Electrical Short Issue

[300cruiser]

I am experiencing an issue with a short on the oil pressure switch wire. Some background: I picked up the car from it's winter resting spot. Started perfectly and drove home for 30 minutes with no issues. Parked the car in the garage at home and left it for 2 weeks. Tried to start it this week and it wouldn't fire. Did some testing and determined there was no spark. Cranks fine and all other electrical in the car appears to work. Chased around the issue with a meter last night and determined that the wire connected to the oil pressure sending unit is a dead short and is pulling down the voltage at the coil. Disconnect the wire from the sending unit, car starts and runs perfectly. I plan on tracing wires this weekend but looking for tips on where to start. Any thoughts on where the short could be and why it would suddenly short when parked?

 

[CBODY67]

The sending unit is a variable (if for a gauge) or switched (if an indicator light) ground. Not sure where it gets its voltage feed from, which should be switched rather than "hot" all of the time. Might start by using the FACTORY schematics in the Factory Service Manual to trace the circuits for the feed circuit that ends up at that sending unit. Then verify with your meter as you also look for chaffed wire insulation (exposing the protected conductor in the wires) too.

Just some thoughts,
CBODY67

 

[Big_John]

I am experiencing an issue with a short on the oil pressure switch wire. Some background: I picked up the car from it's winter resting spot. Started perfectly and drove home for 30 minutes with no issues. Parked the car in the garage at home and left it for 2 weeks. Tried to start it this week and it wouldn't fire. Did some testing and determined there was no spark. Cranks fine and all other electrical in the car appears to work. Chased around the issue with a meter last night and determined that the wire connected to the oil pressure sending unit is a dead short and is pulling down the voltage at the coil. Disconnect the wire from the sending unit, car starts and runs perfectly. I plan on tracing wires this weekend but looking for tips on where to start. Any thoughts on where the short could be and why it would suddenly short when parked?

Assuming you are talking about the pressure switch for an oil light, the sender will be grounded when the engine isn't running and the light will be on.

When the engine is running, the pressure switch will not be grounded and the light will be off.

So, unless that wire is hooked up incorrectly, it is working as it should and I kind of doubt if that's your problem. More likely, you disturbed another wire while looking around.

What year etc.? And what type of ignition?

 

[300cruiser]

Assuming you are talking about the pressure switch for an oil light, the sender will be grounded when the engine isn't running and the light will be on.

When the engine is running, the pressure switch will not be grounded and the light will be off.

So, unless that wire is hooked up incorrectly, it is working as it should and I kind of doubt if that's your problem. More likely, you disturbed another wire while looking around.

What year etc.? And what type of ignition?

Hi John - '67 300 with factory points ignition. Had a Pertronix but I removed it and re-installed the points during the troubleshooting. The points were shorted to ground with the gap open which is what started the chase to find the short.

The short is actually in the wire, not the sending unit. All is good as long as I disconnect the sending unit which I assume is due to the fact that I'm not allowing the short to connect to ground through the switch. I will admit that the wire management under the hood is not the best so I do plan on removing the rolls of electrical tape from the wire groups to inspect each wire individually.

The baffling part is that it ran fine until parked, then the short appeared.

 

[CBODY67]

Some of the underhood harnesses came from the factory wrapped in electrical tape. Are there some that look more recently wrapped?

 

[300cruiser]

Some of the underhood harnesses came from the factory wrapped in electrical tape. Are there some that look more recently wrapped?

I've had the car for 12 years and haven't touched any wiring, but that's not to say the PO didn't make changes. I will start under the dash. Not sure if a mouse got in over the winter and did some chewing.

 

[Big_John]

Hi John - '67 300 with factory points ignition. Had a Pertronix but I removed it and re-installed the points during the troubleshooting. The points were shorted to ground with the gap open which is what started the chase to find the short.

The short is actually in the wire, not the sending unit. All is good as long as I disconnect the sending unit which I assume is due to the fact that I'm not allowing the short to connect to ground through the switch. I will admit that the wire management under the hood is not the best so I do plan on removing the rolls of electrical tape from the wire groups to inspect each wire individually.

