testing sending units

RichJohnson · Post by **RichJohnson** » Tue Nov 11, 2014 1:17 pm

Several years ago someone directed me to a great paper that had all the resistance measurements for military sending units and how to test them.
I lost it between then and now when I need it again.

My M37 has sat for 5 years. I just got it started up (it literally fired off in seconds being jumped from the jeep and it sounded great)

I didn't have any oil pressure reading. While I believe I probably have oil pressure Im not taking it for granted so I shut it down and messed with the sending unit connection. Still nothing.

I have the 0-60 psi oil sender and gauge. I seem to recall that it worked when I parked it (ha ha yeah I know)

Does anybody know the document I spoke of? Or at least what the readings of the senders should be so I can test it.

Post by **wesk** » Tue Nov 11, 2014 1:46 pm

It's right here in our downloads section:

http://www.willysmjeeps.com/v2/modules. ... tit&lid=46

RichJohnson · Post by **RichJohnson** » Tue Nov 11, 2014 2:56 pm

That's it. Thanks Wes.

Its looking like a gage so far. Pulling the line to the sender and turning power on, I don't get any gage movement. Need to get under there and make sure the gage is getting power just to confirm, that could be bad after sitting for this long.

RichJohnson · Post by **RichJohnson** » Sat Nov 15, 2014 7:22 pm

I had one spare gage but it has the rubber connectors so I just installed it and plugged it in anyway.
Well, following the manual, I should get full pressure with power applied to the gage with the sending wire disconnected.
I still get nothing with my replacement gage.

Question, does the oil pressure gage have to have its body shell well grounded or can I bench test the gage simply applying 24v to the power in and then using the DC negative wire to test the gage?

Also, my batteries in the truck have dwindled down below 20v because they are more or less toast. (I jump it with a slave to start it) Will that affect if the gage works or not.

4x4M38 · Post by **4x4M38** » Sat Nov 15, 2014 8:01 pm

Rich,
My understanding is the chassis is ground for nearly everything on the jeeps,
like many other vehicles. The u-shaped bracket on the back of the gauge presses against the back of the instrument panel, and the panel presses against the dash/cowl, with contact assured with the dzus fasteners in the corners.

I believe instrument ground is through the bracket, so you need a return to your ground form the gauge from whatever power source you are using to test.

Wes actually recommended installing an additional ground strap from the instrument panel to the dash/cowl using one of the mounting screws to ensure a good ground. Seems simple enough to do, and if long enough should not interfere with removing the panel for maintenance.

Brian

Post by **wesk** » Sat Nov 15, 2014 8:58 pm

Yes the gauge's primary function for measurement is the routing of power in from wire #27 to the sender and on to ground. But the internal meter driver must be grounded through the gauge's housing to the cluster and then to the dash. Corrosion and/or too much paint destroys these grounds. That is why I recommend the additional ground between the cluster and the firewall and removing all paint and corrosion from the gauge housings, their brackets and the contact area for the brackets on the back of the cluster.

RichJohnson · Post by **RichJohnson** » Sat Nov 15, 2014 10:49 pm

Ok so I can test this gage on a bench by connecting 24v in, ground to the body, and then ground to the sender pin.

I was just thinking about an extra ground to the gages. My M37 hasn't moved in 5 years so no bumping and wiggiling down the road to keep the metal to metal joints clean, so they likely got rusty between the panel and dash. I live by the bay, everything rusts fast and bad.

cabinfever · Post by **cabinfever** » Sun Nov 16, 2014 9:01 am

Yes Rich, that is how I bench test my gauges.