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Possible Battery Short

2K views 23 replies 6 participants last post by  Manta Rallier 
#1 ·
After putting the motor back into my 1970 GT mostly to upgrade to 2.2l, but also added a pemanent magnet starter from OGTS, with a relay, before I connected the battery I thought I might check for potential electrical problems. So I checked the resistance from the lug on the starter solenoid that connects to the battery. I saw 1.5 ohm to ground. That seemed wrong. I pulled the wires (main cable from battery, wire to relay, wire from harness) off the lug on the solenoid. I saw the same 1.5 ohm to ground on the wire from the harness. On my car this is a red wire. I believe this is shown as Red & white on my schematic, 10 gauge, and in the schematic runs from the top lug on the starter solenoid to the middle of the fuse block. The key is in the lock position, and it looks like all the switches are off; headlights are retracted. I even pulled the wire off the alternator (B+) that comes from the harness, and the 3 pin connector. Still showing 1.5 ohm to ground on red wire, disconnected from the starter solenoid. My first question is, does anyone know what resistance you'd expect to see (with the battery disconnected) at that point? I'd expect it to be open to ground. Second, if I have potential short, does anyone have any suggestions about where to look?
 
#2 ·
carbon build up in your ignition switch?
 
#4 ·
First, check to see if everything is off (lights off, dash lights off, blinkers and flashers off, ignition switch off, door closed so the overhead lamp is off, cigarette lighter out, and so on). Then if it persists, recheck after the following:
  • pull all fuses
  • reverse leads on meter
  • remove output connection for alternator (which you did)
I've chased my tail before on this and found something like you are finding... IIRC it was the door open LOL. It seems like it would not be so low, as the bulb pulls only part of an amp so should have a restiance up in the tens of ohms. BUT when you only apply a very low voltage from a meter battery, any incandescent bulb filament resistance will be very low. Incandescent bulbs only have a high resistance once the filament glows and gets hot.

BTW, to get accurate readings for such low resistance values with a standard ohmmeter, connect your 2 leads togehter and read the resistance in the leads. Do that several times and average the lead reistance bvalues that you get, Then subtract that value out of the circuit readings to get the meter lead resistance out of the situation.
 
#5 ·
Also try disconnecting the alternator, both the plug from the regulator, AND the large red wire that attaches to the post (and keep it away from ground!).

During my very recent restoration - electrical circuit test, I had a 1/2 amp current draw with EVERYTHING shut off. Found it was a bad diode in the alternator rectifier section, Said alternator was a 1975 55 amp until, bought as "rebuilt" a few years ago. Changed to another alternator I had in stock (a 1974 45 amp until) and the current draw went away. Cost $135 CAD to get the 55 amp unit rebuilt (new rectifier section and new bearings), tested at 70 amp output. So two wins...
 
#6 ·
Thanks Everyone:

I'm not sure about carbon build-up in the switch; I could cycle the switch a few more times; not sure if a squirt of contact cleaner would actually penetrate and help.

I have the plug going to the relay unplugged from the relay. Plus the harness wire is off the solenoid. I'm pretty sure the relay is out of the picture.

In one test sequence, I did pull the plug from the solid state regulator to the alternator, but that did not help.

I had not thought at all about the door being open. Both were. Thanks, for that suggestion. On the other hand, I've never seen the dome light come on when the car was running. What was interesting, and a bit spooky was that after one door slam, the resistance went to 4.5 ohm; after another slam, up to 5.6 ohm (BTW: zero on the meter is typically 0.2 ohm); back to open in each case w/o change. I would hate to think I'm going to have to chase down where some garage mouse has been gnawing at the wiring. Also, good idea about the lighter; I actually had a USB charger plugged in; although unplugging it did nothing (that device really shouldn't draw anything if there's nothing plugged in and it is designed for automotive use). Pulling the fuses is a good idea. It would be nice if they were a bit more conveniently located. Rotating the headlights a bit and then setting them back caused a change in the resistance, as well. Now it's more like 8.5 ohm.
 
#9 ·
disconnect white plug from fusebox and check. Report findings
 
#10 ·
What do you have for an ignition system OP? Is the original resistance wire to coil + bypassed by any chance?

Put the fuses back in one at a time can check after you install each one.

Hmmm the door closure changes are indeed odd.... 'specially with the interior lamps never working to begin with. The fact that the resitances sdid not drop back down when you opened the doors again could well just be corrosion in the switch contacts.... they are cheap and simple and easy to corrode and get dirty.
 
#11 ·
The ignition is a Crane Fireball XR3000; that points replacement unit has an optical setup in the dizzy. BTW: Dizzy is not in the car yet; I was leaving it out until I did a pre-lube; but I probably will not do that until I get happy with the electrical; no sense in washing out the assembly lube, until I'm good to go.

