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P15 electrical issue


blue p15

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I have a couple of problems for the electrical experts. The gas gauge in my P15 has not worked since I inherited it about 6 years ago. It always reads empty, or less than 1/8, even when the tank is full. I keep track of the miles it is driven, so it is not a huge problem. I just get gas after a certain number of miles. My battery recently died, so I put in a new battery. I noticed the ammeter showed a discharge, even when travelling at 55 mph, so I had the generator rebuilt, and it charges as it should. Here is where it get interesting. I started the car yesterday and let it run at a high idle for a few minutes when I noticed the gas gauge read 1/2!  But when I slowed the engine, the gas gauge went back to empty! I played with it a few times just to make sure. At higher rpm's the gas gauge would come up, and at lower rpm's it would go down. The gas gauge is mimicking the ammeter - it just moves slower than the ammeter. The car is 6volt positive ground. Any ideas what could be causing this?

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There were two types of gauges a single wire and a two wire. Which do you have?

 

Sometimes a dedicated ground wire from the sending unit to the trunk floor will wake them up.

 

I had a situation with mine where the hose feeding the defroster came loose from the diffuser it layed across the fuel gauge wires and the stiffening wire in the duct created an unintentional ground of the gauge. My current situation is that I bought a new 2 wire sending unit (offshore reproduction no doubt) and now my gauge waves at me, showing the fuel level upon first start up, but then working only  intermittan a we drive along.  

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under his profile he list a 47 Plymouth and stock for that is the two wire system.....am not sure if you have the manual for this model but it is well covered for test and repair in the book.....good grounds, clean connections a must.....odds are the sender may have a poor wiper to cup connection on the sender where it goes to ground.  

Edited by Plymouthy Adams
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Check Motor’s Automobile Repair Manual. Your 2 wire type fuel level indicator system is described in detail and troubleshooting is also covered. The sheer complexity of the gauge for this system lends itself to problems. Sending unit impedances at 1 & 2 are also critical for proper operation relative to fuel level. Your wiring for this circuit should be thoroughly checked and grounds verified. I opted for a NOS gauge and a new sending unit and mocked up the circuit on the bench prior to installation proving out proper operation of these devices. If the combination does not work when installed then a wiring fault exists. M

PS my fuel gauge had the same symptoms and it was found to be at fault.

Edited by Marcel Backs
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