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Hi, 

 

I've been trying to hunt down why my oil pressure idiot light is not working.  It may be a bad bulb, but before pulling the cluster I have a question about the Prove Out (PO) circuit.  

 

72 351C Vert with AC (LoL)

 

a. The oil pressure sensor is ok

b. Connector to oil sensor clean, tight, and wire is ok before it enters the firewall. 

c. Temp and Brake warning idiot lights work when the ignition is  'ON'.  

 

From searching past posts, I believe the Prove Out (PO) should also ground the oil temp idiot light to test the bulb however when looking at the 72 wiring diagram I don't see it.  I've attached a screen capture of PO (circled in Red) for the BRAKE warning and also the Temp.  I also looked at the 71 and 73 diagrams and don't see it either.  

 

Question.  Is there a PO ground path oil pressure similar to temp and brake warning lamps.  From the schematic? I don't a separate path for the PO.  Could the path be tied to the Temp PO circuit? 

 

Thanks

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There should be a circuit IF you have a non-tach dash or a 71 without the 3 gauge cluster and without a tach. Otherwise, the proof-out circuit doesn't exist because you have a gauge instead of a light.

Let me check your shorts!

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If you ground the wire at the sensor does the light come on when the key is on? If it does the oil pressure switch is bad, if it doesn't the bulb or a connection is bad.

 

The oil pressure light does not need a "prove out" circuit. When you turn the key on and the engine is not running the oil pressure switch will be closed, providing the negative (ground) connection while turning the key to the on position provides the positive voltage.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Thanks for your feedback. It's 72 non-tach dash, no oil or temp gauges, and all original for sure. I suspected confusion on my part about the PO and combo's and guessed what Don C. confirmed. I just wanted to make sure I understood before I went further. Also confusing was the wire color to the oil sensor Red/yellow (maybe it turned yellow) is different than shown on the schematics. For such a simple circuit I hated even asking this question.

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Check the plug and pressure switch at the distribution block, that could be bad or unplugged.

My car has idiot lights, but when I upgraded my brakes to front discs, rear drums with a separate balance control, it bypassed the distribution block switch, therefore I no longer have an oil light. Actually, I wish I could find a way to have that working if anyone can offer an idea onhow to do it.

Geoff.

 I learn something new every day!

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Hi Stanlover,

 

Can you point me to where the distribution block switch/connector is located? It is a connector under the hood near the firewall or mounted under the dash somewhere? From the schematic, I see C-27 is that what you were referring to.

 

Thanks

c27.png

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The brake distribution block and brake warning light and the oil pressure switch circuit have nothing in common, other than being on different pins in a couple of connectors. C-29 is the connector that plugs into the circuit board on the back of the main instrument cluster and provides 12 volts to the warning bulbs through circuit 640, as well as the feeds to the brake warning light and oil pressure light switches. C-27 is the main firewall connector.

 

Yes, your wire to the oil pressure switch has likely turned yellow with age and having had oil on it. Lightly wipe it with lacquer thinner and it should turn white.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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  • 1 month later...

Where does the 640 wire connect? I know its the warning lamp feed. Ive located it on on the back of the cluster plug but its been cut previously. I need to wire it to somewhere.....not sure where. My brake warning switch had also been removed. I can jump power to the 640 and make my light come on but I would like to connect it to the proper power source. 

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Start by downloading the wiring diagram for your car on the wiki page

https://7173mustangs.com/wiki/

You'll find that it originates at the fuse block.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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  • 3 weeks later...
Posted (edited)

Update: After fixing my stupid oil sensor I was able to reach my hand and small head under the dash and get to the idiot light that was burnt out and replaced.  Least now all the  lamps working without removing the dash.   Background illumination is still iffy, but I'm not suppose to drive the car at night anyway.   Oil lamp works just like it suppose to,  no prove out circuit needed. 

Edited by cv_72mustang
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