Charging system question...

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

Reply to
JS
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Probably. It sounds like a dead cell in the battery which shorts the whole thing out. Test it with a $3.00 hydrometer. If you get one cell with a specific gravity far less than the others, replace it.

-JD

_________________________________ JD's Locally-Famous Mustang Page: http://207.13.104.8/users/jdadams

Note: Due to SPAM problems, I can only accept mail from those known to me. See my website for details.

Reply to
JD Adams

Argh... I was hoping you wouldn't say that. ;-)

Thanks for the tip. I'ma try the hydrometer thing first... not much in life do I hate more than chasing a bad ground problem. Maybe dental work and glass in the pit of a pinsetter (and crawling through it on your hands and knees to get it all out, then cutting yourself open to get it out of your hands and knees)... but that's about it.

I'm hoping that the fact that shutting down and restarting it has something to do with the battery. On the way home, once, I jogged the blower motor in much the same way as I described before and the volt gauge went up, for a while. Maybe applying a heavy load (starting, sudden blower motor draw) does something to make the battery want more juice for a while...

Hey, I can hope, can't I? ;-)

Thanks to Jim and JD for the advice... tomorrow I shall attempt to tackle it.

JS

Reply to
JS

HOw hard can it be?

Use a voltmeter... check output of alt to it's own frame. Should be near

14v. Check voltage from engine block to battery neg.. bet you get a few volts.

So fix the block to firewall ground and the batt neg to fender well ground.

I've never seen a bad cell pull the volt reading down, but I guess I might have missed that one.

JS opined in news:2V0Qa.2556$ snipped-for-privacy@nwrdny03.gnilink.net:

Reply to
Backyard Mechanic

Thanks for the assistance guys...

Put my Tri-Ax on it today, and took the opportunity of checking the stop bolts to ensure proper adjustment to take it out for a ride and warm it up some. By the time I got back, it was nice and warm... operating temperature... and charging at 12.5V-12.8V.

Using my trusty meter (which I'd been using to monitor the battery voltage from the cigarette lighter, temporary homemade digital gauge), I checked the negative to fender and got 0.01V. Acceptable. checked the battery to firewall (using the metal tab that contacts the hood when down with the holes in it on the passenger side, same reading. Checked negative to, say, alternator housing... 1.8V. Same to the intake plentum, valve cover, and AC compressor.

Just for kicks, took a set of jumper cables and hooked up to the negative, then to the AC compressor, and sure enough, battery voltage was 13.7V at idle.

Thanks for the assistance. I still hate chasing a ground problem. Comes from chasing ground loops in audio equipment... that sucks. At least I know what's wrong. I suppose the next time it's cold I'll change the block ground cable.

JS

Reply to
JS

JS responded, graciously

Hey... BTW the REAL problem's in the block terminal end of the big ground cable... but you need the block to firewall strap working good too, or you get the afore-mentioned "ground loop" and only your instruments and - soemtimes- ECM see them.

Reply to
Backyard Mechanic

I was going to check and clean both and replace what was required. I think I'ma just replace the negative cable for piece of mind and check + clean up the ground strap to the firewall.

Thanks again for the assistance.

JS

Reply to
JS

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