how to test oil pressure sender and gauge

[Gigem]

OK, got my truck up and running with the new GM 350 HO crate motor, new 700R4 transmission, Vintage Air, new wiring harness, new gauges, etc. I am a happy dude!

Just got a few kinks to work out.

One is that my electric oil pressure gauge is not working. There is a new sending unit back near the distributor, wired to a tan wire. Gauge is pegged to the max pressure and never moves.

Is there something simple I can do to test that the sender is working, and that the gauge works, so I can find which is the problem?

Thanks - wiring/electrical is not my stong suit. Not sure what my strong suit is, but it surely isn't this!

[88Jimmy4x4]

Part number of sending unit?

I think theres a diff one for gauges and for warning lights

[Gigem]

Originally Posted by 88Jimmy4x4

Part number of sending unit?

I think theres a diff one for gauges and for warning lights

I don't have it handy... This is a new sending unit, I did not re-use the one from the old 250 I6 when I had the warning light...

Maybe I should ask the opposite question - which part number/sending unit should I be using for a crate 350 HO motor?

[hatzie]

Our GM fuel and oil pressure gauges use the same sender ohmage... 0Ω=0 90Ω=60/80psi or Full. When the oil gauge is nailed it usually means you have an open (cut/broken) sender wire or an idiot light sender.

Firmly short the sender wire to a good ground and check the gauge. It should read 0. If it does replace the sender. If it doesn't find the break in the sender wires.
The big can on the left is a gauge sender on the right is an idiot light sender. The GM parts will be similar.

[UATahoe]

I believe there are different gauge senders as well. I am currently sorting through this as well on my engine swap. Below is what was hooked up on my original motor. Its an 87 with the oil pressure gauge. I could have sworn these were for the idiot light but i am told that is not the case and that this is for a truck with a gauge.

[Rich84]

That sender is for the computer..It lets the computer know that there is oil pessure and supplies power to the fuel pump.

[UATahoe]

So i will need an additional sender for my dash gauge? Where would it install? Down by the oil filter? Sorry for the hijack

[Skunksmash]

Yeah there's two oil senders. Whatever you do dont lose or otherwise mess up the (brass?) fitting for he lower one. Otherwise its multiple trips to home depot to rig up something new. What a pain that was.

[UATahoe]

Lol I am screwed then. Sold the old motor and apparently left the sensor and all on the motor. Is the lower unit sometimes surrounded by a metal type cage? Any help on replacement parts, and what made it so difficult?

And I am assuming you are talking about the fitting needed to mount the sending unit down by the oil filter? I also have to get the other fitting to mount the other unit up by the distributor. Left that on there as well.
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[Rich84]

There is a brass T fitting.. Someone should have apart #..The cannister type for the gauge goes in one side and the one above goes in the other side.

[UATahoe]

Oh ok. So both sensors can mount up top by the distributor?
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[Gigem]

Originally Posted by UATahoe

I believe there are different gauge senders as well. I am currently sorting through this as well on my engine swap. Below is what was hooked up on my original motor. Its an 87 with the oil pressure gauge. I could have sworn these were for the idiot light but i am told that is not the case and that this is for a truck with a gauge.

This must be my problem, as I have the same sender as you. No computer. Dude sold me the wrong part. Should the correct sender be available at a local auto parts store, or is that a mail order only deal?
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[Psycho_SWB]

Originally Posted by Gigem

This must be my problem, as I have the same sender as you. No computer. Dude sold me the wrong part. Should the correct sender be available at a local auto parts store, or is that a mail order only deal?
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Any local auto parts store should have it. I bought mine at my local O'Reilly auto parts store.

[Skunksmash]

Originally Posted by UATahoe

Lol I am screwed then. Sold the old motor and apparently left the sensor and all on the motor. Is the lower unit sometimes surrounded by a metal type cage? Any help on replacement parts, and what made it so difficult?

And I am assuming you are talking about the fitting needed to mount the sending unit down by the oil filter? I also have to get he gift fitting to mount the other unit up by the distributor. Left that on there as well.
Posted via Mobile Device

Yep. Its got something like a 30 degree angle on it, where the sensor screws in. Without that angle, the new sending unit won't clear the block and you'll never get it in due to its bell shape. I ended up going to home depot and using brass fittings and teflon tape to "construct" a new little pipe to use. Works fine, doesn't leak. Destroyed the factory piece getting it off. Last guy must have put it back on there with an impact wrench or something.

[Gigem]

Fixed! Had the wrong switch. The right one is way bigger, as pictured above. No way it fits with the distributor in place, not sure the distributor would go back in if I installed it. So I got a 2" nipple and a 90 from Lowes, and we are good. Still a tight squeeze, a 45 would work better but Lowes didn't have any.

Oil pressure on the 350 HO looks great!

On to the next issue...

Thanks all!
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[UATahoe]

And I am actually working off of a Vortec block. I think the location is actually different on the newer motor as well.
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[mrolds88]

The 87 engine we have in Boys truck (350 TBI) has the computer sensor by the distributor and the gauge sender above the oil filter. There should be be a simple pipe plug that is on the driver side about an inch or so above the oil filter lip. I think its 1/4 pipe thread. If I remember right there should be a 90* elbow in there.

[UATahoe]

Thanks for the info! My old motor is gone. Found the plug on the new motor though. Any reason the elbow is needed? Can I just screw in the sensor without a fitting?
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[UATahoe]

Anybody know what the color of the wire going to the sending unit for the dash gauge is?

[Gigem]

My wire for oil pressure gauge was tan in color. I would think that any elbow would just be for clearance/access. Not required to function.

[motornut]

Originally Posted by UATahoe

I believe there are different gauge senders as well. I am currently sorting through this as well on my engine swap. Below is what was hooked up on my original motor. Its an 87 with the oil pressure gauge. I could have sworn these were for the idiot light but i am told that is not the case and that this is for a truck with a gauge.

that reminds me of a knock sensor?
pass side block drain hole?

my 78 had idiots,converted it to gauges using

I wana say blue

[mrolds88]

Originally Posted by UATahoe

Thanks for the info! My old motor is gone. Found the plug on the new motor though. Any reason the elbow is needed? Can I just screw in the sensor without a fitting?
Posted via Mobile Device

I think it was a clearance thing. you could put it straight in but I think you would still need a bushing to go from 1/4npt to 1/8npt

[motornut]

but in the 79 gauges its tan

[UATahoe]

Awesome. Thanks! I pulled up the wiring schematic for the truck. It is a tan wire. I will use an elbow on the top mounting location for the oil pressure/fuel pump sensor.

I will go ahead and utilize the port above the oil filter for the oil pressure sending unit for the dash gauge. 99% certain thats how the old motor was rigged up. Hopefully i can find fittings to work.

[Skunksmash]

Is it me, or did so many of those old seat belt lights fail? So much that often, the seat belt light just stopped working completely.