Elecricial ground problem

[GAWen]

RQB 1600 had a severe electrical system fire 2 years ago. Today, I finally finished rebuilding the factory wiring harness. Yipee.. Nope. I still have a 5.2K Ohm ground, with all fuses pulled and all relays disconnected. Can anyone give me some guidance. I have reached the end of my patience and knowledge.

Thanks,

George RQB1600

gandcwendell@gmail.com

[Avanti83]

George

To be sure we are on the same page, How and where are you measuring the resistance? Such as, across the battery cables with the battery disconnected or ??

Bob

[GAWen]

Battery out of the system, measuring between the battery negative and positive leads.

[Avanti83]

My first thought is that not all circuits are fuse protected unless you rewired them that way. There are a couple of fuseable links that carry unfused power and could show resistance potentially, so you may need to disconnect the alternator and starter leads for starters if you need to resolve the question.

If it continues to worry you, you may want to start disconnecting each accessory.

I spent two days hunting for a similiar issue in a friends 63 vette and it turned out to be a cigarette lighter socket that had a small problem from a shorted lighter element that had been removed.

Bob

Edited May 25, 2011 by Avanti83

[GAWen]

Cigarette lighter is disconnected, doors are closed, and all switches are in the off position. The rear window defroster blower is removed, the power antenna is removed radio is removed. Damned if I know where to look. So many thousands of dollars invested, and still a broke Avanti...

Edited May 25, 2011 by GAWen

[WayneC]

Why are you measuring resistance?

I'm not really all that confident working on electrics, but I tend to agree with Avanti83 that you should take the alternator out of the system; the starter I'm not so sure about.

Are you measuring across the battery cables with the ignition key both on AND off?

Do you get the same resistance in both instances?

The battery positive cable is connected at the far end to the starter solenoid, and from there a smaller wire goes to the ammeter to connect the rest of the electrical system, to eventually flow back to the battery... perhaps you are measuring the resistance (ohms) of the ammeter? You could probably test that theory by probing across the ammeter posts with your ohmmeter.

But... I question whether the test you are doing is definitive, since an electrical fire issue is due to a short, not a resistance, I think. A short is the absence of resistance, so measuring resistance may not tell you much. In fact, I'd be concerned if you DON'T see any resistance measuring across the disconnected battery cables, because that's a dead short (direct connection to ground with too-low or no resistance), a high-amp draw, which is what your fuses protect you against (when one blows, it immediately takes the resistance on that circuit to infinity). The battery works against the resistance of each circuit, producing higher amperage in a circuit when a device on that circuit goes bad (like bearings going out on a heater blower motor) and the fuse is inline in order to be sacrificed if the circuit amperage rises above the amperage rating of the fuse.

Electrical draws are generally measured using the ampere scales on a multimeter connected between the negative battery post and the (disconnected) negative cable, with expected readings in the under-50 milliamp range when all devices are turned off.

Start with a high amp scale and work down from there, because applying too many amps across the multimeter can ruin an inexpensive multimeter or at least blow the internal fuse on better-quality multimeters. Either remove fuses one by one, looking for a drop in amps, or start with them removed and insert one at a time, looking for a jump in amps. If there's still a high amp draw with all fuses removed, then there's a problem with an unfused circuit (starter or alternator are likely candidates). The concern is that a device on an unfused circuit requires more amperage than the wiring connected to it can carry, and I'm not sure how you'd measure that without activating the device.

Those thoughts are off the top of my head; if I get a chance tomorrow I'll take a long look at the circuit diagram. I've never tried the test that you are doing, so I don't know what the ohm readings would normally be.

But, I repeat, I don't claim to know a whole lot on the subject of vehicle electrics.

Out of curiosity, where did your electrical fire start?

Edited May 25, 2011 by WayneC

[GAWen]

The fire started in the main body wiring harness under the hood, approximately where the horn relay is located. The fire then traveled thru the main harness to the dash area. EVERY ground wire was destroyed along with a considerable number of power leads.

George

[GAWen]

Played with the electrical system some more this morning. The resistance is not in the ampmeter unit. Decided to get brave, bold and daring, and connected the battery (with all fuses pulled), no sparks, that was a good sign. No fire, even better sign. Turned the key, and voila, the engine rotated 4 times and rumbled to life. After 2 years, finally. Now to continue with totally solving the electrical system. BTW, I did find a bad wire, touching the alternator case,, at the alternator plug-in connector.

George

[adamderosa]

Sorry to hear about your troubles. Do you have a good, clean, ground strap connection from the engine to the frame?

[GAWen]

Sorry to hear about your troubles. Do you have a good, clean, ground strap connection from the engine to the frame?

Not only one ground strap, but three ground straps.

[WayneC]

As I said, I'm no expert, but I'd like to learn more, to understand why you're testing ohms instead of amps.

You haven't told us what reading you were expecting.

The ohm reading you are getting between the disconnected battery cables does sound high, but I don't know what it is supposed to be, do you? In other words, why do you think that reading means it's broke? Seems to me that if the reading were zero ohms (which is what the reading would be if you measure at opposite ends of a wire that's not connected to anything), that would be a dead short... so if you connected the cables to the battery, it would smoke the wiring or the battery, just as though you connected a cable right across the battery posts. Therefore you don't want a zero-ohm reading; what reading do you want? Are you looking for an infinity ohm reading? That would seem logical, it would mean nothing is using any power, but every device would have to be disconnected, including the clock and any keep-alive circuitry like in an aftermarket radio, and any courtesy lights that might have faulty switches (glove box, trunk light, etc); a bad alternator diode can cause a leak to ground, also.

