Oil pressure - gauge, wiring, sensor/sender - Lesson learned

[SublimeMudTime]

So I was having an issue with my oil pressure reading fine at some times and then it would go into a spike mode where it would bounce up to 90psi or all the way to the max.

I posed the issue to my mechanic and he said it's gotta be the pressure sensor/sender.

Rather than following the TM troubleshooting flow chart I went and ordered a new sender unit.

Yesterday I went to the shop where the truck is and popped the doghouse to pull the old sender. I grabbed the wiring and on the sending unit the rubber shell and a tiny nut and plastic washer just came off.

Turns out the nut holding the rubber shell on the sender had backed off causing an intermittent connection. So I did a test of making the connection and firing the truck up. The gauge worked fine. So I pulled the wire from the sending unit and saw the gauge go to the max.

UGH! Who would have thought that the sender unit acts as a variable pull down resistor. And for those who do not know what a pull down resistor is, here is a good explanation: http://playground.arduino.cc/CommonTopics/PullUpDownResistor


So for all the rest of us new HMMWV owners I have some advice to repeat that others have stated here: READ THE TMs to save everyone's time and some cash. Stop looking for a quick fix or make assumptions that these things are just like a normal vehicle. Had I done that, I would not be sitting on a NOS spare pressure sending unit.

 

[SublimeMudTime]

And to leave a breadcrumb for the TM beginners, the troubleshooting flow chart on that is in 9-2320-280-20-1 on page 2-356.

 

[papakb]

With the exception of the fuel level gauge and voltmeter, The gauges in M series trucks are really just current meters and the sending units are just variable resistors controlling the flow of current through the gauge. It does make troubleshooting a bit easier though. An open connection reads one way and shorting it to ground reads the other. Anything in between is controlled by the sending unit. Keep in mind that these gauges are made by the lowest bidder and definitely aren't the best available.

Kurt

 

[Csm Davis]

Lots of these gages are Stewart Warner and not cheaply made there are specifications they were built to. But go ahead and change them out with civilian gages and see how well they hold up 30-40 years from now

 

[papakb]

I will admit that the Stewart Warner gauges are the best of the bunch and the Dixon gauges aren't bad but the Faria stuff is junk. They would fail all the time in the 151's and I've replaced all but 1 in my HMMWV.

 

[BlueRoadster]

Stupid question but where do I get the TM? I have an 85.

 

[Warthog]

There is a Technical Manual tab at the top of every page. You have to drill down to find the correct manual.

http://www.steelsoldiers.com/showthread.php?78861-TM-s-for-the-M998-series

 

[papakb]

You can find almost every manual you need for the HMMWV on this site: http://www.imfmotorpool.com/HMMWV_M998_Manuals.html

 

[BlueRoadster]

Thanks guys.

Grant