sixtyninestang

Hi, I am experiencing a problem with surging voltage on my 87 GT. From what I've read on here, the underdrive pulleys on it could be the cause, however this problem had not occurred until after changing the alternator. I replaced the alternator after it tested as being junk. I first cheaped out and bought one at the local parts store, but the problem didn't go away. I brought it back and they tested it and said it was good. Long story short, I got a refund and went and purchased a stock Motorcraft. I had the battery fully charged, replaced the alternator, and everything seemed fine. Last night I noticed the headlights fluctuating with high idle (more throttle, brighter lights). Other than the underdrive pullies, the motor is stock. I'm not running any auxiliary items requiring more power (stereo, lights, etc), and I have no interest in doing the 3G upgrade. Any suggestions? Thanks guys.

Tony71502

I was running underdrive pulleys long before I did the 3g upgrade. Never had an issue.

People will comment "remove the underdrive pulleys". That just hides a different problem.

Check your positive and negative cables for corrosion or loose fit. That's just a starting point. Somewhere in there is an electrical problem.

sixtyninestang

Thanks for the reply Tony. I cleaned the cables while changing the battery, but I hadn't thought to check them. Are there any other places I might want to check for corroded/loose connections?

Tony71502

Grounding straps. There is one that connects the cylinder head to the firewall.

im_stuck_in_iraq

3g upgrade?? mine kinda do that too

Tony71502

Alternator upgrade. I ran underdrive pulleys with the stock low-output alternator for years. The only reason I did the 3g alternator upgrade was because I was installing a mini-starter, battery reloc, and other junk... decided to do the entire system at once.

im_stuck_in_iraq

word.

How hard is it to do that battery relocation? pretty big gauge wires cant really splice them and twist together and run in the back. I think mine has the main one and a smaller one. jw

ttocs

if your voltage is fluctuating while idleing, yes that is your underdrive pullies. It might have just manifested itself with the new one but put the stock pullies back on just for ****s-n-giggles and see what happens. I ran underdrive pullies for a little less then a year and even with the stock alt pully on it still had voltage problems anytime it idled. I swapped the stock pullies back on after my alt died 7 months later. By slowing down the alt, you also slow down its ability to cool itself and it does not take long for it to become heat-soaked and then sooner or later that heat/cold stress will get to the voltage regulator or to the coils inside and cause a problem.

It can also lead to your battery failing prematurely because anytime your charging system is under 13v you are doing damage to your battery by undercharging it. When this happens it causes a chemical reaction called sulfation which after long enough can cause failures.

Seriously slap the stockers back on the car and I bet you will see a change in voltage but will not feel the loss in HP that you will get.... Only take 30 mins right to find out right?

Project 5.GO!

The output varies a bit on alternators, and depending on the idle speed, and what is being run on the car while it is idling. The stock alternator can either be barely staying ahead, or breaking even without underdrives. One person might put on underdrives and it brings them to break-even point and they think they are the best mod in the world and even say they can FEEL the measly couple hp they get, and the other person might put them on and not be able to keep a charge at idle.

On a car with a decent alternator, sure, spend the 60 bucks for the placebo mod so you feel faster, but on a car with a barely sufficient alternator, its asking for problems.

+1 on removing the underdrives.