Oil Level Sending unit issue

[cruggles]

I'm having and issue with my sending unit on my ’84 911, it has never worked since I got the car. I have ready the excellent thread by DRACO on “Oil Level Sending Unit: How to Service / Repair / Maintain”, but still can’t nail it down.

The issue is the tiny wrapped coil wire inside the sending unit has been blown like a fuse and even after re-soldering it, it happens again. I think the issue is the amount of voltage being sent to the sending unit overloading the tiny coil wire. I have a new sending unit standing by, but I don’t want to connect it for fear of frying that one too. When I re-solder the tiny coil wire, the ohm resistance reading is the same as the new one so I don’t see how it should behave differently if connected.

Matching the earlier pictures in the above thread, and my other 911, I should be getting about 7 volts at the signal (green/gray) wire at the sending unit. However, on this car when I check the voltage at the sending unit signal wire to ground on the chassis, I get the full battery voltage, around 12.5 volts. I had the gauge checked and it tested fine at NH Speedometer.

I am running out of things to test. Is the ground different than straight to the chassis? That’s how the ground wire that connects under the sending unit mounting nuts is connected on my car. The wiring diagram indicates that wire just grounds to the chassis at the rear of the car.
The voltage from the G (signal) terminal on the back of the gauge reads lower (around 8 volts) than the other end of the wire, which is reading over 11.5 volts. Since they are grounded in different places, would that affect the voltage?

What am I missing, this is a pretty straight forward circuit!?

Thanks,
Chris

[Walt Fricke]

For the same resistance, if the volts go up, the amps go down, don't they?

The voltage at the oil tank sender input can't be higher than the G terminal voltage on the back of the instrument. Voltages don't magically step up.

You have some wires crossed somewhere to cause this.

[hienz]

Did you check the oil sensor in the oil tank which located under the right rear wheel?

[darrin]

sounds like your unit is bad and has a short -- the sending unit is fed 12.5 volts and the wire coil in the sending unit creates variable resistance (similar to a dimmer switch) depending on how high the oil is -- the dash gauge is reporting the resulting voltage as the oil level.

Replacing it should solve your problem

[cruggles]

Originally Posted by darrin

sounds like your unit is bad and has a short -- the sending unit is fed 12.5 volts and the wire coil in the sending unit creates variable resistance (similar to a dimmer switch) depending on how high the oil is -- the dash gauge is reporting the resulting voltage as the oil level.

Replacing it should solve your problem

Thanks for the replies guys.

Can you confirm that you are getting 12.5 volts from the sending unit wires at the sending unit on your car too? I just want to make sure the circuit up to that point is to spec before connecting the new sending unit.

I'm away from the car so I won't be able to test it for a few more days. I'll let you know how it goes...

Chris

[cruggles]

So I got to the it figured out. Upon closer inspection it was a wiring issue. Once I traced it back to the correct signal wire form the gauge, with the matching roughly 8 volts instead of the full 12.5 volts, it all worked. I was even able to fix and re-use my original sending unit. The creative wiring by the previous owner made it all the more confusing. I even fixed the issue with the license plate lights while I had everything unwrapped.

Thanks guys.

Chris

[timmy2]

Sounds like the PO swapped the wires for the sender and license plate lamp coming from the housing harness.
Easy enough to do...