dropTopTx

Well I think I found my grounding problem from last week. Now my electrical knowledge leaves much to be desired so Im looking for someone to confirm what I found was the cause of my electrical fire! :eek:

In the photo below you might notice the nut on the far post of the fuel gage is not tightened down. It was actually flopping around. The one on the inner post was not completely tight either. While adjusting the timing I had a major cloud of smoke and then the dash harness was cooked almost all the way to the sending unit on the tank.

So the question is
1) Could these lose wires have caused the short
2) Is the fuel gauge and the sending unit likely damaged?




[Modified by perryth, 9:41 PM 2/3/2004]

magicv8

Here's my guess:

Since both fuel guage wires are toast, the current burned through the guage (and likely damaged it) trying to get to a ground between the guage and the tank. If the wiring at the tank is not damaged, you will find that the harness between the guage and the tank provided a ground for the current. In that case, the sender is likely not damaged. :lurk:

dropTopTx

Thanks magic. A few follow-up questions

Was the fact that the wires were not tightened down enough to short this out or should I look for something else that as the root cause?

The brown wire (bare and completely broken in the pic) goes to the sending unit. From the sending unit a ground goes back to the frame ground at the license frame. The pink is the power source for the gauge? Did something happen upstream on the pink?

Whats the best way to test the fuel gauge to determine if its damaged?

jerrybramlett

I think gauge failure started this mess.

The sender is a variable resister that not designed to take a full 12 volts across it. That little red tag on the left terminal of the gauge says something like: never touch this terminal with 12 volts or you'll ruin the sender. At one end of the range the sender has essentially 0 resistance. The high end is about 30 ohms. You can test it with an ohm meter after removing it, but I'm betting it's a goner.

You can still buy the tank sender from Delco for a reasonable price. The gauge you're going to have to send out for repair.


[Modified by jerrybramlett, 9:38 AM 2/4/2004]

GaryC

Perryth,

This is my take on the problem:

I don't think that the loose nut cause the electrical fire. A loose connection will result in a higher resistance rather than a lower (shorted) resistance. Now, a loose connection could cause the lug to short against something else but, in your case, it doesn’t appear that the lug of the pink wire (12-volt line) could touch anything near it.

Now we are assuming that the wires melted due to a short somewhere which caused a high current flow. I was looking at the wiring diagram for your fuel system and I don’t agree with ‘magicV8’ that a short between the gauge and the tank caused the problem. The reason is that the sender unit in the tank has a range from 0 ohms to 32 ohms. A 0 ohm value is achieved by shorting the wiper arm on the sender unit to ground. So, a “short” to ground is a valid value. This means that a zero ohm reading at the tan wire side of the gauge is OK. The circuit is able to handle this “high” current load.

I think the problem was caused by the gauge itself. I think that the gauge malfunctioned and caused the 12-volt line (pink wire) to be shorted directly to the sender wiper line (tan wire) via the inside of the gauge. And, if the sender wiper was closer to the 0 ohm side (empty gas tank) then the 12-volt line was connected to a very low resistance because if the gauge was shorted (0 ohms) and the wiper was close to ground (0 ohms) then the 12-volt line could have been connected to less than a 1 ohm load.

Well, that is my guess, I hope that you find your problem.

GaryC

[Modified by GaryC, 11:21 AM 2/4/2004]


[Modified by GaryC, 11:22 AM 2/4/2004]

greg454

:iagree:
The only other possibility I could think of is if there was some loose piece of metal behind the gage that was able to touch both lugs on the back of the gage.

Greg

Dyna

I agree with GaryC and JerryBramlett. the gage is the root cause of the incident. Note that a healthy gage couldn't possibly carry enough current throught itself to cause those hefty copper wires to overheat thier insulation. Thus, the gage had to have something going wrong with it internally to initiate the meltdown.

Dyna

2 Ag R8Cs

:withstupid: It's all been said I can't add anything else.

58Mike

If the nuts were loose and the guage could move around in the case, you might have developed a short if the two posts from the fuel guage contacted the case. If that happened, you might also have a fried ground wire somewhere.

- Mike Greene

dropTopTx

The verdict is in. Sent the gauge out for repair and got word back today. He found that the tabs attached to the studs had turned and grounded out. Says its a pretty common problem.

Glad to have found it cause I was going to be very nervous about firing it up again if he hadnt found something definitive. :skep:

Thanks for all the info :thumbs:

GaryC

Glad to hear that you found the source of the problem.

Now of course you will need to replace the burnt wires but, you may want to do one other thing before you reinstall the gauge. You may want to check the condition of the sending unit in the tank. The high current draw could have damaged the windings on the sending unit also. At least the windings from where the wiper arm made contact and the ground side of the windings. A quick test would be to measure the resistance from the spade lug connection on the sending unit where the tan wire was connected to ground. Depending on the location of the wiper arm (i.e., how much fuel there is in the tank), the measurement should be anywhere from 32 ohms (give or take a few ohms) down to 0 ohms. If the sender was damaged, it would probably read an open.

Good Luck,

GaryC



[Modified by GaryC, 10:47 AM 2/10/2004]

dropTopTx

UPS man dropped a sending unit at the front door today. Dropped the new wiring harness a few days ago. He's been a regular around here lately....

I am curious so I will do the test as described just to see. I ordered the new one based on the better safe than sorry approach.

At this point there are very few parts that have not been restored or replaced and in the last week two of them went out. The starter was acting questionable long ago but I thought I could get some more time out of it but I only got about 10 starts since putting everything back together. The fuel/temp gauge was one of the few others that is not new :rolleyes: