1999 Ford Mustang



February, 26, 2008 AT 8:58 PM

Electrical problem
1999 Ford Mustang V8 Two Wheel Drive Manual 5000 miles

This car has basically been rebuilt front to back. Everything on it from the drivetrain, to the transmission, to the engine, to the suspension, has been replaced with top of the line performance pieces approximately 5000 miles ago by a reputable performance shop. The chrome Tuff Stuff alternator has the same 5,000 miles on it and the battery is a brand new Optima Red Top.

Here is my question. Under load or at approximately 1500-2000 RPM, the battery light comes on and if these RPM's are maintained or higher, the battery will eventually wear down and die. However, if I shift or keep the RPM's below this threshold, no battery light comes on an all appears fine. The alternator, even though basically new, was taken off and tested by a reputable alternator repair shop. It tests out fine, but they thought the belt might be slipping. So they put an OEM pulley on it to replace the chrome plated pulley. Put it back on the car, same thing happening. Took it back to them and told them to rebuild it, replace everything they could to include voltage regulator. Put it back on the car and replaced the old battery with the Optima. Still doing the same thing. There is no belt slippage whatsoever.

Of course, before I did the alternator thing and battery thing, I load tested both. Then and now I am seeing the same thing. 14.4 at idle, 12.2 with engine off, back down to 12.2 when at 1500-2000 rpm's and drops off from there under sustanined load. Checked what I could at far as grounds, but not sure where all are.

Help, this is very frustrating and I do not see any complaint like this on your website. I see complaints of the opposite. Meaning problems that are obious battery or alternator related symptoms.


5 Answers



February, 26, 2008 AT 11:19 PM

You sure its not computer controlled voltage regulator-I'm away from my database-I'll get back to you ASAP

I've you checked all the fuses involved w/the charging system.



February, 27, 2008 AT 12:19 AM

Okay am back -you might want to check wire to wire from and to the alternator-12.2 volts for an open voltage-meaning no load on battery-is kind of low. Load test the battery again if it doesn't hold 9.6 volts in 10secs while using starter as a loading device-replace it.

see below Charge System Diagram



February, 27, 2008 AT 6:41 AM

Here's a few tests to perform....



February, 28, 2008 AT 11:12 AM

I have done all the tests which all check out, grounds appear fine, cannot find fusable links off of the grey wires per your diagram. Do see the the grey wires and can trace them back to the fuse box on the fender well, but no fuses apparent in the wiring. All fuses in the fuse box and under the dash are fine.

Could it be the female part of the connector that plugs into the back of the voltage regulator on the alternator? Also, I have been told that possibly my model year car has a split regulator system where there is one residing in the ECM. Is this the case? If so, how do I check if it is bad? Replace the ECM and how would I do so when it has a specialty tune on it due to all the modification I have made to the car?

Thanks again,

DP in Georgia



February, 28, 2008 AT 11:18 AM

The alternator and battery are fine, by the way. I can eliminate these from being problematic. It is definitely in the wiring somewhere. Since it is happening cosistently at 2,000 RPM's and suddenly started happening, wouldn't that mean something just broke that controls the flow of power based off of RPM's. Again, the voltage regular on the alternator is perfect, the alternator tests out perfect off the car, and battery is a brand new Optima Red Top. The belt does not slip, tension is good, belt brand new. I can only imagine if it is not a split regulated system with a second regulator being in the ECM, then it has to be some kind of relay or switch somewhere else.

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