Techno Duck

I think my oil pressure sender just took a crap, but i just want to make sure its not a wiring issue.

The gauge pegs past 5 as soon as i turn the key to 'on' but without starting the engine. I start the engine, red low pressure light goes out and the gauge still stays pegged. Turn off car, take out key, gauge returns to 0. I am assuming this is an indication of a bad sender?

I am wondering, if i leave the lead on the sender disconnected, it should peg the gauge right? As per ClarksGarage, a high resistance value indicates a higher number on the gauge. I need to go get a variable potentiometer to test my theory..but hoping someone can verify for me.

I know the leads are not reversed as the gauge was not touched at all.

Coincidentally i did clean the passenger side frame rail today of leaking powersteering fluid... low pressure water and Simple Green. I dried off and cleaned both leads on the sender, still no change. Just a coincidence i guess, as i didnt see any indication the sender was going bad. Oh well..

KuHL 951

It's been over 4 years but when I bought the silver 951 (the 1st time) the gauge read 0 because the OP sender was fubared. The car came with a new sender I installed before I bought it and it read normal again. I know if you cross the wires at the terminals or the positive sender wire grounds out it will peg high like yours also.

schwank

When I cross wired the sender it would peg at 5 all the time. This was a new sender. Switched the wires back and all was fine.

If you didn't actually change anything and it stopped working, well... you know the answer to that one. It ain't crossed wires!

MM951

My old 85 n/a pegged the gauge when it failed. New one solved the problem , IRRC they are ~$40 at paragon.

I just broke the terminal on my 84 .. I need a new one too !

Techno Duck

Did some testing with a variable resistor, pretty sure its the sender though there is always that paranoia of a wiring issue. I was able to get the gauge to read different levels at difference resistances.

Mike, with the wire not attached to the sender does the gauge still peg to 5? Thats what has me concerned i guess. I would have figured if the wire was not connected it would sit at zero.

Another strange thing is the red low pressure light stopped working while i was testing with the variable resistor. Just ordered a new sender from Lindsey Racing.. only $30, but the $13 2nd day air sorta kills the sweet deal .

J Berk

I know it's not a Porsche...but years ago we had so much trouble with oil pressure senders on my dad's boat (that boat had twin 350 chebies) that we put a mechanical gauge right on the blocks. Took all the guess-work away!

CarbonRevo

Ok, so since we're on the topic of pressure senders...how do these act when they start failing? What causes them to fail?

My car ran with great oil pressure. Shut it off for 3 minutes then try to re-start it and it hasn't had oil pressure since (even when cranking for 30 seconds+). The gauge reads 0 to about .25 bar when the key is on, ready to crank and when cranking. Sound like a dead pressure sender?

FRporscheman

Sounds like the sender is electrically broken (not sending a signal / open circuit). One time I had the same symptoms (pegged at 5) and it turned out one of the terminals had snapped off the sender body!

MM951

From my experience it will be pegged @ 5 with one wire disconnected

CarbonRev- I don't think they really fail over a period of time.. They *may* act flakey , but mine went out all at once. When I took it off it had something loose inside rattling around. I would switch 2 the wires and see if the gauge pegs @ 5. If you switch it back and it is back at zero, I would say you really don't have oil pressure

Techno Duck

Mine gave no indication of problems until i happened to look down and saw it was way past 5 bar a little earlier. I was driving to the Porsche dealer actually..to order the stupid backlight bulb for the digital clock that is burned out (and driving me crazy). Anyhow during the ride over there, it did come back down to 5 bar a few times, then back up to past 5. When i let up off the throttle and came back down to idle, it went down to maybe 4.5bar... so it definietley was on its way out. When i went to start the car when leaving the dealer, i put the car to 'on' but didnt start it. The gauge was resting at 2 bar. So it makes sense i guess, at idle iit was at 4.5 bar, and with the car off it was at 2...so that is on par with what my pressure usually is when warm and at idle (3.5 bar). By the time i got home, it was pegged to 5+ bar all the time.

It was funny because i was actually thinking to myself that the oil pressure gauge works well on this car...dosent bounce around on occasion like my old 944 did.

I am assuming the sender died completley while i was testing it with the variable resistor as the low pressure light no longer comes on when i put the key to 'on', but dont start the car. I am hoping the new sender cures this problem.. i really DO NOT want to deal with any wiring issues. Having rewired an early '85 interior and replaced the dashboard on a later car...ive had more than enough wiring experience on 944's to have nightmares about it.

ike229

Be careful with the terminals on the sender, i thought i was being gentle when tightening down one of the nuts..... And, i broke it (a few days ago). Live and learn i guess....

