Hi everyone, I am really stumped on troubleshooting my Oil pressure light not functioning. I am 99% that it used to work, but to be honest, I have no idea when it would have stopped.
Here is the deal, over the winter I decided to pull the dash and clean up the gauges, repaint a little, LED bulb swap (just for the gauge illumination backlights), and build out a new dash face. Well, I pulled everything, cleaned it all up and it is ready to go back in - but then I noticed my Oil light on the standard 68 is not illuminating. Great, I forgot to double check that bulb and it is probably burnt out.
Here are the troubleshooting steps to date:
I pull the cluster back out and check the bulb and it is good.
I pull the wire from the sending unit and ground it - light does not illuminate. The wire to the sending unit does have positive voltage.
Confirmed cluster wiring to schematics.
I apply 12v to the wiring in the cluster - light bulb illuminates. GREAT, this would indicate that I didn’t screw up the cluster when cleaning it up.
Tested continuity of both connection pins at connection in dash with the sending unit wire ground - good measurement.
Test continuity of 1 connection (wire 640) to ground - good measurement. Other pin (wire 31) failed continuity test.
Turn key to ON and test voltage of both connection pins at connection in dash (going from pin to ground) and both returned positive voltage?
THIS IS ODD TO ME. I would have expected 1 to have current (wire 31) and the other (wire 640) to remain a ground to complete the circuit.
I put a incandescent bulb back in the backlighting thinking perhaps for some random reason it needed more draw? Didn’t change anything.
I got to this idea because the LED bulbs I am using, when swapped into the AMP indicator would cause the light to always illuminate my best guess was due to lack of resistance impacting amperage test. As soon as I put an incandescent bulb back in, the AMP light went back to standard operation.
So at this point I don’t want to put the cluster back in the car without getting this issue resolved, but I am stumped on what else to test…HELP?
Thank you,
Chad
Also posted at MercuryCougar.net: http://www.mercurycougar.net/forums/showthread.php?119137-68-Oil-Pressure-Light-Troubleshooting
The way the circuit works is that wire 640 supplies 12 volts and the bulb is grounded by wire 31 going to the sender. You need to look for 12 volts with the key in the run position, on wire 640 (Red Yellow) stripe. You need to ground wire 31 (white red stripe) at the sender and then check for continuity to ground at the cluster.
Thanks Bill. Further clarification:
With the key off - I ground the wire at the sender and both pins tested positive for continuity - that seems somewhat odd that they would both test positive for ground.
With key off - removing the ground to the wire at sender, both pins did not register continuity with ground.
With key off - wire at sender not ground, testing continuity between pins was successful.
With key on - wire at sender not ground, both pins tested positive for 12v.
With key on - wire at sender ground, both pins tested positive for 12v. Both pins went to infinity for continuity.
Based on that, and the earlier test of testing continuity between the 2 pins, guessing those two wires are touching somewhere? Would that make sense?
Thanks!
Additionally:
With the key off - I tested continuity between wire to sender and ground - infinity.
With key on - I tested continuity between wire to sender and ground - infinity.
With key on - I tested voltage between wire to sender and ground - 12v.
Royce is helping me out via the other forum…more test results:
With key off - sender to ground continuity is infinity.
With key on - sender to ground continuity is infinity.
With engine running - sender to ground continuity is infinity.
Valve cover bolt to ground continuity is 0 (just to rule out that I didn’t have a bad “ground” point).
This would seem to indicate a bad sender, right? But why would taking the wire to ground through other means (direct to ground) not illuminate the light, as following instructions in shop manual?
Often times, the wire 640 is used on other lamp circuits, which may give false readings to ground if a bulb on the other lamp is still in place. Why is that? The lamp itself has only about 1 ohm resistance so the best way to trouble-shoot these kind of lamps is to remove all bulbs in the dash cluster other than the dash lamp bulbs (completely different circuit).
If your measurement show the oil sending line from the bulb to the sending unit (plugged in) shows infinite resistance, then the sending unit is bad or you have a broken wire. You should see something like 50-60 ohms at the sending unit. You can directly measure the sending unit resistance by probing the signal post to a good engine ground. Don’t use teflon tape on the sending unit threads, as that can block the ground path.
Sunday night I just still did not feel right about what I was seeing, so I disconnected the engine to gauge harness and tested from there - no longer have an issue of both 640 and 31 feeding 12v. Most of my testing is without the gauge installed, just testing at the pins of the harness connector. I pulled out the wiring schematic of the engine to gauge harness and it looks like a PO had things wired a little goofy, spliced in larger gauge cables, and things just didn’t look right. So I am resetting - I ordered the replacement harness from WCCC to run the wires right, replace the bad sensor, and then see where things are.
Spliced in the correct wiring harness and ran wires to the correct connections and the oil light actually works. I am unsure how the old harness was ran so wrong - there are only 3 wires! Unfortunately the sender is bad, so I will need to replace that prior to being able to confirm the car runs with the wiring I just installed, but feeling much better about it! Thanks for the comments and test ideas, I think we will be back on the road soon.
I am back to report success of the road test. Everything ran as it should, lights operated pre and post startup as they should. In my case the problem was indicated during the testing of pins feeding the OIL light (without the cluster attached) of wire 640 and 31 both returning 12v. After cleaning up the harness and replacing the engine to gauge harness completely, and replaced oil sender - things tested correctly.