Edited May 25, 2011 by WayneC

[GAWen]

As I said, I'm no expert, but I'd like to learn more, to understand why you're testing ohms instead of amps.

You haven't told us what reading you were expecting.

The ohm reading you are getting between the disconnected battery cables does sound high, but I don't know what it is supposed to be, do you? In other words, why do you think that reading means it's broke? Seems to me that if the reading were zero ohms (which is what the reading would be if you measure at opposite ends of a wire that's not connected to anything), that would be a dead short... so if you connected the cables to the battery, it would smoke the wiring or the battery, just as though you connected a cable right across the battery posts. Therefore you don't want a zero-ohm reading; what reading do you want? Are you looking for an infinity ohm reading? That would seem logical, it would mean nothing is using any power, but every device would have to be disconnected, including the clock and any keep-alive circuitry like in an aftermarket radio, and any courtesy lights that might have faulty switches (glove box, trunk light, etc); a bad alternator diode can cause a leak to ground, also.

I was hoping to have a reading close to infinity, as every parasitic device is currently disconnected or removed from the vehicle. I was able to get the vehicle to start 2 times, and then nothing, no clicking, no smoke, I suspect that the neutral safety switch may be activated.

[WayneC]

It's good that there was no smoke, I once read an article that claimed that letting the smoke escape from the wires is a major cause of electrical problems

Forgive me if I ramble, I'm thinking out loud (by typing). My usual disclaimer still applies.

Does your car have an automatic transmission? (I assume it does, if it has a neutral safety switch, I don't know if manual transmission Avanti's have a neutral switch).

Did you read 5k ohms with the ignition key off, or on?

Consider trying your ohms test with the ignition switch in each position (off, on, start).

If the readings vary, there might be some clues there, though I'm not sure what at the moment.

Did you unplug the wire/wires to the alternator?

As I think about it, 5k ohms is a huge load... when you are expecting infinity (open circuit) with the ignition turned off, that means something is allowing a path back to the negative side of the battery, but only at the expense of going through a huge resistance; it's not as simple as a light bulb, clock, or a coil resistor. Maybe there's a way to check the ignition switch itself, to see if it completely opens the circuit through it to infinity ohms.

The alternator diodes put up a big resistance to flow in one direction only (my guess is in the thousands of ohms, which just happens to be what you are reading), but the diodes probably don't stop flow entirely, just effectively, which is why I asked if you removed the wiring from the alternator to take it out of the un-fused electrical circuit. If diodes were the issue, it might even be possible that just reversing the ohmmeter leads on the battery cables could change that high ohms reading???!!! Does your alternator have "field" and "output" wires going to it? Maybe remove those leads at the alternator and jumper them to each other for your ohms test?

(just thinking out loud again, follow my suggestions at your own risk ).

Edited May 26, 2011 by WayneC

[Charlie Kile]

It's good that there was no smoke, I once read an article that claimed that letting the smoke escape from the wires is a major cause of electrical problems

Forgive me if I ramble, I'm thinking out loud (by typing). My usual disclaimer still applies.

Does your car have an automatic transmission? (I assume it does, if it has a neutral safety switch, I don't know if manual transmission Avanti's have a neutral switch).

Did you read 5k ohms with the ignition key off, or on?

Consider trying your ohms test with the ignition switch in each position (off, on, start).

If the readings vary, there might be some clues there, though I'm not sure what at the moment.

Did you unplug the wire/wires to the alternator?

As I think about it, 5k ohms is a huge load... when you are expecting infinity (open circuit) with the ignition turned off, that means something is allowing a path back to the negative side of the battery, but only at the expense of going through a huge resistance; it's not as simple as a light bulb, clock, or a coil resistor. Maybe there's a way to check the ignition switch itself, to see if it completely opens the circuit through it to infinity ohms.

The alternator diodes put up a big resistance to flow in one direction only (my guess is in the thousands of ohms, which just happens to be what you are reading), but the diodes probably don't stop flow entirely, just effectively, which is why I asked if you removed the wiring from the alternator to take it out of the un-fused electrical circuit. If diodes were the issue, it might even be possible that just reversing the ohmmeter leads on the battery cables could change that high ohms reading???!!! Does your alternator have "field" and "output" wires going to it? Maybe remove those leads at the alternator and jumper them to each other for your ohms test?

(just thinking out loud again, follow my suggestions at your own risk ).

At 5k you will only be drawing 2.8 miliamps or .0028 amps.

Good chance if you have a clock that's what you're reading.

The sleeping computer in my 84 draws more current than that.

If you are in a humid area just the relitive humidity with all those connections might cause them to colectivly be the problem.

Charlie RQB3921

[WayneC]

At 5k you will only be drawing 2.8 miliamps or .0028 amps.

Good chance if you have a clock that's what you're reading.

The sleeping computer in my 84 draws more current than that.

If you are in a humid area just the relitive humidity with all those connections might cause them to colectivly be the problem.

Charlie RQB3921

Dohhh! You're right, Charlie... amps=volts/ohms.

I've been trying to understand why George was measuring ohms rather than the more usual method of measuring amps, which is why I asked that question. I figured he wanted to get everything debugged before he connected the battery. I've always used the amps method with the battery connected via the multimeter but I should have thought to do the calculation... 2.8 milliamps is a negligible loss in an automotive electrical system, even 50 milliamps is acceptable.