The path I'm going down now, is to open the circuit from that eyelet on the solenoid at various points and see if the path to ground is upstream or downstream of the test point. The lugs on the fuse block are a convenient place to start. E.g., I removed the lug which on the wiring schematic is the NorthEast corner of the 4 lugs in the middle of the fuse block (the one to which the line from the battery/solenoid first connects), which resulted in a open to ground, where I had been measuring the short, previously; so it is NOT the case that there was a short, say where the harness went through the firewall, between the solenoid eyelet and the point where it attaches to the fuse block. We also know the problem can't be downstream of any fuse, because they are all removed. It's tedious, but I think I'll get there this way. Probably next weekend.

Also BTW: When the motor was out, I got to fussing with other things, and replaced the front turn signal bulbs with LEDs, that go both amber and white, and switched to clear lenses. I thought I could use the white, sort of as a daytime running light. When I did that, I replaced the flasher unit with one that's designed to prevent hyper flashing with LEDs. So I had the thought that maybe that new flasher unit was my problem. But, after removing the flasher the problem remains. On the other hand, it seems like every time I get in and out of the care the resistance bumps up a bit. Strange.
 
#12 ·
A short to ground will be zero ohms. Or as close to zero as your VOM will read. 1.5 ohms sounds like a bulb filament, OR A BAD ALTERNATOR RECTIFIER. Have you disconnected the large wire from the alternator post, as I suggested?
 
#14 ·
What about the white plug on the fuse box?
 
#15 ·
WRT the white plug. First, there are two white plugs; one for the instrument panel, and one to the steering column. I think the more likely is the steering column connector. However, I'm somewhat reluctant to pull that connector. I recall the last time I did that, I think when I pulled the instrument panel, the housing seemed really flimsy. It seemed like in trying to re-insert one plug, the box was beginning to crack, and the retaining tabs on the connector receptacle were pushing through the slots on the box. And I would really rather not get myself into a situation where I feel I have to replace that box.

I pulled individual wires from the lugs and tab connectors on the fuse block one at a time (re-connecting after each test). When I pulled one tab connector on the hot side of fuse #4 I saw open to ground from the eyelet on the starter to ground. It looks like that could either be the wire to the white steering column connector or the yellow instrument panel connector. Which would be a good reason to pull the white column connector, if I could do it gently enough. I was about to make that attempt, but when I reattached that tab connector I saw open from that eyelet to ground.

Kind of annoying, but somewhat consistent with the fact that the resistance value kept changing when I was chasing the problem. I'm not sure there's much to be done now. I guess I'll wait and see if/when it reappears. Probably the first symptom will be that the battery drains with the car parked in some particularly desolate location.
 
#17 ·
I pulled individual wires from the lugs and tab connectors on the fuse block one at a time (re-connecting after each test). When I pulled one tab connector on the hot side of fuse #4 I saw open to ground from the eyelet on the starter to ground. It looks like that could either be the wire to the white steering column connector or the yellow instrument panel connector. Which would be a good reason to pull the white column connector, if I could do it gently enough. I was about to make that attempt, but when I reattached that tab connector I saw open from that eyelet to ground.
If I am following your description correctly, there are 2 hot wires at this point: One to the ignition switch via that white connector in the column, and the other to the parking lamp switch via the yellow connector to the instrument panel. Wiggle the park lamp switch and make sure the instrument lamp rheostat is not in the full on position, and rotate it back and forth, and watch the resistance readings to see if therre is any correlation there.
 
#16 ·
You're indicating a short in the ignition switch. It should be removed and cleaned
 
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#18 ·
Probably I will not chase this until something shows up again, although there is something to be said, for cleaning or replacing the ign. switch.

If there were still some kind of varying resistance to ground still showing, I would manipulate the dimmer for the panel lights; but it's not showing up now, so that's not an option. While the problem was present, I did cycle the park lamp switch a couple of times, with no consistent change.
 
#19 ·
Easy test... cheap.... painless...

Measure the resistance.
Measure it again
Measure it a third time.
Note the resistance.

Cycle the ignition switch 10 times.

Measure the resistance.

If the resistance changes, voila
 
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#20 ·
I started putting things back together. I re-installed the fuses, and connected the wires to the top lug of the starter solenoid (main battery, blue to starter relay, red wire to fuse block); no distributor, no connection to alternator.
4.0, 4.0, 4.0 ohms
cycle key 10 times
4.1, 4.1, 4.0 ohms

I think I'll try connecting the battery and see if anything lights up. Plus I should be able to get a reading from the ammeter. If it really is a 3 amp draw, it's not like it's going to do a lot of damage.
 
#21 ·
That was interesting, and I think I now know what the problem was. When I connected the battery, I got a bit of a spark, heard a clunk from the neighborhood of the clock, and then the clock started ticking. I think when the clock winds down, it closes a contact so that it will use battery power to wind it up, and then it runs on the spring. Since the car had been powered down for so long, the clock was unwound, and ready to pull power from the battery to rewind. I bet that was it.

Disconnect, then reconnect of battery -> no spark, and no appreciable current draw measured on ammeter.

On disconnect of battery, solenoid lug to gnd ~ 250k ohm.
 
#23 ·
Yeah, that's how the clock works.
 
#22 ·
Thats pointing away from the ignition switch.

Hmm

Does it change when you put your turn signal lever in the "Right hand" turn position?
 
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