I just put in a new Year one crate engine and purchased a new Oil pressure sending unit (the holes were different size). Shortly after running my oil gauge which seemed to be working fine, pegged out all the way to the high side. Everything with the motor seems fine so i suspect it is either a gauge problem or sending unit problem.
Anyone have troubleshooting ideas?
Do new and older sending units operate on different voltage or resistance could i have gotten one that is not compatible?
Thanks
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yes, try your old one

 

Uh, I said I could not use the old one because the hole is a different size....

 

sorry , I missread your post

 

Plumbing supply co may have an adapter you can use to put your old sender in the new hole.

Just bring both senders to them and ask for an adapter. Probably an NPT standard to metric.

hth

 

I think there are 2 major ones. 1 is for up to 60psi and the other is 80psi.

Where did you get yours and what did you look up for it?

 

I recall that the older cars (for example, my '74) had an oil pressure gauge that maxed out at 80 psi. It is 1/8" NPT. And at some point, Chevrolet changed over to a 100 psi gauge. So, yes, the sender has to match the gauge.

 

With the engine NOT running, switch the key to the "on" position. Pull the sending wire off the the oil press sender and ground it, either with a jumper lead or hold it to ground with someone watching the gauge. If the oil press gauge pegs out to max oil pressure, the gauge is good.

 

I have a similar problem. This just started happening, when I first start the engine the gauge is pegged out after it warmed up it started working normally. The other day it just stayed pegged out. When I turn the ignition off it slowly goes back to 80psi mark. Today I picked up a BWD Oil Pressure Switch Part # S769 from O'Reilly's. I installed it and it is still just staying pegged out. I had recently had the heads off for a valve job off and after putting it back together had a leak from the left side valve cover gasket and the wire got pretty oiled down. Could this be the problem? I read in another tread to not use teflon tape as the treads on the sensor are the ground, the new sensor had teflon tape on em and I installed it that way. Should I pull it back off and remove the teflon? I will give LI C3's suggestion a try in the morning

 

Did you have a different reading on your gage after you took the teflon tape off. Mine also has teflon tape on it and my gage doesn't seem to be reading correctly. Thanks

 

I haven't had time to pull it yet. I don't think that is my problem though. I pulled the wire connector off and it stayed pegged out. I grounded that and it still was pegged out. It must be shorting out somewhere between the connector and the gauge or the gauge went bad.

 

Here is the run down on the 1975-1982 Oil pressure gauges:

The oil pressure gauge works on the exact same principle as the fuel gauge.. 0 ohms is 0 pressure and 90 ohms is 80 lbs.... 40 lbs is 45 ohms input... The difference besides the face, is the dampening oil inside the gauge. The fuel gauge has a thicker oil and responds slower than the oil pressure gauge.....

The gauge requires power, ground and a input signal. When a oil pressure gauge pegs past 80 lbs it is missing ohms input... when grounded out it will go to zero and when the resistor is blown on the back of the gauge it will read, but the highest reading is about 1/3rd... or about 28 lbs on the face.

So.. go to the sending unit on the engine and ground out the wire to the block.. The gauge should go to zero... Then leave it unplugged and touching nothing and the gauge should peg.

This will eliminate, or point to the dash unit as the problem.

Now if the sending unit came with all kinds of Teflon on the threads this may be part of the problem. You may want to clean this off and put Teflon tape on the first three threads only... Then screw it back in and see what you get. Teflon is a lubricant used to make the NPT threads bite and not a sealant. Some factory senders are coated with a liquid form of this stuff and it can prevent the sending unit from grounding to the block..

You can test the sender and see if if it is working with an ohm meter in the 200 setting.
With the engine running see what the out put of the sender is.. If there is no out put then the sender is defective... (or not grounded properly).

Willcox

 

So if it stays pegged grounded or not that would indicate a break in the wire somewhere between the sender and the gauge correct?

 

Or too much Teflon Tape on the threads.....

 

Not sure if this was directed to me or not but, if I ground the wire at the point it connects to the sending unit (but not attached to the sending unit) and see no change it has to be something with the wire? I am not very good with electrical stuff

 

Correct - I should not answer so early in the AM!!

 

when grounded out it will go to zero and when the resistor is blown on the back of the gauge it will read, but the highest reading is about 1/3rd... or about 28 lbs on the face.


If the resistor is blown can it be replaced or does one just get a new gage>

 

Found my problem. It was in the connector itself. I guess with all the jostling around with the headers taking the heads off and then back on I must have hit the connector and it pulled the wire loose from it or was not making contact. New connector and now reading 60psi

 

good deal...

 

Depending on the year it will either have a wire wound resistor or it will have a ceramic one...They are both around 90 ohs so you can put a new one in there. Issue is you can't buy a new on so you'd have to visit your local electronics store of hit the internet to come up with a resistor.

I've used the ones from Radio shack in the past and I crimp a loop on the end wires that will pass over the stud.

Willcox