Oil Pressure Sender Voltage W\ Ignition On

[slow&rusty]

Hey Gang - With the ignition on, car not running am I supposed to see 12V at the (disconnected) wire to the oil pressure sender?

A new development as of yesterday I started getting a dancing needle on the oil pressure gauge.

The gauge does peg "5" with the car running and the wire disconnected.

Thanks!
Yasin

[djcdmv]

http://forums.pelicanparts.com/porsche-911-technical-forum/600677-oil-pressure-stuck-4-not-pegged-5-searched.html

Does this help... Third post.

[slow&rusty]

Thanks for the response and I am intrigued to check the resistance as per Kidasters post. My oil pressure on the gauge does register "2" at idle and "4" at higher rpms, where as before it would peg the gauge at "5" any rpms over 1,500.
So something is not right.
Yasin

[DRACO A5OG]

Check the connector, it may need to be re-soldered and the terminal cleaned and polished. This fix my eratic druck bounce

[Walt Fricke]

Yes, you will see 12V. Yes, if you disconnect the OP sender, the gauge will peg. The sender is a varying resistance (basically a fancy rheostat), and at infinite resistance it won't bleed off some of the current flowing through the gauge mechanism to keep the needle below maximum.

So take Graco's advice and check the connector. If you still get erratic readings, probably best to get a new sender. Alas, they don't last forever.

[slow&rusty]

Thanks for the reply Walt. I disconnected the single wire to the OP sender and getting a meagre 7.3volts, the connector looks clean too...so this is driving me batty! Any suggestions?
Thanks - Yasin

[Walt Fricke]

Actually, I don't think I have ever measured the voltage where it attaches to the sender. A gauge can be made to work in such a way that the sender wire does not send battery voltage out. One of the tricks in designing these gauges is to make them independent of varying voltage supply, as you might see voltages ranging from 12 to 14+. The gauge might start out with some voltage dividing. Maybe a Wheatstone bridge?

It is not likely that there is a voltage drain between the gauge and your sender,k other than at the sender connection. But you could run a wire externally from the back of the guage to the sender, and see if the gauge behavior changes at all.

Generally, I think that if a guage reads 0 when you ground the sender wire, and infinite when the sender wire is "open," then the sender might be OK and things should work. I tend not to suspect problems with the guages. A jumpy guage, if the wiring is in good shape, is most likely due to a sender which is wearing out.

[slow&rusty]

Thanks for the response Walt. I have a few more diagnostic checks before tearing things out of the car and will report back.
Cheers - Yasin

[Walt Fricke]

What I meant to say was that if the gauge reads 0 with the sender wire grounded, and pegs with it disconnected, then the GAUGE is probably OK. Not the sender - different testing needed for the sender.

[Sniff]

Originally Posted by Walt Fricke

What I meant to say was that if the gauger reads 0 with the sender wire grounded, and pegs with it disconnected, then the GAUGE is probably OK. Not the sender - different testing needed for the sender.

Walt, I'm hoping you can help me as you seem to know your electrical stuff regarding Porsches. The problem I am having ('89 911 Turbo Cab) is that my oil pressure guage just started reading zero all the time. I disconnected the wire from the oil sender and the pressure guage pegged to 5. When I grounded the wire the pressure guage went to zero. This means my guage is good. However, you hinted at a test to determine if the sender is bad. Can I ask you what that test is? I've read multiple threads on replacing the sender and it's more involved than I initially thought. I'd hate to go through all that trouble (plus the cost of a new sender) if it's still good and the problem is somewhere in the wiring.


On that note, would I have a wiring problem if I got the pressure guage to peg at 5 (sender wire open) and 0 (sender wire closed)? Any help you could provide would be greatly appreciated.

[Autoban]

I used to work for VDO and was the Supervisor in charge of the production for these sensors in the late '70s (so I made probable a lot of the sensors we are using these days). Knowing how they are made my first guess is that the wire on the internal resistor is starting to wear through and has some shorts that make the gauge jump around. It is a variable resistor and I don't remember the resistor value specification in 1 Bar increments for these senders, but if you can find the values you can test your gauge.
Buy cheap fixed resistors of the correct values and connect them (with clips) between the wire of the sender unit in the engine compartment and ground. Start with the 0 bar resistor and then check your gauge (with ignition on) to see if it reads close to 0 bar. Repeat the steps in 1 bar increments all the way to 5 bar. If your gauge is way off or jumps around, it's the gauge. If your gauge reads fine, the sender has to be replaced.

