928 Oil Pressure Sender Dead after 15 Minutes?

[928_Owner]

I have a 1982 Porsche 928 which I'm cleaning up because I need to sell it (don't drive it much any more and it takes 1/2 of my garage).

For the past several years, the oil pressure gauge would either read zero (when the engine was not running) or max pressure when the engine was running. I assumed that it was a defective VDO oil pressure sender. I replaced the sender with another unit from Pelican Parts (mfg. by FAE, part # 928-606-203-04/323).

The oil pressure gauge worked great for about 15 minutes. No pressure with engine off. About 3 bar with engine running. Then the indicator went to max reading just like the one I replaced.

Has anybody seen one of these fail after only a few minutes of operation?

Thanks,

Keith

[Maleficio]

You may have a wiring problem instead of a bad sending unit. The pegged needle indicates a short to ground, which is what the sending unit does when pressure is maxed out.

Disconnect the sending unit and read for short to ground on the wires, maybe you'll see something. Remember, one of the wires IS ground ( I think.....), you may have a problem on the other wire.

[MPDano]

Mine pegged when the wires were crossed. Put them the opposite way and all was good. To me, sounds like it's a bad connection like Maleficio says, but those units are cheap too. Try wiring then unit.

[Linderpat]

it sounds like one of the connections slipped off from the vibration of the engine. If it worked for a bit then stopped working, that might best explain it. That happened to me once too. Just crawl back under, pull off the connections (which screw on IIRC) and reconnect. There are little rubber boots that fit over them - so you may not be able to see the problem until you slide those boots back.

[Landseer]

Or a short in the wiring harness or the 14 pin connector.

How about posting some pictures of your car?

[neil30076]

Oil pressure comes into the 14 pin on pins 2 and 4 - 2 is blue/black and 4 is blue/white - if either shorts out or is open circuit you will get what you are seeing.
Note on the diagram, the actual pins are shown on bottom right, the diagram is in logical order, not physical.
If you re going to pull the plug to check - DISCONNECT the battery first!

[neil30076]

This was the 14 pin on my 82 - and everything worked fine!

[MPDano]

Neil, great Diagram. You should post that one in the "Most Common Issues" Thread.

[Maleficio]

Originally Posted by neil30076

This was the 14 pin on my 82 - and everything worked fine!

Good Lord!

[neil30076]

Not quite my words, but close - remade the whole harness, now everything on the pod even reads correctly - no suprise there!

[928_Owner]

Originally Posted by neil30076

This was the 14 pin on my 82 - and everything worked fine!

That wiring harness is a mess! It looks just like mine, or should I say the one I replaced about 10 years ago. The vendor who made the harness clearly used the wrong kind of plastic insulation.

FYI,

I'll investigate all of your recommendations tonight and Saturday when I can get my head under the hood.

Thanks,
Keith

[MPDano]

Originally Posted by 928_Owner

The vendor who made the harness clearly used the wrong kind of plastic insulation.

I'm guessing that it's just what was available back in the early 80's in Germany.

[JhwShark]

Originally Posted by Landseer

Or a short in the wiring harness or the 14 pin connector.

How about posting some pictures of your car?

+1...many of us have re-wired the harness, the oil pressure wires may be just like the pic. Mine were bad, as were others in that harness.

Where in the bay are you??

[neil30076]

And easy to check, disconnect battery -ve, pull the 14 pin plug, and carefully remove the cover -

[MPDano]

Stress on "Carefully." The Plastic is more like Brittle Glass. With all the heat over 30 years really takes a toll on this Connector.

Also, carefully can be in reference to all the wires and prongs will come flying out.

[Maleficio]

Surprisingly, my 30 year old connector is in great shape. In fact, it looks new. Considering the PO's mechanical prowess and concern for the car's condition, it isn't new.

[MPDano]

Originally Posted by Maleficio

Surprisingly, my 30 year old connector is in great shape. In fact, it looks new. Considering the PO's mechanical prowess and concern for the car's condition, it isn't new.

