rwhp/653rwtq @6600

For safety reasons on the stock bottom end:

9.9:1 AFR
18* at WOT

 

We have seen no less than 1/2 dozen with the identical issue & have done all described & still have not solved the issue. We are just as stumped as the GM techs at the dealerships here....

 

What is the symptom? We saw "service charging system" on the DIC, and it seems to be RFI affecting the new-fangled digital control circuit on the alternator. Shielding the control lead to the alternator helps.

 

Hey, what if I run a straight wire from the battery and put a resistor on it to make it 5v?

 

A "resistor" won't make it 5V. You could either use a voltage divider circuit or a zener diode but even then that's not how I understand the circuit works. I'm under the impression that its a 'gated' signal (pulses) that control the output. You won't be able to read the "voltage" level of this with a standard VOM. You would be roughly reading the average voltage over the time of the sample rate of your meter. If it were me I would bypass the existing wire with a shielded, twisted pair. Carry the signal on one of the pair and ground (electrical ground at the alt end) the other. And ground the shield on one end (chassis ground) Of course this would involve cutting, etc. Unless you can come up with the proper pins/sockets to temp it up for trial. Be a shame to hack up the wiring and that's not the trouble........... I still say get someone who understands electronics and have the wiring diagram and KNOWS what's supposed to be going on hook up a scope and see what's happening. If there is a problem with the signal or voltage the scope will show it. But FIRST one has to know what they're looking for. I would think that GM must have a test procedure for this circuit. Its in a book or computer somewhere and probably with sample waveforms or reference levels.

 

On a highly modified car with over 800 hp, I don't think so. I'm surprised the dealer is even touching it.

That much power on the stock bottom end, the alternator charging will soon be the least of your issues.