Oil pressure gauge or sender? Help an electrical dunce

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studawg

studawg

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Im an idiot when it comes to a lot of things, but especially automotive electronics. I cant figure out if my oil pressure gauge is bad or what. Lets just assume I actually have good oil pressure.

Here is what I know:

1. When I start the truck, the oil pressure goes up to about 25%, then slowly goes back down over the next 20-30 seconds to rest at the base line. Revving the engine or idle speed doesnt make a difference. It doesnt move after it gets back to zero.
2. To test the gauge, I grounded out the sending unit wire to the battery, and the needle does not move at all when I turn the key on (not running).
3. My test light will light up when grounding the sending unit wire to battery ground with the key off, but when the key is on, it doesnt light up. (I have no idea what this means.)

Any help? This is probably not enough info to make any conclusions.

Is there an Auto Electronics for Dummies book? I know somebody is going to tell me to "check continuity" with an "ohm meter" or something and I have one of those, but I have no idea how to use it. Im hoping there is some random thing that people with small minds can do like turning my hazards on while in 4th gear with key on/engine off and it will tell me if its the gauge or sender.
 
i would remove the sender and install a temp mechanical gauge to verify actual conditions.

 
that is good advice

I have a "T" adapter and run a mechanical oil pressure gauge as well as the standard gauge.
Remember this is a 1/4 BSPT thread, it is NOT a standard NPT pipe thread. Adapters are available on either Amazon or McMaster Carr.

It could be your sender, or it could be the gauge itself. Grab a copy of your FSM in the Resources section above, the book has some troubleshooting steps to test each of the gauges.

1975 FJ40, FJ43, FJ45, FJ55, BJ40-43, HJ45 Chassis-Body-Manual​

and Section 12 page 29 should allow you to test both the gauge itself, and the sender.
 
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Oil pressure is too important to FAFO IMHO. About 1985 my oil pressure started to read erratically. I tried 2 new senders with no success. I couldn't find a meter so I bought a lighted direct read gauge and mounted it in a custom plate clamped on the steering wheel housing. I ran the meter tubing inside fuel hose from the back of the meter to the engine side of the firewall - that way it the tubing fails I don't get a face full of hot oil. I keep a pipe plug to screw into the engine in case of tube failure in the ash tray. Every thing still there and working flawlessly.

There should be some sort of testing procedure in the manual
free downloads Land Cruiser Factory Service Manuals (FSMs) - https://www.cruisercult.com/factory-service-manuals
Coolerman's wiring diagrams and parts
 
bhsdriller said:

An adapter i've used that lets you keep the original sender. The 1/8" fit I don't believe it's 1/4" as mentioned above
Click to expand...
That is a good looking piece there. I just ordered one. I had made my own at the hardware store, it works but doesn't look as nice...

20231210_113700.webp
 
PRO TIP: Check your fuses!

Duh. Im even dumber than I thought. I was also having an issue with my rear taillights. They didnt work. Brake, turn and reverse worked, but no tails. Checked the fuse and waddaya know, that fuse does the tails and gauges, and it was blown. I replaced it, turned on lights, and still no worky. Checked the new fuse, and it had blown. Uggh, ive got a short somewhere. Realized my rear side markers arent installed and the power wire was resting on the body. Now I have taillights. But gauge is still suspect. It did not pass the ground test. Bought a new multimeter and Im going to test the back of the gauge cluster today. So still diagnosing....
 
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