We often post our problems on here, and I think it's nice to give up a few solutions in the mix. I've got my notes on the fix of my RDK failure, and here's what I've found:

My failure mode was "TIREPRESSURE [ + ] LOSS" in red on the display. The two rear, and occasionally one rear tire arrow would flash. Message would reappear at about 30 minute driving intervals.

Investigation: Set air pressure to 3 bar(49PSI). Since I was getting the arrow for two rear tires, and not the front, it eliminated the control mechanism because the signal lines are strobed into the control mech. Since it was well defined to the rears, the control box was probably working well. The WSM has a breakout of possilble causes based on display mode, and reoccurance. Mine fell in the catagory of "sensor at wheel, pickup, or associated wiring". There is a second diagnosis path for if the ABS is also on, as the right rear wheel has a rotation sensor for both ABS and PSD on >88 cars. I had no ABS or PSD issues, so it was furthur defined to the wheel bung, sensor pickup or associated wiring.

Jack up rear, first visual check of wiring from hub, through suspension, into base of spare well. All good. Visual inspection of sensor shows the right side as being kinda far away from wheel bung, left side is okay. Sensors are located on the back of hub, facing the rear of the car. Easily visible with wheel on. I had the right rear show on the display more often than the left, sometimes it was both. Coorelation? I think so.

Remove right rear wheel, clean bungs, had some crap on one of them that came off with thinner. Install wheel and measure gap @ 14mm. Seems high, but no spec in WSM. It says "if the fault is in the sensor, take the car to an authorized dealer for service". Not bloody likekly. Test with car on jacks, still getting fault on right side. Run for a few minutes, nothing on left yet.

Remove wheel and loosen two 10mm nuts holding sensor to suspension brace. I notice that the sensor is not a parallel angle to the rotation of the wheel. Possilble that the sensor mounting plate was damaged by a tire jockey, or service tech, moving the sensor away from the bung. Bend mounting plate slightly, and cut notch in mounting plate hole with a die grinder. Remount sensor on brace, and fit wheel. Slowly rotate until bung is over sensor. Measure 9mm and tighten nuts on plate.

Remove spare tire, and uncover wiring to RDK. Found both sealed jack/plug connections partially out. Clasp plastic broken on one of the connectors. Open both connectors and spray CFC cleaner into holes, and around plug. Scrape clean with small round file, and reconnect.

Test with car still jacked. No message. Set car down, drive for 18 miles, no messages. I'm calling it a fix. Time will tell if it comes back.

Most probable cause: Dirt, grime, water in connectors in base of spare well. RR sensor plate damaged by service tech in the past. BTW I have Aluminum Nuts

Do you have Aluminum nuts? Jeez, saved a few grams there.

 

Great Job!

When this last came up, someone had sugested looking at the brackets, if I remember right!

I think Ern said those Alluminum Nuts on the wheels cost like 65.00 bills each!

How many do you want?

 

Well done Doc. This is a rare story.
I went through that whole procedure years ago and failed to get a fix so I bypassed it as most do. But I'm inspired by your success and I think the time has come to try it again. I'll do it in the new year. Will post.
Curtis. I've been told the wheel nuts are titanium which might justify $65 though I don't believe it. I don't think they are straight aluminium either, it would be too soft. Most probably an alloy. Anybody know for sure, and BTW does anybody know a workshop test to ID titanium.

Colin 89GT

 

Curious... do those set off the metal detectors at the airport? How does your wife/SO like them?

Couldn't resist... it's Friday!

 

All the Porsche alloy wheel lug nuts are an aluminum alloy sell for $5 to 6 dollars and really need to be installed and torqued with a torque wrench 130 NM about 98 ft lbs. The rattle gun boys love to overtighten and the next poor fellow especially using the lug wrench often will shear off the top of the lug nut. But Doc your choice of materials for prosthetics is a personal one.

 

It's not the wheel stud nuts that sell for $65.00, it's the nut that holds the sensor in place. Look THAT up in your Funk & Wagnels Jim!!

 

Last time I checked a year or two ago......they were $35-$40 each. Ounce for ounce they may be one of the most expensive parts on a 928.

 

"130 NM about 98 ft lbs"

Thanks Jim, for some reason I was thinking it was 100 ft lbs on the lugs.

Yea, I was talking about the wheel sensor nuts.

Hey Ern, how many nuts did you have to replace when they tried to reseal you senors?

 

None,
The guy at Zims was able to turn the nut around and get it to hold after the thresds on the inside stripped. Some gorilla had way over torqued them at some time in the past. IIRC the MSRP is around $65.00 on the nuts.

 

My, my. It seems my nuts have stirred a bit of hyperboyle on the list. Well, I know that the lug nuts were Al because I had them all off before. I was referring to the sensor plate nuts as being Al, of course. That's where I was working in reference. The only thing I can think of is the magnetic resonance they may encourage in the HF sensor on the plate. Al is of course, conductive, but it is not inductive to magnetism permanently. It would seem the makers could have used brass, or something else as a bit of less costly. Oh well, they came off and went on fine.

Drove for about 80 miles today, still no faults. If those of you that want to T-shoot the system again, PM me and maybe I can provide some individual insight based on your failure mode. Roger tells me that his car has 2.5 bar wheeel bungs all around. Mine has 2.5 in front and 3.0 in rear. It's easy to have low pressure in back.

 

ColinB... workshop test for Titanium... take the items, nuts,
in this case, and write on glass with them. Titanium will leave
a 'silver' streak; steel, magnesium, aluminum/aluminium,
tin, tungsten, etc, won't. Nut streaks, who knew??

docmirror... thanks for the RDK feedback. Sounds as if
you nailed it.

G'luck.

 

macreel
Thanks for the titanium tip. Something to file away for most of a lifetime then bring out to impress someone just as I am now. Nuts, ****, what is the board coming too? Randy will be slapping our wrists any minute now.

Kind regards. Colin. 89GT

 

Colin... slap just wrists? Well, we'll promise not to ENJOY it.

We drive 'sharks', so must be a bunch of masochists, eh?