Electrical Gremlins

[Jim Craik]

I seem to have what I hope is a small electrical glitch.

I was out driving today, freeway, headlights on (I usually keep them on as it is a very small black car), as I just left stop and go traffic getting on the freeway I still had the cooling fans on.

I first noted trouble when I turned on the turn signal and the tack dropped to zero! I quickly checked the gauges and the volt gauge showed no charge.

My first though was fan belt but the temp gauge was not rising. I turned off the lights and the tack came back, I turned off the fans and the volt gauge showed about 2 or 3 volts (normal is 12 to 14).

I headed for home but home is at the top of an 8 mile hill and on the weekend there are very slow cars and as it is hot I turned the fans back on.

After only a few seconds, the tack dropped out so I turned the fans back off, shut it off on a down hill so I cold bump start after it cooled down.

I made it home but I am not sure what to look for as a cause.

The motor is a Roush 342R, with a 100 amp alternator. Does anyone have any thoughts?

 

[Randy V]

Jim - I'd check the battery voltage and connections.. If your battery is under 12v, you may have lost a cell.. Use your home battery charger (no more than 10 amps is best) and leave it on for a few hours. You should come back to a battery that has at least 13v. If not, you should consider having it load-tested at the local garage..

 

[Lynn Erickson]

when the alternator is working it does all of the work up to a 100 amp load. it should show 14.2 to 14.5 volts on the voltage gauge. if the voltage goes down below 14.2 volts it means you are running off the battery which at first will be about 12 volts under load. as the battery power is used up the voltage will go down until systems stop working. I would say it is the alternator is not working. easy check is to start the engine and then disconnect the ground wire from the battery if the engine stops then the alternator is not working. or check with a volt meter, a fully charged battery should be 12.65 volts, with alternator running it should read 14.2 or higher

 

[Seymour Snerd]

Jim --

My first thought is to remember that you had a major electrical failure a while back that involved smoke and melted insulation on heavy red wires near the battery, so my first question would be: what was that about and how was it repaired? And what collateral damage might have occured in the wiring harness that runs forward from there that they might not have noticed?

My second thought is that you describe only two phenomena neither of which in and of itself is clearly a problem. The reason I say that is that a) the tach reads zero when the engine is running fine and 2) the voltmeter reads zero when the engine is running fine. Clearly in this case the tach and the voltmeter are both "wrong" in some sense because 1) the engine didn't stop (while the tach says "0 RPM") and 2) if the voltage at the battery dropped to 2-3 volts or zero the engine would have shut down. So at least from the engine and ignition system's point of view none of this happened.

So I suspect there is a problem in the 12V supply to the tach and voltmeter. I further suspect that it's a loose connection of some kind, since you have at least one instance where the phenomena occured with no "immediate cause" (you turn on the fans and later the tach drops out again).

Given what you said I don't particularly suspect the battery itself, since there's not a likely way it would jump from 12 to zero to 3 volts all by itself.

However, there is a complex (and therefore unlikely) alternative scenario that could involve the battery but it would require the battery and all of its loads to have been disconnected from the alternator, coil and ignition box while you were driving. IOW, it requires the engine to be able to run with no help from the battery. In that scenario the battery could be dead or nearly dead but the engine would still run. However, for that to happen the battery would have to have drained since the last time you started it, and furthermore if you went to start it now it wouldn't. Now, having said I don't believe that one much, it does have the interesting property that it kind of matches the wiring layout (alt, battery, ign are all near each other and separated from everything else), so in that sense it's kind of plausible. And it also corresponds to a scenario where the guys who went in to fix your heavy-red-wired melted-insulation problem didn't quite completely fix it.

Unless of course I've misunderstood, but if I understand the sequence right, except for when you shut it down, the engine is running all this time, right?.

But I think the upshot is it's time to take it back to the guys who (supposedly) fixed your earlier problem. I think they missed something.

The other thing to look for is a loose ground near the dash (eg there is a collection of them right behind the center switch panel). Having a ground open up can cause really strange-looking interactions like what you saw between the fan, tach, turn signal, etc. And a loose ground up in the dash might not cause the engine to shut down as long as it didn't involve the ignition and fuel pump relays.

 

[Jim Craik]

Thanks for the input guys!

I will check the connections and the grounds when I get home.

Alan, you are correct about the earlier electrical problems I had (you have a great memory). I think this could be connected to them but I want to check for other simple explanations before I get into that can of worms again. (I have put approx 1,000 trouble free miles on her since the repairs).

One thing I failed to mention before, when trying to drive up hill and through town with only the rad fans on the voltage kept dropping and the temp was going up (as the fans slowed?) so the alternator was not keeping the car running and I was running on the battery only.

 

[Seymour Snerd]

Thanks for the input guys!


One thing I failed to mention before, when trying to drive up hill and through town with only the rad fans on the voltage kept dropping and the temp was going up (as the fans slowed?) so the alternator was not keeping the car running and I was running on the battery only.

I'm not sure exactly what you mean by "keeping the car running" but to me that symptom sounds like the alternator was keeping the engine running but everything else was running off the battery, which was slowly dying.

Thinking some more about your earlier issue, keep in mind that if during that event the "big red" wire that got so hot did so in the middle of the wiring harness, it could have melted the insulation on some/all the other wires in the harness. This means that potentially any pair of wires in that harness could have melted together, which could leave you with an intermittent short in there. Off hand I can't think of a short circuit that would cause your symptoms, but it's something to consider. It might call for untaping and pulling apart the harness between the engine and the dash to inspect the condition of each wire's insulation.

A basic test would be to verify that at each relevant point in the entire system that is supposed to be "+12V", when the ignition is on but engine off, the voltage reads "12.4" (or whatever) everywhere and with the engine running all "+12V" points read "14" (or whatever) when the engine IS running. This would tell you if any of the +12V points are missing out on their connection to the alternator.

If it weren't for all the "now it's 3 volts, now it's zero volts" stuff I would thinkg that we're simply talking about the alternator having been disconnected or failing.

One more thing to check before everything else (especially before measuring voltage relative to the frame): all the "big black" connections: ground cable to frame at the rear crossmember, ground cable to engine block, ground cable to battery. Any of those being loose could cause very odd things to happen.

 

[Ian Anderson]

My memory says that there was a recomended upgrade on the wiring loom inserting 3 relays for main power draw units.

Now I'm at the office so cannot spent too much time searching

But if your car did not have the modification done I would think your main swithces could be trying to fry themselves (perhaps ignition switch or even switches for lights fans etc.


Also if you have managed to lose voltage somehow (probably a short) and knowing that Power /Volts = amps drawn your current loadings could have been exceeded hus showing up the other "events"

Ian

 

[Jac Mac]

Seems strange that a 100Amp Alternator couldnt keep the car running even if the battery was dead & Lights on, also you didnt mention whether the car was started on the battery before & or after the problem..IE was the voltmeter telling the truth?

 

[Seymour Snerd]

My memory says that there was a recomended upgrade on the wiring loom inserting 3 relays for main power draw units.

You're probably referring to the 2-relay fix to protect the two highest settings of the heater/AC blower.