Chasing a short circuit on 280SE 4.5

[Bezant]

Hi guys,


I recently tore stuff up to fix my heater and AC. Both went well and are working now but I have a high current draw short on fuse #2.


I am not sure how long it has been shorted since I haven't used anything on that fuse in some time, but I think it was working before I took stuff apart to do the climate controls.


That fuse has a lot of connections, radio is wired there (should not be but is thanks to a PO, was working fine), cig lighter, horn, wipers and windshield washer.


I unplugged the radio and cig lighter first thing as those were what I had to remove for the climate control job, but nothing.



I also unplugged the wiper motor and the round relay under the hood which I believe is the AC changeover relay on the wiring diagram, but I was still shorted.


All of the dash wiring to me looks to be in great condition and I can't see any fatigued wires.


I realize chasing a short is a pain, but I feel like there must be something obvious I'm missing.

[meltedpanda]

are you blowing a fuse or just a draw?
There has to be a good ground from either the radio or the cig lighter that has been disturbed or not reconnected . IIRC the ground is a connection to the dash pod metal frame . Try running a temp ground to that circuit , see what happens

[Bezant]

Blows fuse right away. The radio doesn't have a ground wire, I believe it grounds through the body. The cig lighter is just a two prong plug.

[Tomguy]

Unplugging the items themselves reveals the devices (radio, wipers, etc) are not the short but does not rule out the wiring TO said devices. For example, the (+) wire to the radio itself is pinched on a bolt or rubbing against the dash. Unplugging the radio wouldn't rectify that short.

When I see issues like this myself, I go back to the initial "Split" from the fuse that goes to each individual harness. Essentially you disconnect every wire that comes off fuse 2 and unplug the other end, and use a multimeter to ohm it out to ground. Anything less than infinite ohms (no resistance) means there's a short somewhere "Downstream" on that wire. You can then trace that part of the circuit to find out exactly where, by following that part of the harness. It could be almost anything but on an older car - especially one you've just worked on, it's almost surely 1 of 2 things:
1) Internal short in the harness due to aging wiring. You moved it just enough to crack the insulation so now the (+) is grounding out.
2) Pinched or pierced wiring (harness stuck in between two mounted/bolted bits, running behind a screw that's gone through the harness itself, stamped steel penetrating the harness, etc.).

[Bezant]

Originally Posted by Tomguy

Unplugging the items themselves reveals the devices (radio, wipers, etc) are not the short but does not rule out the wiring TO said devices. For example, the (+) wire to the radio itself is pinched on a bolt or rubbing against the dash. Unplugging the radio wouldn't rectify that short.

When I see issues like this myself, I go back to the initial "Split" from the fuse that goes to each individual harness. Essentially you disconnect every wire that comes off fuse 2 and unplug the other end, and use a multimeter to ohm it out to ground. Anything less than infinite ohms (no resistance) means there's a short somewhere "Downstream" on that wire. You can then trace that part of the circuit to find out exactly where, by following that part of the harness. It could be almost anything but on an older car - especially one you've just worked on, it's almost surely 1 of 2 things:
1) Internal short in the harness due to aging wiring. You moved it just enough to crack the insulation so now the (+) is grounding out.
2) Pinched or pierced wiring (harness stuck in between two mounted/bolted bits, running behind a screw that's gone through the harness itself, stamped steel penetrating the harness, etc.).

Thanks Tom, that was my thought as well. I guess I'm just hoping to avoid major wiring harness surgery and find something simple that I overlooked.

[meltedpanda]

thats my point - run some temp ground wires. See if it helps. For example ,I do know that the dashpod has to sit just right to produce a good ground for it and other items to work Had to do this in my 72.

[Bezant]

Originally Posted by meltedpanda

thats my point - run some temp ground wires. See if it helps. For example ,I do know that the dashpod has to sit just right to produce a good ground for it and other items to work Had to do this in my 72.

Maybe I am just not electronically savvy enough, but this doesn't make sense to me. Knowing I have a big short and high current draw from somewhere, I don't see what an extra ground wire would help.

[Mike D]

Not saying this is the problem but what Panda is saying is that a poor/corroded connection can result in additional resistance causing an excess of required current which might lead to the blown fuse.

[Bezant]

Originally Posted by Mike D

Not saying this is the problem but what Panda is saying is that a poor/corroded connection can result in additional resistance causing an excess of required current which might lead to the blown fuse.

Got it. I haven't been thinking in that direction because it's pulling a LOT of current even with the items off, and my wiring is overall in really good shape.

[97 SL320]

You could try a low amp ( like 2 ) self resetting circuit breaker and a compass or one of the kits with the same parts. The magnetic field set up by the cycling current affects the compass from the power source to the location of the fault. I haven't used this method but the theory is sound.

[Frank Reiner]

Originally Posted by Mike D

Not saying this is the problem but what Panda is saying is that a poor/corroded connection can result in additional resistance causing an excess of required current which might lead to the blown fuse.

Originally Posted by Bezant

Got it. I haven't been thinking in that direction because it's pulling a LOT of current even with the items off, and my wiring is overall in really good shape.

The basic relationship between available voltage (E), current (I), and resistance (R) always applies: E=IxR

For any given battery voltage (E), as the resistance (R) in a circuit increases, the current (I) in the circuit will DECREASE. Hence, an open circuit (infinite resistance) will have zero current, and a short circuit (zero resistance) will have infinite current, aaand: pop goes the weasel.

[Tomguy]

I think the theory is if the ground is bad, but the devices on the circuit are still consuming the same amperage, the resistance of the bad ground supply itself acts as an additional current draw, thereby blowing the fuse. The question then is, is it a dead short to ground, or is it just a high draw?

[Mike D]

Theory doesn't always correlate to reality.

[Bezant]

I got in tonight with a light wired to the fuse, but couldn't find the short I took off the washer pump switch and played with wiring harness all over the place.

We do think the short is on the power side of the fuse, which would be washer pump, wiper motor, wiper switch, horn, instrument cluster, and cigar lighter.

[Bezant]

Found it tonight! Had to pull the fuse box down and isolate wires. The horn wire was chafing against one of the black crossbars in front of the condenser.