Low voltage under boost

86SS86LS

Member
The Bat reading on my scanmaster is dropping from 14 to the 11's when I start to get into boost. I first noticed it at night when the headlights and interior lights would dim at 15 pounds of boost. I have a feeling that this happens at the same time my SMC kit starts spraying. Any ideas? I am seing a few degrees of knock at above 15 psi and I suspect it is related to the fuel pump not getting enough voltage. The battery and alternator are both new and I never see less than 13.5 Volts during normal driving.
 
Shut the kit off and try 8-14 pounds of boost and check your voltages.

This assumes you are normally tuned for no KR at this level or put some 100 unleaded or a can of xylene in 4 gallons of gas in the tank for the test.

Otherwise put a hose with regulated compressed air on the SMC kit input hose barb and idle the car and see what happens to the voltages when you activate the kit.

Most likely it's not that however but it can be simulated and tested.
 
We've seen this problem a number of times, and the usual cause is the fact that parts store alternators cannot provide the voltage required when these cars are under boost. Even the new 'premium' alternators commonly sold are inadequate.
A Delco alternator or an aftermarket hi-output alternator is the solution.

HTH,
Rick
 
I adjusted the turn on point of the SMC kit up, and ran at a few seconds under 10 psi of boost. The voltage dropped to the 11's even without the alcohol spraying.

Yes, there is a hotwire kit.

The alternator is the stock replacement from Advance.

I've noticed that the negative battery cable is rubbing up against the passenger header. Could this be contributing to the problem?
 
Under boost things can stress and move under the heavy torque.

If a cable is loose or moving that could be a problem.

If the cable is simply up against the header sooner or later it will melt and be a permanent problem shouldn't be intermittant unless loose/stressed or broken inside the jacket.

I'd also check the belt tensioner with a couple of blip throttle moves while looking under the hood, perhaps there's a belt problem at WOT.

Another test could be to run a gauge on the alternator output stud directly and check the voltage under boost.

If it doesn't drop down it could be the feed cable to the battery that's stressed or bad or loose.

That would be my next test, a light bulb such as a trunk light could also be wired into the car for easier visual readings while driving under boost. Make sure you give it a good ground too.

If it dims you know it's the alternator itself not providing enough. Sometimes it's hard to read a digital voltmeter in the car under boost.
 
salvage - That was a great idea - I ran the multimeter wires from the alternator directly into the car and I taped the multimeter to the dash. Sure enough, as I got into boost the multimeter voltage dropped from 14.4 to the 12's, right along with the scanmaster readings.

So my problem is alternator related:

I plan on getting it checked at Advance. How reliable is their diagnosis? The alternator is a month old, the test sheet that came along with it indicates that it tested 150 amps at 6000 alternator rpm. If the test comes out good can I assume that the alternator is sufficient?

When pushing on the belt I can only get it to deflect around 1/2". It seems to be tight enough. Is there any way to check for a slipping belt?
 
Advance did the in car alternator test, showed around 35 amps at idle with the headlights on, the battery also checked out fine. Without putting a load on the alternator, I doubt the results are really telling me anything, he said that his bench tester did not have the ability to verify amp output under load.

I tried the prybar on the alt fan and the belt would not slip.

I did notice that volt light is not lighting with the key in the RUN position. However at idle and while cruising, the alternator is charging just fine. Could this be related at all to my voltage drop under boost?

I am going to try a replacement alternator and see if that helps. Are most people under the opinion that a properly functioning 120 amp alternator is adequate or do I need to look at upgrading to a 200 amp?
 
I did notice that volt light is not lighting with the key in the RUN position. However at idle and while cruising, the alternator is charging just fine. Could this be related at all to my voltage drop under boost?


Nope. If it's charging and the light is blown then the resistor on the alt lamp socket is causing it to charge as normal.

Make SURE you have CLEAN connections EVERYWHERE! Even the little ground going to the fender from the neg cable.
 
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