Ok, so I have this 2008 GMC Acadia that I am working on. The alternator is putting out 19.8v

The dashboard information screen displays SERVICE CHARGING SYSTEM

What'* my best option?

With the Key On Engine Off, the battery voltage indicates 12.5 volts......When the engine is running, voltage indicates 19.8 volts.

I have one OBD code indicating PCM/ECM/TCM EXCESSIVE TEMPERATURE.

I guess from the high voltage, the computer is getting hot.

I removed the alternator and took it down to the auto parts store to have it tested, but the stores test machine basically says it is not capable of testing it.

Is battery fully charged?

Is this a gage reading or a voltmeter reading across the battery?

The reason why they probably couldn't test it is because this is controlled by the ECM/PCM...there is also a battery current sensor involved in this system....

When I turn the key to the RUN position, everything is as normal. Bulb check and needle guages do thier sweep test. As soon as I start the engine, the entire guage cluster goes black and the words SERVICE CHARGING SYSTEM are displayed.

I did the volt meter test across the battery and across the battery emergency jump start terminals under the hood. I also used a quality OBD2 scan monitor diagnostic tool. Hooked it up to the DLC under the dash and it too was indicating running voltage @19.8v in Real Time.

The clowns at Autozone CAN test the alternator, BUT it has to be installed on the vehicle with engine running. Obviously, if the alt is putting out 19.5v It will damage something. I think vehicle electronics are supposed to operate at levels no higher than 15.4v max.

The CEL light is NOT on, but my scan tool indicates one OBD code has matured. I forget the actual code number for it, but it basically was indicating ECM/PCM/ECU EXCESSIVE TEMPERATURE.....something like that.

As I said, the alternator is controlled by the PCM,

AND

input from the BCM, which reads the input from the current sensor(that is the sensing loop that goes around one of the battery cables)......

Think you are going to have to do a TSB search, and you are not only going to have to access any specific PCM codes but also "B" codes that may be in the BCM.....

It could be the PCM, a bad input to the PCM or BCM, BCM or possibly a reprogram update to the PCM......

The trouble trees to the specific B and P codes will have to be followed.....

Here is some good reading.....

http://www.esghotwire.com/0213_pit.pdf

Ok, here'* an update....and thanks Tech II for that info link.

This alternator CANNOT be tested on a stand-alone test machine. It has to be tested in-vehicle while under load, meaning accessories running ie: air conditioner, radio, lights, wipers, etc....The PCM (and BCM-Body Control Module) infact controls the amperage output of this alternator. Operating voltage should be at or fluctuate right around 14.6v.

In my case 19.8v was way too high, the result was the PCM/BCM shut off power to certain electrical systems, mainly the guage cluster and the trans control module (not mentioned earlier, but the vehicle trans went into (Limp Mode).

I replaced the alternator......the internal power regulator was the fault, and the PCM did not recognize the info.

Now that everything is back together, the vehicle has no high or lo beam headlights. I've researched this, each hi and lo beam light has its own individual fuse.....so basically 4 fuses.....in-line per circuit....upstream from that, there are two relays. One for hi and one for lo.

I doubt that all four fuses burnt out at the exact same time.....I doubt that both hi and lo relays burnt out at the same time.......

What to look for now?

Well, when you turn the low beams on, do you have power out of both low beam fuses?

I'll have to check that later on this afternoon.

Where is the BCM located? I've searched online as to it'* where-abouts, to no avail.

Well, I removed and checked the fuses with a continuity tester and they are good. No power to the fuse terminals though.

I pulled the hi beam relay and the low beam relay. I rigged up a contraption to test the relays and they both work. The electrical terminals for the relays on the fuse box itself has B+ battery power. So I guess whatever is happening has something to do with the 'switched' power source for the relays.

I am going to pull the dimmer and headlight switch and test them. (how difficult is it to remove the multi function switch (the blinker stalk) for testing purposes? Any special tools needed? I've got the basic steering puller and steering lock compressing tool (GM Style).

So you pulled the low beam relay, and you have power on two terminals?

If yes, insert relay, and have someone manually turn the low beams on....with your hand on the low beam relay, it should click...if it doesn't then the BCM is not grounding the relay.....

You would need a wiring diagram of the BCM, to see if you are getting a signal from the headlight switch(all the switch does is give a ground input to the BCM) and a ground signal from the BCM to the relay......

Good way to check the relay and light circuit is this.....you have 4 terminals(terminals are numbered on the bottom of the relay)....85 is bat to coil of relay....30 is bat to the switch of relay...86 is from the BCM(when BCM grounds this line, relay is turned on)......87 is the output of the relay switch to the fuses.......quick check is wrap a paper clip around terminal 86, have end of clip stick outside the relay...insert relay......now just take a ground and touch the end of the clip....low beams should come on.....this verifies the circuit........so the problem is either no output from the BCM, or no input to the BCM from the headlight switch....