Ok, hopefully someone can help me out with this. I have bought this 1989 f350 crew cab with a 7.5l (460) 4x4 with a zf5 speed. I have done all kinds of work to this thing since I got it but my issue right now is a voltage drop. I already replaced the alternator (tried 2 different ones to make sure I didn't just get a bad one but seems to have same results)which has a external voltage regulator but on the alternator itself, also the plug going to alternator, terminals, optima yellow top battery, and starter relay. I also checked ho many amps I was using . 25-30 just running, around 50 with lights and defrost with fan on. And I checked my 96 f250 with a 5.8 and got around the same results . The problem is when first start up the voltage is 14.8, as I turn on the lights, defrost and fan it starts to slowly drop. Lowest I have seen to be 12.5. If Ibring the engine to 2500rpm or higher it stays around 14.2. I can't seem to understand. The alternator is a 75amp and I don't go over 50 what is dropping the voltage?

 

The 2G alternator you have is pretty weak, especially at idle. It's unlikely the alternator can output its full 75 amps at idle engine speed.

The recommended thing to do is to upgrade to the 3G alternator - it's much better.

 

2G alternators put out 65A *at best*.

A 130A 3G from a '93 Taurus will put out more than that at idle if called on.

There's a reason so many of us have switched from that horrible pos.

 

Welcome to FTE btw.

Everything has to be clean and tight when it comes to electrical connections. Cables get bad and start to act like resistors instead of conductors. Your truck, and its connections are 27 years old. Even a nearly invisible layer of skunge at a body or engine ground will interfere with good charging. A lot. Not saying this is your problem for sure, but it's a safe bet. It's free to check, won't hurt anything to clean up and may help. Trust Me.

As a quick test, connect a heavy jumper wire or cable from the battery negative terminal to a clean, unpainted spot on the alternator case. Measure voltage again at idle, and under load with lights at different RPM etc and see how it compares.

http://www.aa1car.com/library/charging_checks.htm

http://www.aa1car.com/library/2002/cm10220.htm

http://www.aa1car.com/library/voltage_drop_testing.htm