I recently acquired a 1988 Mustang LX 5.0 5-speed coupe. I drove the car 6 hours home from Virginia with not much of an issue. The only issue that I noted was that the voltmeter was reading low by the time I got home, headlights on most of the trip back. Car never felt underpowered and didn't try to die due to lack of voltage, and the headlight lenses are too bad to project much light on the roadway anyway.
A couple more trips locally with the lights on and wipers on, and the voltmeter was about on the line before the red area. The car still ran fine. I'm figuring it's needing an alternator at this point.
I swap the alternator with one that functioned the last time I drove the car I sourced it from. I also cleaned the battery terminals. What I noticed was that upon startup, the voltmeter seemed to be normal at first, then slowly fell.
I took the original alternator to get tested... 14V output, all was fine.
I swapped the orignal back in after the trial used one did not help. I also checked all of the wiring per schematic and all was well. No shorted wires, no brittle or cracked insulation. The car has 72,000 miles on it, but I figured I'd check what was exposed and check for continuity and shorts. All seems well.
I take the car for a test drive after this, using a digital multimeter hooked into the cigarette lighter to monitor voltage constantly. Upon initial cold start, the meter read 13.8V. As the car warmed some, it dropped to around 13.5V, then 13.3V where it seemed to stay pretty solid. Just before getting it to highway speeds, the car would only muster 13.0V without any accessories or lights, hitting the brakes caused the voltage to drop to 12.7V, regardless of engine RPM.
I was cruising at highway speeds with the meter showing 12.8V, when I decided to turn the car completely off. Restarting the car within 15 seconds, I was shocked the see the meter stabilize at 13.3V at engine idle immediately after restarting.
It stayed between 13.1V and 13.3V the rest of the 5 minute drive home, until I got to the last two blocks, when it dropped down around 12.5V. Restarting when I got home provided the same results - charging at 13.3V.
I know 13.3V is low.. but the alternator checked out ok on the test bench (alternator removed), and will charge at 13.8V on a cold start. Is this a battery issue?
I don't mind putting money into the car, I just don't want to throw parts at it that don't need to be replaced. If there's a wiring issue elsewhere, or a weak regulator, a battery isn't going to help.
What concerns me is the wiring into the dashboard. The car *will* not charge without the dash cluster in, I found this out the hard way in an '88 GT of mine. One wire runs from the positive terminal of the starter solenoid, through the ignition switch (connecting the circuit in the start and run positions), then to the alternator warning light in the dash (light bulb with a parallel resistor of unspecified value), and then to the regulator. This circuit checked out in the cold testing. I've only had the alternator warning light flicker a couple of times on the way home from a friend's house when the voltage started to get pretty low. It comes on at startup as it should. I do know, though, that if this circuit isn't connected, the car will not charge. I do have another cluster I could probably swap in just to see what happens, but that's a pain in the ass and I'm hoping to avoid that. If it does decide to work with the other cluster, I think I'll just make my own.
Another interesting thing, when driving and the volt gauge starts to read low, turning the blower motor to high drops the tach a notch and it does not come back up until the blower motor is turned off. Not just a bounce, but an actual 50-100 RPM drop that holds. Obviously the volt gauge follows suit.
If anyone has a clue as to what this could be, please let me know. I think I'm going to try to stuff another known decent battery in it and see if that helps matters any.
JS