This is the big thing is that "most" 928's are driven into the ground and maintenance was almost nil in these cases. Those that got lucky and scored on some great deals with documented maintenance is rare.

My Euro was a complete basket case with zero maintenance, basically a red-headed step child's hamster. Neglected beyond belief. Now, my "S," although no "documented" maintenance was in very good shape with obvious maintenance upkeep.

What a lot of 928 owners did to keep the Rubber and Plastics in tip-top shape is to open the hood after their run. The 928 is so well sealed in the Engine Compartment, that it traps heat very very well. Hence, the baking of Harnesses, Rubber Hoses and Plastics.

[Maleficio]

Originally Posted by MPDano

This is the big thing is that "most" 928's are driven into the ground and maintenance was almost nil in these cases. Those that got lucky and scored on some great deals with documented maintenance is rare.

My Euro was a complete basket case with zero maintenance, basically a red-headed step child's hamster. Neglected beyond belief. Now, my "S," although no "documented" maintenance was in very good shape with obvious maintenance upkeep.

What a lot of 928 owners did to keep the Rubber and Plastics in tip-top shape is to open the hood after their run. The 928 is so well sealed in the Engine Compartment, that it traps heat very very well. Hence, the baking of Harnesses, Rubber Hoses and Plastics.

Raising the hood is exactly what I do with my Volvo XC70 in the summer. That engine bay traps heat like a convection oven, even after I removed the belly pan.

[Linderpat]

Originally Posted by MPDano

....What a lot of 928 owners did to keep the Rubber and Plastics in tip-top shape is to open the hood after their run. The 928 is so well sealed in the Engine Compartment, that it traps heat very very well. Hence, the baking of Harnesses, Rubber Hoses and Plastics.

that's what I do, and apparantly the prior owners did too, altho mine is a low mileage car. My prior two sharks were Florida cars - everything baked to glass under the hood. Took alot of work to replace all that rubber. I won't buy one from there again.

[928_Owner]

Ok, I bet you guys thought I had died and gone to Porsche heaven. Well not quite...

Yesterday, I started the engine to move the car to the driveway where I could work on this problem. Well, when I started the engine, the oil pressure gauge seemed to work normally!

With the car in the driveway (and engine off), I measured the electrical resistance from the pressure sender terminal (wire harness wire removed) to ground. The electrical resistance was about 30 ohms.

Using Neil's diagram, I traced the oil pressure sender gauge wire to the plug/socket on the passenger side of the engine compartment next to the +12V battery terminal. On the plug, I removed pin 4 (with wire) and again measured the electrical resistance (to ground) and measured 31 ohms. (I did another resistance test from the terminal on the oil pressure sender to pin 4 and measured less than 1 ohm). Therefore wiring resistance loss from the oil pressure sender to plug on the passenger side of the engine is about 1 ohm, or less). THis is what I expected to measure assuming this portion of the wiring was good. See attached photograph.

I removed my DVM and put the wiring back in place. With the engine running, the oil pressure indicated about 3 bar. After again driving the car for about 15 minutes the oil pressure indicated (on the instrument panel) max (5+ bar).

I again removed pin 4 and measured the electrical resistance (to ground) and again measured about 30 ohms, with the engine off. I turned the engine on and the measured electrical resistance went up to 165 ohms (engine idle). See attached picture. When I turned the engine off, the resistance again fell to about 30 ohms (zero oil pressure), as expected.

I noted that with pin 4 (and wire) remove with the engine running, the oil pressure gauge indicated max. Therefore when the oil pressure gauge sees an open circuit, it will indicate max (5 + bar).

At this point, it appears that the new oil pressure sender is functioning Ok. The wiring from the oil pressure sender to the plug/socket on the passenger side of the engine is also Ok.

At this point, I'm assuming that the most probable problem is an electrical issue with the instrument cluster (or a ground problem with the instrument wiring). I've also noted that the voltage indicator indicates in the colored region between 14 - 16 volts with the engine running, although the system voltage is less than this (using my DVM).

Keith