Wormhole

I broke the terminal too, it's super sensitive. Then I had to solder the connector on and made a mess of it..

Techno Duck

Put the new sender in (major PITA) the other day. Ordered it from Lindsey, and is of the FAE brand. Oil pressure gauge worked again though light did not work still. Figure it burned out. Anyway i noticed two things...

1. With the ignition on but engine off, the gauge reads 1 bar instead of 0 bar. Turn car on, goes to 4.5-5 bar..which leads me to #2

2. The oil pressure has not dropped below 4 bar. I did quiet a bit of driving, highway and mixed street. Using 15w50 Mobil 1, its between 30-45 degrees out (i know, a heavy oil to be using). It hasnt dropped below 4 at all.

Normally i wouldnt think twice, but with these two occurances i am still thinking something is off with the gauge. I tested the resistances again with a variable resistor and the gauge seems to be working fine. Its hard to be exact as i could only get a 5k ohm variable resistor.. so it was tough to dial in exact resistances. I also grounded the warning light lead directly to the chassis and the light worked...and mysteriously works normally now. Must have 'jiggled' something in the right way. Just the oil pressure readings still seem 'off'. I guess the fact it hasnt dropped below 4 is just because ive been using 15w40 in this relatively cold weather. Im just surprised i havent seen it drop below 4 yet. Ive only put about 1k miles on this car since buying it so im not really all familiar with its 'normal' operation. Maybe the air / oil front mount cooler really makes that much difference over the n/a's water / oil cooler.

What i want to try next is ground the gauges wire directly to the chassis aswell. If im correct, this should give me a perfect zeroed reading on the gauge. If it still reads at 1, i am thinking my gauge uncalibrated itself. Can anyone comment on this? Does that make sense? If you leave the lead disconnected 'free floating' in the air, it pegs to max. So if i ground it directly to the chassis with no resistor, it should goto zero right?

I dont think its a wiring issue as the gauge remains steady. No bouncing around or anything like that. I suppose i could check this by testing for continuity between the sender lead and on the engine block.

Is there anyway i can test the sender itself? Maybe take a resistance value from the connector post on the sender to the engine block? I think it should read something like 0-1 ohm with the car off..and with the car running around 184 ohms (on cold engine)...see the chart i copied off Clarks below.

1 bar - 29.6 ohms
2 bar - 65.3 ohms
3 bar - 98.9 ohms
4 bar - 133.6 ohms
5 bar - 184 ohms

The other thing i was thinking of trying to verify gauge operation is hooking up an early 944's oil pressure gauge directly to the sender and running it right off the battery. The senders are the same for early / late cars, so i should get similar results with the other gauge (i think). So many things i want to test..if only there wasnt freezing rain right now..

Excuse my electrical ignorance. I have taken three electrical engineering courses and still am very flaky with diagnosing things...im a mechanical engineer . Ive rebuilt two engines and am licensed to operate steam, gas turbine and diesel ship plants, but ill be damed if i can read a wiring diagram to save my life (or fix my car)!

CarbonRevo

Very weird. I'm interested to see what your fix is. I'm having oil pressure problems as well (gauge says I don't have any). I suggest buying a mechanical Oil pressure gauge. It's what I'm doing to cancel out all of the electrical gremlins there may be and I'll know if the mechanical gauge reads 0, that I really do not have oil pressure. I suggest getting a mechanical gauge, see what it reads and compare the findings with your electrical gauge. There's one for sale currently somewhere, but I have dibs!

And if worse comes to worst, I'm going to permanently mount the gauge in the car. Tacky in my opinion, but I'd rather be tacky, then engine-less.

Techno Duck

I was thinking of testing with a mechanical gauge but the problem is figuring out where to hook it up. There is one listed in the factory manual but it requires removal of the pressure sender..which i dont want to do based on the PITA factor. Dave Lindsey emailed me this morning after i asked him if he ever saw faulty sensors before. He told me to try my test with grounding the gauge directly to the frame and seeing if it reads zero. If it does, he told me i can send the sender back and has no problem exchanging it. Its freezing outside right now, so i am going to try and test it this weekend.

Im also sure that the factory mechanical pressure gauge is $1000 or something insane like that!

Cory, have you tried testing the gauge using a variable resistor? Its pretty easy, just need to goto Radio Shack and get a variable resistor (potentiometer) and a couple alligator clips. Remove the ring terminal from the sender and connect the potentiometer in series to a ground om the chassis. You can then use the resistance readings (veryify with a multimeter) and see if the actual gauge works still, or atleast be able to isolate it being a gauge or sender issue.

Edit - Special tool VW1342..and you also need adapter 999.105.013.02 and 901.101.175.01