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here's what I have observed in my 1997 ETC with 55k on it....

For a while now, I've noticed that if I go wide-open throttle, the headlamps will quickly dim at night. Originally, I thought this to be a result of the nose quickly rising up, aiming the lights into the air. Tonight I decided to go WOT while the voltage indicator was up, and my observation (and fear) was true - there is a significant decrease of voltage at WOT.

Here's what I observed: If at 50mph I floored the car, it would kick down, and very quickly the voltage would then drop from about 14.1 to about 12.1, but no lower. Once I was off the gas, voltage went right back up to 14. If i slowly increased RPMs without a kickdown, i'd keep the same 13.5 volts right up to the redline.

This leads me to believe two things, and this is where a true caddy "guru" can chime in - Either the belt was slipping, causing a sudden drop in voltage at WOT, or, a sudden WOT command causes an increase or surge of power to the ignition & fuel delivery systems, leading to a sudden draw of power. (Or the voltage regulator is compensating for a huge and sudden increase of output from the alt when the transmission kicks-down and RPMs fly up?)

I checked the belt tonight, and it's on very tight. The tensioners aren't making any noise, and the belt is in good shape. The car typically runs at about 13.5-13.8 volts, and has a new delco battery. To me, this says the charging system is healthy.

I'd like to believe my 2nd hypothesis, that there is a huge draw of power to the ignition & fuel systems, maybe to insure that a high-RPM leaning condition won't occur (I imagine that'd be a terrible problem in a big aluminum V8!). Regardless, the car runs great, and accelerates quite hard. I don't notice any knocking or other symptoms in the upper-end of the power-curve. (Just the throaty scream of an American V8 at WOT.... nothing in the world compares to that sound!)

If anyone has some spare time and wants to double-check my theory, let me know!

Thanks for all the help!

-Scott
 

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'98 STS (RIP @ 206,xxx miles)
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Scott:
What you are describing sounds like the serpentine belt would be your 1st suspect based on the voltages you mentioned. The voltage going to 12.x tells me the alternator is not carrying the load (for some reason) and belt slippage at high RPM is what I would be praying for. Maybe a quick dose of belt dressing would confirm the belt is slipping at higher RPMs if the system stays at whatever voltage you normally see.

Good luck,

Jim
 
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