1988 3.2 Electrical Troubleshooting Help?

neilschelly · 04:57 AM

I came home last night, and as I was backing down my driveway, I glanced forward to see the alternator light was on and the gauges were all dead at zero after a 90 minute drive with no issues. The car is a daily driver, so this was definitely unusual. This morning, it started up in the same way - gauges all at zero and the alternator light on. Always starts up fine. Here's the symptoms I'm looking at:

Before starting:
* Voltage at battery is around 12.4V. (measured with a high-end Agilent DMM)

After starting:
* Voltage at battery around 11.8-11.9V
* Gauges are all down at zero.
* Alternator light is on.

After revving the motor a bit:
* Voltage at battery up at 13.4-13.8V
* When I rev the motor, the gauges come to life.
* Alternator light is on.

During driving (20 minute drive to work)
* Car drives normally.
* Watching voltage on a cigarette-lighter gauge varies in the 11.9-12.8V range, seemingly randomly. Often boosts when I turn off something big like defroster, headlights, heater blower, etc. Always seems to settle around 12.2V eventually.
* Turn signals don't work, inside or out.
* Gauges are a little wonky, but mostly work. Tachometer seems to read about 400-800 RPM low. At idle, they bounce a bit more than usual.

After driving, idling:
* Voltage at battery still up at 13.4-13.8V

Theories:
* I think a bad voltage regulator would be causing over-charging circumstances, so I'm not focusing there.
* I'm guessing the alternator is original, so perhaps it's getting tired and needs a refresh?
* The weather in New England lately has been crazy warm (40-50 degrees) and crazy humid due to all the snow melting the last 2 weeks, so there's a good chance some electrical connections aren't making solid contact due to all that extra moisture/humidity and the oxidation that may be coming with it.
* Could the alternator belt be loosening up? It's probably about 30k miles old and just over 2 years old now. Is that tension adjustable?

Anyone know any good starting points? I haven't dug around the engine bay yet focusing on electronics, but are there any particularly good places to check voltage closer to the alternator?

Thanks in advance!
-N

Chuck.H · Chuck.H's Garage

Sounds to me like your alternater is not charging - but you want to rule out a short or a bad battery... not always the easiest thing to do.

You can try charging the battery externally and then it should run the car fine for a while before the problem returns. If you can't do this then something is bringing the voltage down (rather than not charging).

Chuck.H
'89 TurboLookTarga, 370k miles

pete3799 · pete3799's Garage

I would say it's time for an alt. rebuild.
Turn signals may be a different issue.

ischmitz · ischmitz's Garage

agree to above comments. The brushes of the alternator are about gone and charging happens sometimes but intermittent. Get the alternator out and inspect the brushes. I bet they are worn to nothing. Either get new brushes and DIY or have the alternator rebuilt.

neilschelly · 01-16-2014, 03:31 PM

More information and a fix:
* Car still drove normally to and from work today, 20 minutes each way.
* The fuse for the turn signals, hazard lights, and reverse lights was busted - not sure how I missed then when I checked last night. L/R parking light circuits worked fine when the car was off.
* When I flipped the turn signal stalk, all the gauges dropped a bit closer to zero.
* When I switched into reverse, the gauges all dropped to zero until the reverse light switch was released.

I replaced the busted fuse when I got home from work today, and everything appears fine now.
-N

neilschelly · 01-22-2014, 02:06 AM

That fuse has busted another few times now, so something is definitely not right still. It's predictable though. Whenever I go into reverse and tap the throttle, the fuse blows. If I just put it in reverse without any throttle, it stays intact.

I haven't dig in to figure this one out yet. Too cold to go digging for some electrical gremlin lately. I just keep replacing the fuse.
-N

DCinVA · 01-22-2014, 04:19 AM

Predictability is a huge help in diagnosing the issue.
I would be looking for an intermittent short in the back up lights wiring. Given that it happens when in reverse, and when you bump the throttle, I would look very closely at the wiring at the reverse light switch on the trans. When you bump the throttle the transmission moves slightly, which could move the wire and short it out.

Just a place to start looking. You might also want to pull the tail lights and check the wiring there. Look carefully for worn insulation, especially near areas where the wiring goes through the body.

Good Luck

neilschelly · 01-22-2014, 04:24 AM

Both are on my todo list, but it's also roughly 3 degrees out and windy right now. I replaced the fuse and drove to work. ;-) The forecast isn't looking too much better, so I may just be buying a lot of fuses soon.

The reverse light switch seems the most likely to me since I can idle in reverse with nothing happening, and that was what I was looking to test next. I'll see if I can gently increase throttle without setting off the fuse, which to me would indicate that if I keep the engine mounts from flexing, it'll be fine. We'll see how that goes tonight when I get home.
-N

ischmitz · ischmitz's Garage

Throttle linkage in the shifter tunnel has worn through the cabling for the reverse light switch...

rick-l · 01-22-2014, 11:20 AM

Quote:

Originally Posted by ischmitz

Throttle linkage in the shifter tunnel has worn through the cabling for the reverse light switch...

Fuse Box 1 fuse 8 powers the back up lights, turn signals, guages and ..... charge indicator light.

If this fuse blew and then the fault was removed it is conceivable the alternator light is back feeding all that stuff on the circuit when you rev it high enough to kick the alternator on.

rick-l · 01-22-2014, 11:24 AM

Does the alternator light vary in intensity as you turn the turn signals on? Id say back up lights also but that is probably where the fault is.

EDIT: with the fuse blown of course

neilschelly · 01-23-2014, 02:43 AM

Just hitting the throttle gently doesn't seem to have an effect. I'm definitely able to use reverse sometimes without the fuse immediately failing, but if I sit long enough, I think it probably always will.

Maybe this weekend, I'll be able to get under the car and inspect the actual reverse light switch itself. It's still cold, but I think it's at least supposed to stay dry.

rick-l: Yes, the alternator light does vary in brightness somewhat as I'm driving with the fuse blown, and it definitely varies in brightness more with the turn signals in use, and even more still when I switch into reverse. I'm not sure I follow what you're suggesting by "conceivable that the alternator light is back feeding all that stuff on the circuit..." Are you suggesting the alternator is dumping 12V to ground when the circuit fault is active after the fuse has blown?
-Neil

rick-l · 01-23-2014, 06:44 AM

Normally what happens is the the regulator gets power through the alternator warning light (and fuse 8) to provide field current during startup. After the car starts field current is provided by the diode triple in the alternator and there is battery voltage on both sides of the warning light and it is off.

If there is not battery potential on one side of the light, like when the fuse is blown, and just a load there the load will be fed by the diode triple in the alternator through the light.

The reason you have to rev it high is there is no field current through the light and the alternator relies on residual magnetism to start.

So the bottom line is you probably have an intermittent short on your back up lights that takes out the fuse and for the time being your alternator is fine.

neilschelly · 01-23-2014, 10:58 AM

Ah, understood. Yes, I had already determined the alternator was fine once I found that the fuse was the initial problem, but I had no idea how that bulb worked and was definitely curious how it was wired up because of it's odd behavior. Thanks!
-N

DCinVA · 01-23-2014, 02:27 PM

Excellent post Rick I

You have tied it all together so it all makes sense.

Nice work!

Thanks

darrin · 01-23-2014, 02:34 PM

very cool troubleshooting! -- though probably a moot point, I just wanted to add that if your alternator belt's too loose, it's rather easy to turn the fan by hand when the engine's off -- you tighten the belt by removing the bolt holding on the fan pulley and transferring a shim or two that's between the pulley halves to the outside of the pulley (so that it's between the pulley and the fan pulley bolt).