g741.org

As I get more and more into this fellow club members M37 electrical, we are running into some issues. The latest is that we were not happy with the oil pressure readings, so we ordered a new sending unit with Douglas connectors. We now learn that the actual gauges in the early trucks were resistored down in amperage. I can't find anyone that sells a sending unit with Douglas connectors that is resisted down and I would like to use the original gauge if I can for appearance on the dash panel.
It is a 120 psi gauge and the original readings were zero shut off and 20 psi at the high end and 15psi after warm up. Those could be actual numbers, I have no history with this truck. We bench tested the original sending unit and felt it was bad and ordered the new unit. We have not installed it yet but plan to this weekend.

What will running a non resisted sending unit read on a resisted gauge? Will it read at all? What options do I have to get these two compatable? I'm not so much concerned with get 100 percent accurate readings as I am giving the truck owner a comfort level that he has oil pressure or a major drop during a use.

you have to use a 60 psi sender with a 60 psi gage, and 120 with 120. I am not aware of any resisters in the original system. I added a Autometer mechanical pressure gage on my steering column, sometimes disagrees with lower pressure with my older 60 gage. I like the probably more accurate and easy to read Autometer.

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Hook a mechanical gauge up to test.
The resistor was an external device when the M 37 was introduced and they were burning through the stock of 6v senders and gauges. You can use the part number to determine what you have. Most likely 24v.

So, the more I read, it implies that there are two types sending units and gauges for the oil pressure, one being up to 60 psi and one being up to 120 psi. The early gauges are external resistored and the later style gauges are internal resistor, but both are resistored. So, my new 120 psi sender should work with my old 120 psi gauge, as long as the gauge itself is working, which it appears to be.

Think I got it understood now. Until I read some more.

Thanks for the comfirmation.

For an engine where oil pressure never sees 60 psi, the 0-60 gauge is best.

Most 120's were used on bigger trucks that carried more running pressure, but a lot of the 120's were used in G741 rebuilds when no 60's were available.

PoW