About two months ago I had a new AC delco alt. installed. Today No charge output. I Brought it Back and they istalled a new alt. agian. The volt on the dash reads 13.0 to 13.1 while on the Highway. This seems low/ The mechanic said the battery reads 7/10 ths of a volt higher then the gauge. (13.6 to 13.7)The want to install a new pcm board. Another dealer ship said it sounds like a dirty ground????? Any one have any Idea

Two things I am no fan of- replacing parts until you stumble on a solution, and blaming the computer/PCM/ECM/whatever right off the bat. I personally think that a voltmeter driven by the on-board electronics would be more accurate than .5-.7 volts. Simple and stupid stuff first- if you have a multimeter, check battery voltage with ignition off. That should read around 12.6v. If not, try recharging the battery, then check again. If it still doesn't come up to that, you'll need to test the battery. You're at 12.6? Good. Check the cables- clean and tight, both ends, both cables. I have what I call the "2 Basic Rules"™- 1) you have to hook it up and 2) you have to turn it on. A loose or corroded cable violates rule 1. Start the car (rule 2), and check your battery now. You should see at a minimum 13.8v, 14.2-14.3 optimum, 14.65 or better is overcharging. Do you get that? If yes, you have no problem, or we have to dig further for what is causing you to see a problem. If you see low voltage, we can continue in a more linear fashion. Now- here is where it gets a bit tricky. The PCM on your car monitors voltage, and not only gives you your voltmeter, but tells the alt. regulator what output is needed. I am more sympathetic to what the dealer called a 'dirty ground'- that is, likely something is corroded or loose and causing the PCM to act a little wonky. What I would do before spending several hundred dollars on a new PCM is check all of the grounds in the engine compartment, check the plug on the alternator if a wire is cracked or broken, maybe even pull the trim cover off of the PCM and see if it's excessively dirty inside. Check for cuts, burns, chafes, wires where only some of the strands are broken (can cause a bigger problem than you think, especially on newer cars like yours). Like I said- simple and stupid, because that is what causes most electrical problems.

Thank you HotrodElectric

At 8:03 the Battery read 12.72 volts with car not running.
After starting the car the Batt. read 14.43 volts/ At the dash 13.7.
After Five Mins of running with all acc. off 14.20 with the ground on the engine/ with the ground on the batt 14.16 /and with the ground on the strutt tower 14.12.
with the ac on high is reads 14.10 on the engine/ at the batt reads14.08/ on the strut it reads 14.08/ at the dash reads 12.9 to 13.0 volts

Your numbers as you check with a meter are good. I have to say I have a hard time believing the dash voltmeter is that far off- not that you're fudging, just that amount of inaccuracy surprises me. It could be the PCM, but I would want check out other things such as the instrument cluster first. At this point, I have to bow out because my specific knowledge on later model Caddies is limited. I do have a copy of 2000SI, email me later and I'll lookup what I can- there may be something to check neither of us know about.

heacox@cox.net