Juergen

[Sniff]

Originally Posted by Autoban

I used to work for VDO and was the Supervisor in charge of the production for these sensors in the late '70s (so I made probable a lot of the sensors we are using these days). Knowing how they are made my first guess is that the wire on the internal resistor is starting to wear through and has some shorts that make the gauge jump around. It is a variable resistor and I don't remember the resistor value specification in 1 Bar increments for these senders, but if you can find the values you can test your gauge.
Buy cheap fixed resistors of the correct values and connect them (with clips) between the wire of the sender unit in the engine compartment and ground. Start with the 0 bar resistor and then check your gauge (with ignition on) to see if it reads close to 0 bar. Repeat the steps in 1 bar increments all the way to 5 bar. If your gauge is way off or jumps around, it's the gauge. If your gauge reads fine, the sender has to be replaced.

Juergen

Juergen,

My oil pressure (druck) gauge is not jumping around. It stays pegged at 0. I thought I ruled out the gauge being bad by removing the wire from the oil pressure sender. When I did this, it pegged to 5. When I grounded the wire, it pegged to 0. This is the test that has been referred to in multiple threads on the topic. What I'm trying to find out is how to determine whether my oil pressure sender is working. It's somewhat involved to remove that thing and I don't want to do it if I don't need to. I've read about testing the sender's resistance while the engine is running but wanted to get Walt's opinion.

[bazar01]

On my 964, I took it out and bench tested it. Here are the resistance values of the VDO OP sensor. It is not a good idea to run the engine just to test for resistance. Who knows, you may have zero oil pressure.

0 bar - 9.5 ohms
5 bars - 168.9 ohms.

I used compressed air, mechanical pressure gauge and adapter fitting to pressurize the VDO oil pressure sensor.

[Sniff]

My engine oil light works fine and goes out right after I start the car. That tells me my engine does have oil pressure. Also, when I was driving the car and noticed the oil pressure went to zero (luckily I was less than 1 mile from the house), the oil light never came on, the oil quantity gauge showed normal, and the oil temp gauge was right where it needed to be. After I pulled into the garage I did a quick oil level check with the dipstick (while idling) and everything checked out. Also, there are no odd sounds from the engine or excessive temp readings.

I am pretty sure my oil pressure sender is bad, but I want to test it first while it's still on the car. If it tests good, I'll know I have a problem somewhere in the wires. Still looking for someone to tell me what the easiest way to test the sender is.

Sean

[Autoban]

Originally Posted by Sniff

Juergen,

My oil pressure (druck) gauge is not jumping around. It stays pegged at 0. I thought I ruled out the gauge being bad by removing the wire from the oil pressure sender. When I did this, it pegged to 5. When I grounded the wire, it pegged to 0. This is the test that has been referred to in multiple threads on the topic. What I'm trying to find out is how to determine whether my oil pressure sender is working. It's somewhat involved to remove that thing and I don't want to do it if I don't need to. I've read about testing the sender's resistance while the engine is running but wanted to get Walt's opinion.

Sorry, I misunderstood. Does the pointer move off the stop up to 0, or does it stay all the way down? I drove my car to the airport this morning, but I can't remember if the 0 is a little above the pointer resting point or not. However, we know now the resistor values:

0 bar - 9.5 ohms
5 bars - 168.9 ohms

Get a 165 ohm resistor and place it between the wire of the sender unit and ground and see if your gauge moves up close to the 5 bar mark. Just for cross checking I would also get a 80 ohm resistor to check if the gauge indicates in-between.

Juergen

[Sniff]

Originally Posted by Autoban

Sorry, I misunderstood. Does the pointer move off the stop up to 0, or does it stay all the way down? I drove my car to the airport this morning, but I can't remember if the 0 is a little above the pointer resting point or not. However, we know now the resistor values:

0 bar - 9.5 ohms
5 bars - 168.9 ohms

Get a 165 ohm resistor and place it between the wire of the sender unit and ground and see if your gauge moves up close to the 5 bar mark. Just for cross checking I would also get a 80 ohm resistor to check if the gauge indicates in-between.

