1972 AMG M35a2 oil pressure gauge problem

[Reaper651]

Open circuit causes gauge to peg, not shorted.
Ground the sending unit wire, as in shorted, makes it read zero.

Wow my Deuce must be a weird one then.

 

[CNKILLERCLOWN]

I believe the other wire is 32, can any of you fellas take a peek and tell me which one of them is the power and the sender? This truck is a 1972 AMG if that info is pertinent.

 

[jbayer]

The sending unit is the variable resistor to ground.

 

[CNKILLERCLOWN]

The sending unit is the variable resistor to ground.

That makes perfect sense then. The wire flopping in the breeze gives me a pegged reading when pulled from the sender. Next question is who do you guys recommend me throw some coin at to get a replacement sending unit. The fella I bought this truck from said it would always read zero no matter what. He put a mechanical gauge on it to make sure the pressure was good. (Around 45 to 50 when hot) this gauge however is under the hood making it impossible to use while moving.

 

[jbayer]

I thought the same, it must be the sending unit. I disconnected the wire to the sending unit and the gauge pegged, with it connected it read almost zero when running.
Then I happened to come across another gauge panel for a great price, and just for ha ha's, swapped the gauge out.
Now it reads the same as the direct reading one I tee'd off of the sending unit. 55 PSI cold, 45 PSI hot.

 

[sandcobra164]

Open circuit causes gauge to peg, not shorted.
Ground the sending unit wire, as in shorted, makes it read zero.

I'm trying to understand how electrical circuits work. My understanding is that if you want something to move, you need to create a path from positive power to a ground or neutral to create a movement or transfer of power of some sorts. The gauge is sitting at 0 when no power is added to the positive side. If you switch the power on, why does the gauge peg to 60 or 120 (depending on oil pressure gauge and it makes no difference in this case) if the other end of it is ¨open¨ or not grounded. Electricity is funny stuff and understanding it is all I'm trying to do. Please explain your expertise on the subject, I'd love to hear it.

 

[CNKILLERCLOWN]

I may just buy a set and replace them both then.

 

[jbayer]

Pressure to the sending unit opens the circuit a varying amount.
With no pressure there is no resistance. some pressure some resistance.
Measure from sending unit post to threaded end with an OHM meter you get zero resistance.(With sending unit off vehicle)

 

[Reaper651]

Ahh I get it now. So he has an open in the sender wire somewhere, not a short. Sh2t, I read the troubleshooting TM wrong.

 

[CNKILLERCLOWN]

I just found a mechanical one from a M923A2, I am just going to plumb it in and do away with the electrical one all together. Thanks for the help guys, I appreciate it.

 

[sandcobra164]

Pressure to the sending unit opens the circuit a varying amount.
With no pressure there is no resistance. some pressure some resistance.
Measure from sending unit post to threaded end with an OHM meter you get zero resistance.(With sending unit off vehicle)

My gauges in my truck are having trouble now. When I switch mine on, the oil pressure and temp are bottomed out to 0. Fuel level somehow goes to 1/2 or better depending on when I last topped off on fuel. When I start it, the tach jumps to 800 rpm but that's a mechanical gauge, the air pressure slowly builds up and the oil pressure gauge starts moving to the right to around 90 or so, I think it's good but my temp gauge doesn't start moving till about 10 to 15 minutes into the trip. What is up with that? Do you think I have a bad wire or is the sending unit getting lazy and unreliable? This truck has been a solid runner but I want to head off future issues and I would love some help.

 

[CNKILLERCLOWN]

10 minutes to get up to operating temp is pretty normal on my truck. It also runs maybe 10 degrees cooler than most all the rest I have ridden in. It is however always consistent after we have run it down the road a piece.

 

[sandcobra164]

CNKILLERCLOWN, We're trying to fix your issue and I'm with you as we should have some answers soon. I'm glad to know that maybe my truck is OK with taking 10 to 15 minutes to move the temperature gauge up. Maybe I need to get a mechanical gauge for that as well as the electrical system on these trucks seem crazy at best. I wonder how the air pressure gauge stay's to 90 for about 2 hours after I turn the truck off now. I'll send a PM to jbayer and ask how that gauge seems to stay on after I shut the power switch on. What in the World is my truck doing??? Do these trucks have a time delay module for shut down? If I walk away for 3 days, the air gauge reads zero. I'm so confused on this thing now.

 

[CNKILLERCLOWN]

My gauges drop out to zilch when I turn off the master switch with the exception of the air. It slowly goes down to zero over the course of two days.

 

[sandcobra164]

Does the air gauge have an alternate source of power that you know of? I don't even see wires going to mine but you never know on this military stuff, they may be ahead of their time, I remember when they had to remove alot of trucks for possible radioactive gauges.

 

[CNKILLERCLOWN]

On my truck the air line goes directly to the gauge itself. There isn't an electric intermediary.

 

[rustystud]

CNKILLERCLOWN, We're trying to fix your issue and I'm with you as we should have some answers soon. I'm glad to know that maybe my truck is OK with taking 10 to 15 minutes to move the temperature gauge up. Maybe I need to get a mechanical gauge for that as well as the electrical system on these trucks seem crazy at best. I wonder how the air pressure gauge stay's to 90 for about 2 hours after I turn the truck off now. I'll send a PM to jbayer and ask how that gauge seems to stay on after I shut the power switch on. What in the World is my truck doing??? Do these trucks have a time delay module for shut down? If I walk away for 3 days, the air gauge reads zero. I'm so confused on this thing now.

That's because it takes 3 days for your system to bleed down. The air pressure gauge is manual, meaning there is a small air line to the back of it.

 

[jbayer]

My gauges in my truck are having trouble now. When I switch mine on, the oil pressure and temp are bottomed out to 0. Fuel level somehow goes to 1/2 or better depending on when I last topped off on fuel. When I start it, the tach jumps to 800 rpm but that's a mechanical gauge, the air pressure slowly builds up and the oil pressure gauge starts moving to the right to around 90 or so, I think it's good but my temp gauge doesn't start moving till about 10 to 15 minutes into the trip. What is up with that? Do you think I have a bad wire or is the sending unit getting lazy and unreliable? This truck has been a solid runner but I want to head off future issues and I would love some help.

All looks right to me.

 

[Valence]

The sending unit is the variable resistor to ground.

Ahhhh, thank you so much for that one sentence clarification. Fixed my pegged gauge too. Cleaning and reseating the connectors on my new sending unit fixed it.

Thank you again for taking the time to post that.