Oil pressure indication
Thread Starter
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
Oil pressure indication
My 77 has indication problems. The gage will not go over the first line which I would assume would be 20psi. Here is what I have changed. New printed circuit card. Tried two differant oil pressure gauges both read same and I have swapped resistor blocks behind the gages with differant color resisters. I have tried two differant sending units both from differant parts stores. New engine hooked up direct reading gage and engine has super pressure. Checked grounds. With sending unit unplugged gage goes to full scale high so I know engine and sending unit grounded. I even went as far as hooking up one of my aftermarket sending units to my air compressor and grounding it to the car and putting 125 psi to the sender and my gage still reads the same. Is it possible I may have to go and buy a special GM sending unit or should I just add a resistor to my wire that goes to the p/c board until my factory gage reads the same as the aftermarket gage. This has got me very confused.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
First question would be did you try grounding the wire going to the sending unit to the engine block?This is the other half of the process of checking the gauge and gauge wiring.If this sends the needle on your gauge to full scale LOW then you need to ground the same wire to the case of the sending unit and see if the needle reacts the same.If it does go to full scale low again then I would think you need to buy a GM sending unit.GM #14039612
Last edited by ...Roger...; Oct 10, 2008 at 02:03 PM.
Thread Starter
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
First question would be did you try grounding the wire going to the sending unit to the engine block?This is the other half of the process of checking the gauge and gauge wiring.If this sends the needle on your gauge to full scale LOW then you need to ground the same wire to the case of the sending unit and see if the needle reacts the same.If it does go to full scale low again then I would think you need to buy a GM sending unit.GM #14039612
just went out to the garage and grounded wire to engine block and it does go to zero. Do I get this sender from GM or will willcox have it?
7T1 vette
yes I did hook up the good old fashioned gauge to the block and engine has great pressure.
I just came back from my local Oreilly auto that sold me the Borg Warner switch that the book references for my car and the GM switch are different. He can no longer order the GM switch but it cross references a different switch than what he sold me. He ordered the Borg Warner exact cross match for me and it will be here Saturday [tomorrow]. So I will see if this switch works
Last edited by wnmech; Oct 10, 2008 at 03:27 PM.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Just for fun since you have changed the resistor on the back of the gauge put a voltmeter on the green sender wire at the engine(the one you grounded)and turn the key to on....you should show around 8.5 volts.Now turn your key OFF and switch to the ohm scale....you should show around 33 ohms.And again just for fun check the sender you have in your block now....put your red lead on the male spade coming from the sender and ground the black lead on the engine block....a good sender with a proper ground should read around 1.5 ohms.
Drifting
Joined: Aug 2007
Posts: 1,924
Likes: 6
get an aftermarket 'mechanical' gauge . hook it up (I have a permanent secondary mech gauge underhood) and read it there.
2nd opinions are always good
Racer
Joined: Dec 2003
Posts: 275
Likes: 6
From: NY
I can't wait to hear the end of this one. I have a '71 with the same symptom. Shows about 1/2 press at start then goes to zero when warm. I teed an external oil press guage into the car. The external gauge showed approx twice the press., never went below 25lbs. I have removed, disassembled,cleaned, lubed and re-installed the gauge. No joy? Engine runs fine. ??
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Thread Starter
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
Thread Starter
Drifting
Joined: Jun 2005
Posts: 1,523
Likes: 1
From: grand prairie texas
I made a little progress today with my indication problem. I am getting 6.7 volts down to the sending unit thru the cluster/pc board. I removed the indicator and the block resistor [blue] is reading 90 ohms. I removed the gage from the cluster and hooked it up siting on my fender. I was reading now almost 9 volts from the terminal that goes to the sending unit. I dummied the wire from that terminal down to the sender and I have good indicated oil pressure which is exactly what my direct reading gage said. So next question do anyone out there know what is proper voltage down to the sending unit? Alsodo you think possible the voltmeter gage could be causing my voltage drop?
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
I can't wait to hear the end of this one. I have a '71 with the same symptom. Shows about 1/2 press at start then goes to zero when warm. I teed an external oil press guage into the car. The external gauge showed approx twice the press., never went below 25lbs. I have removed, disassembled,cleaned, lubed and re-installed the gauge. No joy? Engine runs fine. ??
