Topic: Oil pressure problems sendiing unit

[1FSTLS1]

My  oil pressure gage is pegged all the time.  Even pegs when I turn the pump slow with a drill.  It is a 455 olds and 455 olds sending unit.  Do I just need a different sending unit?  Any one else have similar issue?

[78ta]

I believe the oil pressure gauge is just like the fuel gauge in that, if you have an open in the wire, it will read max and if it is shorted, it will read minimum.

I believe with the key on, if you disconnect the wire from the sender, the needle should go to max and if you short the sending unit wire to ground, the needle should go to minimum. If the gauge responds correctly, it is probably the sending unit.

You said "Even pegs when I turn the pump slow with a drill." I assume this means that it goes back to zero when you don't turn the pump with a drill? As I said above, if the needle responds, it's prob the sending unit.

[Rick]

Randy is correct.  If it's not acting that way, then there's another problem.

[1FSTLS1]

It is a brand new sending unit specified for the 455 olds motor

[ponchonutty]

What's it do when it's unplugged?  My car had aftermarket gauges in it then I installed a new sending unit then had to fix a bad spot on the circuit board on the gauge cluster.  When I did that both the water and oil gauges worked.  So, about 50 miles later the oil gauge started acting up.  It would show things OK when first started up but the hotter the engine would get, the worse the reading would be.  It would flop around and peg then go back to around normal.  I ended up needing to get a new sending unit again and now it's fine.

[brian c]

Out of curiousity...when you installed the new sending unit, did you use teflon tape around the threads? If so, you should remove the tape from the threads and try it again.

[1FSTLS1]

Out of curiousity...when you installed the new sending unit, did you use teflon tape around the threads? If so, you should remove the tape from the threads and try it again.

Explian?

[Rick]

The sending unit relies upon the block for the ground needed to make the circuit work.  If you use teflon tape around the sending unit threads, you are "insulating" them from the block.  It would have the same effect as if the sending unit was unplugged.  Sometimes a thread will "bite" through the tape but it's not reliable.  The sending units going into an engine should not have teflon tape used on them.

[1FSTLS1]

Ok.  I got another sending unit.  The old one measures from the tab to the body about 2.2k ohm and the new one measures about 550kohm.  Both make the gage peg out.  Put on a mechanical gage and I am just about 40 or so.  So the parts store had another sending unit that was about 4 times the size of the one I got and I dont even know if it will fit on the block.  So now I am stuck with this crapy gage with plastic line and oil running through my car not to crazy about it.  Oh well today was the first drive with the new motor. 

[78ta]

I'm not sure when the sending unit changed but I think they did change ratings at some point. You may need to ask for a sending unit for a 81 Pontiac instead of the whatever year Olds 455. It may be that the 78 gauge doesn't jive with the resistance range of the Olds 455 sending unit.

The above answers are correct but another reason for not using teflon tape is that pieces can tear off and get stuck in the orafice of the sending unit plugging it up and causing innacurate readings.

[ponchonutty]

I've never had an issue with teflon tape myself.  I think what you may have issues with is that maybe the sending unit you have gotten is for an idiot light and not a gauge.  I know when I ordered mine I had to state that I had a gauge.  Not sure if a sending unit for a Pontiac is the same for an Olds or not?

[1FSTLS1]

Yea they showed me another one and said it was for a gage and it was about 4 times the size of the one I have.  Does any one have a picture of the sending unit?  Did any of these old cars have lights instead of gages? 

[Rick]

Unless you had the rally gauges, you got idiot lights.  Most gen 2 Firebirds didn't get the gauge package, so most of them had idiot lights.

Since the 455 Olds engine was never offered in the Firebird line from the factory, I don't know how a stock sending unit on one would work with the Firebird gauges.  There was a difference in the early and late Firebird gauges though -- the change happened in something like 1980, and later gauges had different resistance.

That could be a problem, since they didn't use the Olds 403 after 79 -- otherwise, I'd think the 403 sending unit would be the ticket... 

[78ta]

There was a difference in the early and late Firebird gauges though -- the change happened in something like 1980, and later gauges had different resistance.

That's what I was referring to above about the change. Just wasn't sure when it happened.

This is starting to make sense.  The non gauge sender works the differently than the gauge equipped sender. With the idiot light(non gauge) sender, the circuit is open all the time until the pressure is "not satisfactory" according to the chassis service manual. At that point, the contacts inside make connection and the light comes on. Not sure if it reacts if the pressure is too high as well as too low. It just says if the pressure is "not satisfactory".  Never thought about that. Anyway, the fact that you said that the "other" sending unit they offered is 4 times the size of the one you're trying to use, seems to indicate that you're using a non gauge sender.

Here's a pic of an NOS sending unit. I believe these were the same for all GM rally gauge cars through the 70's anyway. The current service replacement or reproduction is about the same size but has a cone shape to the top of it. I believe it might be close to 4 times the size of an idiot light sending unit.  It's hard to get an idea of the size of it from the pic of just the gauge and box so I included a pic of the engine in my 70 442. You can see the sending unit just below the upper radiator hose on the front right side of the engine. I'd say it's pretty close to the same size in diameter as the inside diameter of the upper radiator hose. Weird comparison but, hopefully, you get the idea. Should be in the same place you're putting yours if you have an Olds 455. By the way, if you don't have it already, you will need a brass fitting with a 45 degree bend in order to mount the rally gauge sender to the engine. You can see how mine is tilted. The angled fitting is to allow the sending unit to clear the intake. You install the fiiting and then screw the sender into that.

[1FSTLS1]

I think this is figured out I have the sending unit for a light.  Sucks the one I need is so big.  I guess I will get it or keep aftermarket gages.  My water gage is broke and I dont have a working spedo.  Maybe I will replace them all in time with something aftermarket.  Thanks for all the help guys.  Picture is worth a thousand words.