New oil pressure sender fails after 1 hr?

pauloregon · 06-01-2008, 02:12 PM

944 people....

I installed a new oil pressure sender in my 85.5 944 NA last weekend as part of my oil cooler reseal project. After reassembly, the car ran fine on it's test drive with the oil gauge showing 5 bars at start, 4 a bit later, etc. Normal behavior.

Later that afternoon, as I was putting the car back in the garage, I noticed that oil gauge had pegged above 5 bars. When I turned off the car, the needle stayed high until i tapped at the instrument cluster and it dropped to zero.

Using the Clark's Garage test for the gauge, I was able to move the needle up and down using a variable resistor as described, so the gauge is fine.

Is it possible for these senders to fail just after install? Heck, my old sender was sort of working after 23 years of daily use! The new sender from Pelican was an FAE. Should I use the VDO sender instead?

Any advice is appreciated before I spend $$$

phoenix_iii · 06-02-2008, 08:55 AM

When installed backwards they peg at 5 (Actually like .25inch above, if not more). Anyway, it's because it's getting current/signal.

Basically, I'm saying electrical/ground issue. Check the wiring for exposure/cracks in shielding, etc.

Anyone happen to know what GROUNDS the oil pressure sender (instrument panel)? Mine is a little jumpy (new sender just installed), I'm 90% it's a ground issue.

pauloregon · 06-02-2008, 09:01 AM

I have heard about the pegging when installed backwards. I did a lot of testing of the gauge yesterday using a variable pot (15 turn 1k ohm precision for those wanting to know...Radio Shack $2.69) and have proven that the gauge is good.

I know that the sender is installed correctly but I am starting to wonder about the connection between the sender and the blue/white wire to the gauge. It may be a bad connection. As the resistance between the gauge and ground increases, the gauge goes up to 5 bars. In theory, a pegged gauge would have high/infinite resistance to ground which may be my problem.

When I drove to work today, I had normal pressure 4-5 bars for a few minutes then it pegged again. The good news is that after a 30 minute drive on the freeway, the engine didn't blow up or burn oil so I probably have ok oil pressure.

I'll check wiring again tonight when I get home.

aj951 · 06-02-2008, 09:13 AM

Did you use thread sealant on the sender threads? The sender relies on the connection between the sender body and oil cooler housing for the ground. I have read that you are not supposed to use thread sealant on the thread and to rely on the crush washer to make the seal.

Also in your previous post you stated that the gauge was pegged even after you turned the car off. That sounds like the gauge itself was mechanically stuck and you tapped on it to make it fall back to zero. When you used Clark's procedure for checking the gauge, could you get the needle to stick by going above the resistance level equivalent to 5 bars?

A.J.

pauloregon · 06-02-2008, 09:22 AM

Hi aj,
I didn't use any thread sealer or dope on the threads. I too heard that the crush washer alone does the job, which it seems to be doing.

Interesting idea, I didn't think about trying to peg the meter using the pot. I tested smooth 0-bar to 5-bar motion based on Clark's resistance table for my car, which worked just fine.

I noticed that the crimp connector on the blue/white wire was looking sort of old, maybe even frayed. I'll look at doing some reworking of this connection tonight, maybe replacing the connector.

Paul

phoenix_iii · 06-02-2008, 10:50 AM

Would love some pictures. If you email me I will host them, or you can put them online somewhere. =)

pauloregon · 06-05-2008, 12:55 PM

Update: I checked the resistance between battery ground and the sender body and found it to be 0 ohms, so this is not an issue of the sender not being grounded.

I replaced the blue/white lead from 6" back from the sender in case the o-ring or last few inches of wire were bad. This did not fix the problem.

Given that the gauge was proven to be good, and the wire is good, it has to be a bad sender. Can't think of anything else. Pelican says "order a new one" so I guess that's my next step. I wonder if FAE senders (vs. VDO) is the issue here.

pauloregon · 06-06-2008, 09:47 AM

Good news: Pelican contacted me and explained that the FAE sender is under warranty, so they will replace the sender for me. I still question the quality of FAE parts, but I continue to be impressed with Pelican Parts customer service.

bazar01 · bazar01's Garage

I bought the VDO OEM sender from a dealer just to be on the safe side. Just can't take the risk when it comes to oil pressure indication.

pauloregon · 06-06-2008, 01:50 PM

bazar01, you have a good point. how much does a Porsche dealer charge for a VDO sender?

bazar01 · bazar01's Garage

You have a PM Paul.

pauloregon · 06-14-2008, 03:09 PM

Update: I got the 2nd new FAE sender from Pelican today and installed it in record time. No other changes made, gauge shows pressure and everything looks good. So, it was a defective sender.

I plan to run with this one for a while and if it too fails, I'll give up and buy a VDO sender.

It's funny that after doing this job a couple of times, I can do it really fast! It took 15 minutes to remove the PS pump, hang it out of the way and remove the sender. The entire swap job took about 45 min with one of the front tires up on a ramp.

BTW, the 15/16" stubby open ended wrench made the job really easy. I bought a set of stubbies from Harbor Freight for $12. Easier than buying a full-sized wrench and cutting it.