jrp27540@gmail.com

I bout a project car; 1988 lx mustang a few days ago. It has a hand full of minor issues that im trying to work out.

The previous owner installed underdrive pulleys, as well as re-located the battery to the back of the car. Currently the car has an impossible time keeping the volts up. Im assuming that is due to the under drive pulley installed on the stock alternator. The voltmeter on the dash moves up and down with the RPMs. At idle, it just cant charge. If not the underdrive pulley, could it have something to do with the way he wired the battery up to the back of the car? I havent got a volt tester on the batter/alt yet, but im almost positive it has to do with one of the two combinations.

I have also noticed that the fuel pressure is low to the point the car wont start after a while due to not enough fuel. He put in a bigger fuel pump, that im assuming requires more volts to run. Since the car is barely running off a dead battery and an alternator that can't keep enough volts pumping, wouldn't you expect that to cause issues with the fuel pump not supplying enough fuel? Just trying to make sure there isnt another problem going on with the fuel pump...

Thanks for any thoughts/advice.

v8miata

No one has replied so thought I would throw in my extremely cheap $.02.

You are right, you need to check and see if the battery and charging system works first.

A battery should always be in the 12.2-12.8 volt range. You can actually use this to get an idea of the battery health over all. After charging and sitting over night, check battery voltage.

A battery reading at 12.2 volts is shot and at it's end of life.

A battery reading at 12.5 volts is about halfway through its life.

A battery reading at 12.6-12.8 is a healthy battery.

Above 12.8 volts after sitting over night is an indication of a fault inside the battery.

This isn't a hard set rule, I have seen a battery that read 12.8 volts after sitting over night but couldn't produce enough CCA's to start the car.


Your alternator should be putting out 14.4 volts. Unless alternators are extremely cheap for the car, (my subaru alternator runs $300) I let them continue to live as long as they are producing above 12.8 volts of power.

I don't think you are suffering from a voltage issue, probably an amp delivery issue. Your car is a 12 volt system, if voltage drops too much nothing will work. You are (assuming) able to start the car so the voltage is adequate and the CCA's to turn the starter motor.

So as long as you are hearing the fuel pump engage, it is probably working fine, but you need to get a fuel pressure gauge and check it out if that is a concern.

The question is, what else could be causing the car not to start again? Assuming (again I know that is bad) the car is hot at that point? or has it sat and completely cooled off?

88 orangepeel notch

+1 ^^^

We need to know the exact volt reading on the battery while at idle. Your right, the UDP are not helping you. The stock 75 amp alternator is barely adequate at idle on a stock Mustang. Also, don't trust the stock gauges, they're well known to be inaccurate.

And for the fuel pressure, how do you know it's low? Did you check it with a gauge and if so, what was the pressure at ?

Oh, and welcome to the forum. Hope we can help get your Mustang figured out and running good.