Oil guage indicating backwards

[1955FordF350]

I have resurrected a very solid 1955 F-350, this truck has the old 239 V8 "Y block". I had an oil leak after rebuilding the engine and suspected the oil sending unit to be the fault, I replaced the sender with a new one and it is now indicating backwards (as the engine revs pressure drops). the truck is running a 6v negative ground system which is why I think this is happening but I want to make sure this is the case before spending money on a new sender. this sender is not OEM so my suspicion is that it is meant for conventional flow rather than electron flow (polarity is the issue) as most vehicles after 1956 switched to negative ground rather than positive ground. What I want to try is getting an OEM sending unit so if anyone has one or knows where to get one please let me know.

[dmsfrr]

New replacement/reproduction parts are most often made 'off-shore' and usually unreliable at best.
Look for a used original from '55 or a few years earlier, hopefully still in working condition of course.

[1955FordF350]

I tried to find one but they don't exist NOS.....

[miker98038]

I’m not familiar with the trucks, let alone a 350. What I’m reading is it’s got an oem electric oil gauge as opposed to the idiot lights I’m used to in the cars. That right?

If the gauge has two electrical terminals, and with the wires off neither reads to ground, I’d try swapping the wires between terminals. I’ve no idea where it will read accurately, but the needle will go the other way if it’s magnetic. If it’s thermal it won’t make a difference.

Either way, I’d try a quality (lol) mechanical pressure gauge, even if temporary under the hood, and see what the pressure really is.

[1955FordF350]

It is a pressure switch for a gauge yes. single prong not 2 terminals. 78 psi cold start drops to around 47-50 at op temp.

[KULTULZ]

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I’d try a quality (lol) mechanical pressure gauge, even if temporary under the hood, and see what the pressure really is.

What the man said ...

[ms fowler]

This is an example of where knowing your general location would be helpful. It is the beginning of flea market season. Spring Carlisle is just a few months away and a few weeks after that is Ford Nationals at Carlisle--billed as the largest Ford-branded show anywhere.
If you are within driving distance, it would be a worthwhile trip.

Also--agree on installing a mechanical gauge to be sure that the pressure is dropping with revs.

[miker98038]

78 psi cold and 50psi hot seems ok to me. What weight oil are you using and what’s the ambient temperature range at your location? That description is different than pressure falling as engine rpm increases.