The baffling part is that it ran fine until parked, then the short appeared.

I'll have to study the wiring diagram a little later and see if I can make sense of it.

I'm wondering if there is some issue with the ignition switch, but I don't want to point at that too much as it's more of an edumacated guess as a place to start.

 

[mgm1986]

Since you've had the car for 12 years this may be unlikely unless you recently replaced a sensor.

I rewired my engine harness and found the Oil Pressure Switch wire and the Cold Temp Switch wires had been crossed at the firewall wiring harness block. Only after replacing both sensors did this issue appear as it sparked when I connected the Oil Pressure wire to the new switch. I suspect the old switches had gone bad so they were no longer completing the circuit.

After correcting this everything worked as expected.

That said, if you haven't made any changes and this started after coming out of storage, I would certainly want to thoroughly inspect the wiring for evidence of critter damage.

 

[300cruiser]

I'll have to study the wiring diagram a little later and see if I can make sense of it.

I'm wondering if there is some issue with the ignition switch, but I don't want to point at that too much as it's more of an edumacated guess as a place to start.

Appreciate the support, John.

 

[300cruiser]

Since you've had the car for 12 years this may be unlikely unless you recently replaced a sensor.

I rewired my engine harness and found the Oil Pressure Switch wire and the Cold Temp Switch wires had been crossed at the firewall wiring harness block. Only after replacing both sensors did this issue appear as it sparked when I connected the Oil Pressure wire to the new switch. I suspect the old switches had gone bad so they were no longer completing the circuit.

After correcting this everything worked as expected.

That said, if you haven't made any changes and this started after coming out of storage, I would certainly want to thoroughly inspect the wiring for evidence of critter damage.

Thank you. I will start chasing wires and grounds this weekend.

 

[Big_John]

I don't have a good electrical diagram for a '67 Chrysler specifically, but I looked at a few others. The pressure light is a real simple circuit, basically +12V power to the bulb and it gets ground from the sender. No oil pressure and the sender connects to ground and lights the light.

Looking at it, I just can't see where it would cause this problem, yet here we are (LOL). The only thing I can come up with is there's some flaw or short in the wiring that is grounding out the power to the ignition. I'm thinking that unplugging the sender shuffled the wiring enough so it wasn't shorted. That's kind of a guess, but it's based on my experiences with similar problems.

So, I think you'll want to trace the wiring from the sender and ignition to the bulkhead connector. If that looks good, you're gonna have to crawl under the dash and try to trace those circuits. Here's a hint... Pull the front seat, it will make this easier.

I'm sorry I don't have a better answer. You will need to study the factory service manual wiring diagram and this is where I like digital copies as I would print the pages out and go to it with highlighters and figure it all out that way. IMHO, don't waste any time with the generic wiring diagrams.

 

[300cruiser]

I don't have a good electrical diagram for a '67 Chrysler specifically, but I looked at a few others. The pressure light is a real simple circuit, basically +12V power to the bulb and it gets ground from the sender. No oil pressure and the sender connects to ground and lights the light.

Looking at it, I just can't see where it would cause this problem, yet here we are (LOL). The only thing I can come up with is there's some flaw or short in the wiring that is grounding out the power to the ignition. I'm thinking that unplugging the sender shuffled the wiring enough so it wasn't shorted. That's kind of a guess, but it's based on my experiences with similar problems.

So, I think you'll want to trace the wiring from the sender and ignition to the bulkhead connector. If that looks good, you're gonna have to crawl under the dash and try to trace those circuits. Here's a hint... Pull the front seat, it will make this easier.

I'm sorry I don't have a better answer. You will need to study the factory service manual wiring diagram and this is where I like digital copies as I would print the pages out and go to it with highlighters and figure it all out that way. IMHO, don't waste any time with the generic wiring diagrams.

Thanks, John. I think I located the source in the bulkhead. Overall it's timely as a few connectors (the alternator) are burnt so I am replacing and cleaning connectors.
Probably should have done this a long time ago.