oil pressure gauge deflects full scale...why?

Formerly Steve Wilkinson · 08-26-2008, 02:24 PM

Yeah, I did a search, because I know I've seen this discussed...maybe I should have used "druck press" as the search term...or "guage"?

Anyway, what did I do to cause my oil-pressure gauge to now deflect to a full-scale reading (i.e. top of the gauge) when I turn on the ignition key? It works fine as soon as the engine starts. If it's a ground problem, it sure doesn't affect the operation of the gauge when the engine is running.

Whatever I did occurred during the installation of a new ignition-switch electrical cassette yesterday, which also involved removal of the steering wheel.

dentist90 · dentist90's Garage

I know my guage pegged to the max when the wire at the pressure sender broke. It's kinda backward that way, but it stayed there... no up and down.
The guage doesn't just read full-scale for two or three seconds during the instrument test-light period, does it? Does ignition switch in postion 1 (accesory) or 2 (run) make any difference?

Formerly Steve Wilkinson · 08-27-2008, 04:51 AM

An '83 SC doesn't have any "instrument test-light" function, like a modern car does. In any case, in key positions 1 and 2, nothing is alive, no lights or gauge indications. Key position 3 (4 being starter activation), everything comes alive, the oil pressure goes to the top of the gauge and stays there for as long as the key is on but the engine off. After engine start, the oil pressure gauge works normally, which baffles me.

dentist90 · dentist90's Garage

That is peculiar. I can't figure out why in the run position the gauge is maxed, but after starting and returning to the same run position it goes to normal function. If I think of something later I'll PM you.

Formerly Steve Wilkinson · 08-27-2008, 08:18 AM

It is a brand-new ignition switch (the electrical part), as I mentioned. Maybe it's just some odd bug in the new switch.

Formerly Steve Wilkinson · 08-27-2008, 04:40 PM

I love it. I have apparently come up with a basic electrical question that baffles the best and the brightest of Pelican's pros.

red 2.7 · 08-27-2008, 05:06 PM

Hi Steve
I think I can help. When your car is not running the oil in your tight engine gradually builds up pressure till you crank her a few times then comes the relief and all is normal. The same thing happens to me :-) BTW Loved the book.

Wally

Brother · 08-27-2008, 05:20 PM

Check the ground on the gauge.

Jim Williams · 08-27-2008, 05:21 PM

OK, I love an electrical challenge...

Some diagnostics - While the switch is on, before the engine is started, go to the engine and jumper the sender connection to ground, leaving the normal connection connected. It seems unlikely that the needle would go to zero pressure, but this would eliminate the sender as a problem if the needle stays high.

I'm thinking of the next step.......

red 2.7 · 08-28-2008, 05:12 PM

Please ignore my last reply it was a poor beer affected attempt at humor. Seriously though many types of gauges go to full scale when "open" or the sender wire is disconnected. Maybe your new ignition switch in the acc or on position is open to unload the battery till the starter runs then goes back to on after the starter is removed. ie when cranking everything goes off except the starter circuit.I seem to recall that happening on one of my other cars.

Wally. in Montreal

Bob Goding · 08-28-2008, 06:03 PM

I vaguely recall this type gauge goes full-scale-open circuit. Might have to get my book out
50 years=fading memory.

dentist90 · dentist90's Garage

Steve's pressure gauge problem is driving me nutz now. I give... what's the answer??

Just for kicks I'd like to know if you disable your ignition (ie, coil lead) so the engine doesn't start, would the gauge go to normal after attempting to start? Or does the actual rise in oil pressure when the engine starts cause the gauge to function normally?
I can't figure why you'd get no ground in Run position until after turning key to start position

Jim Williams · 08-29-2008, 07:23 AM

Formerly:

Still waiting on the report of the results of the first diagnostic test... (my post of 8-27-2008, 8:21 PM)

dentist90 also has a couple of interesting questions.

Formerly Steve Wilkinson · 08-29-2008, 07:36 AM

Jim, I did as you suggested--jumped sender connection to ground--and the gauge indeed goes to zero. I haven't yet tried dentist90's suggestion to disable the coils (there are two). Should I?

ossiblue · 08-29-2008, 07:51 AM

Since you grounded the sender wire and the gauge went to zero, is it possible the sender is getting "hung up" and the engine start, which begins the oil flow, is enough to jolt it back into action? Maybe try turning on the ignition and tapping on the sender with a small hammer or screw driver and see if the needle drops to zero.

Formerly Steve Wilkinson · 08-29-2008, 08:06 AM

No rapping it with a tackhammer does nothing. It stays at top-of-gauge.

dentist90 · dentist90's Garage

At least it works properly when the engine is running, that is the most important thing. Obviously if it was working normally before you swapped the ignition switch internals it must have something to do with the new part. I don't know, but were changes made to the ignition switch since '83?

Formerly Steve Wilkinson · 08-29-2008, 08:12 AM

Can't imagine changes were made, but who knows. It was a switch unit for a broad span of years, maybe even 1973-'89, I don't remember, and it looks absolutely identical, materials and all, to the one that was on the car.

But you're right, the gauge was working normally before installation of the new switch unit and this anomaly showed up on the first start thereafter...

Jim Williams · 08-29-2008, 08:42 AM

Stephan:

What this suggests is that the sending unit has developed an open circuit internally with the internal wiper that contacts the resistance coil. This is occurring at zero pressure (engine not running). When the sender sees oil pressure, the wiper moves to a place on the resistance winding where it picks up a good connection.

If this is the case (?), then it's failure was just coincidental to the ignition switch replacement. If it is not the case, then we'll move on to another test.

You could test out my theory with the appropriate fittings, removing the sender, and testing it using air pressure and an ohmmeter, to replicate oil pressure and read some resistance (I have the numbers recorded somewhere), or just replace the sender with a new or known-to-be-good unit, and see if it corrects the problem.

Mandatory Disclaimer: Or, I could be wrong.

myamoto1 · myamoto1's Garage

My guess is that your pressure sender is on its way out. Do as Jim suggested and take and ohm meter to your sender. If you don't have A/C in the car, it's a pretty easy thing to pull - just make sure you remove the mounting block from the car,before pulling the sender off of the mounting block.