Juergen

I'm not sure where in relation to the zero mark the needle is (at or below). At first the needle bounced ever so slightly (perhaps 1/4 needle width), but now it's just stuck at zero.

[Walt Fricke]

Sean (shortened from the private communication, and informed by previous posts)

You could hook up your ohmmeter to the sender after pulling the wire. It should read zero ohms or close to that (9.8? Come to think of it, these gauges usually rest a bit below zero, so a little resistance makes them come alive).

Start the engine. Ohms should go up. Rev the engine. Ohms should rise steadily to some point, then go no farther (oil pressure stabilized).

It is possible with the sender for the resistance to rise nicely, and then go infinite. This indicates a break in the resistance coil in the sender, about which nothing can be done except replacement. I think this may be the most common failure. In my limited experience it is on the oil lever sender.

I'm not sure how a sender might malfunction so that its output is always a ground or just above it no matter what, but strange things can happen - a wire might break at a solder joint, and end up touching a grounded part. I suppose if somehow really tough crud got into the sender (it has a very small hole) the sender would never rise. Unlikely, though. Oh - one of my bad senders physically broke inside - one of the bent metal supports for the resistance wire, or for one side of the clever double slider or something. Baffling as to how, but there it was when I opened the case up. So that might cause unwanted contact.

I don't have Autobahn's experience, but perhaps he knows of a way that the sender can short out, or almost short out. And not start out OK, and then jump to full scale/infinite resistance/open circuit, as happens with a break in the resistance winding.

Another test of your gauge, if the sender checks out, would be to use a variable resistor - a potentiometer - with, say, a 0 to 500 ohm range. Hook that up to ground and to the sensor input to the gauge. With the key in "run", turn the knob. The needle on your gauge should rise smoothly up to 5 bar, at which point you can stop. Means no discontinuity in the gauge. Might not mean the gauge is dead on accurate, but that's not the point here.

Is the turbo stuff what makes getting at the sender hard?

Some cars are easier to work on than others.

Walt

[Sniff]

Walt,

I really appreciate the reply. It's the A/C compressor that makes removing the sender such a bear (the turbo gear is all on the driver's side of the engine). I also read multiple threads recommending me not to just go in there with a crows-foot at the base of the sender since I would most likely shear off the bolt holding the sender's mounting block.

So, here's what I've done. Obviously removed the wire from the oil pressure sender (gauge pegs at 5) and when I ground the wire the gauge pegs to 0. When I returned home from work this evening, I started up the engine. The oil light went out (a good thing). I disconnected the wire from the sender and attached the red line of my multimeter to the sender's wire prong. I attached the black wire of my multimeter to ground (the A/C compressor bracket). With the multimeter sets to OHMS, I got .01. I moved the black wire around and the reading jumped all over the place. I was expecting an "infinity" reading, but I am not sure that is what I got. I confess I am not the most skilled when it comes to this stuff. Perhaps I wasn't getting a good ground on my multimeter. What do you think?

[JJ 911SC]

Originally Posted by Sniff

... I also read multiple threads recommending me not to just go in there with a crows-foot at the base of the sender since I wouldd most likely shear off the bolt holding the sender's mounting block...

You better believe it...

Had the same problem. The gauge was going all over, but would peg when unplug and go to zero when grounded (the worse ever Not Fail Safe design by Porsche as an open in the line would tell you everything is fine...) which confirmed that the gauge and wire was OK.

Back to the removal, once I took it off the engine and had it on the floor in a vice, the base only came off after I had the air gun at 140Psi, so there is no way it could have come off in-situ without causing major damage. See post 14 of da http://forums.pelicanparts.com/porsche-911-technical-forum/594895-stuck-oil-pressure-sender.html

I put a new Pressure Sender in and its been fine ever since.

[Sniff]

Yeah, I'm thinking I'll just replace the sender. I wish I was 100% sure that the sender was the problem, but even if it's not, I'm doing no harm. From what I understand those things are prone to fail at some point. Perhaps this will buy me another 23 years