Welcome all 928 forum refugees!
User avatar
By Shifted
#50255
For posterity, I am sharing my recent experience with my Antilock light coming on when driving. I'll start with the work that exacerbated the imminent failure, then the symptoms, and then the tests.

Previous work: I replaced all four rotors and brake pads. My existing pad wear sensors on the front are for a different type of pad than the new ones and they don't fit on the new pads. Rather than wait on the right wear sensors before driving the car, I decided to just zip tie them to the wheel speed sensor wire to keep them out of the way but the circuit intact. The car has been on the lift for over a month, so I wanted to get it down and operational. Wear sensors are easy to replace. This only applies to the front pads, the rear wear sensors fit properly.

Symptoms: Prior to the brake work, I've never once had the Antilock light come on after the initial instrument light test at startup. And the ABS has always worked since I bought the car. The first drive, after the recent work, started off no differently. The drive was going just fine until about 10 miles into it. The Antilock light came on. I found a place to pull over and restart the car. The light went off and I drove on. About a mile down the road I hit a bump and the light came on again.

Tests:
1) I unplugged both front wheel speed sensors and tested resistance across their two terminals. They both measured at 1k ohms.
2) I jacked up both front wheels and set the multimeter to measure millivolts and record the maximum voltage detected across the two terminals of each wheel speed sensor. I then spun each wheel. They both recorded about 20mv maximum voltage. And that was dependent upon how fast I was able to spin the wheel by hand. The faster I spun it, the higher the voltage. Both sides were identical.
3) I plugged the sensors back into the barrel plug mounted behind the wheel. Then, I disconnected the flat four way plug mounted to the top rear of each front fender inside the engine compartment. Two of the wires are for the break pad wear sensors and two are for the wheel speed sensors. The driver's side wheel speed sensor showed no continuity when measured from the plug on the wheel well.
4) I unplugged the driver's side sensor again and it tested ok. And the connector started crumbling.
5) I removed the sensor from the wheel and tested continuity on the bench while moving the wires around. They quickly stopped making any connection at all and the plug completely fell apart.

My conclusion is that the wires are failing inside the shielding and my moving the wire when I zip-tied the wear sensor to it broke the connection. When it was stretched out straight to connect my multimeter to it while it was still installed on the car, the connection worked. When the wire was bent to connect to the barrel connector, the connection was broken.

I believe that the sensor itself is ok, but inductive sensors do wear out over time even if the electrical connections are ok. So, a new one is on the way.

However, if you don't want to spend the money for a new one, I found that the wheel speed sensor from the 996J (911) from 1999-2005 and 986M (Boxster) from years 1997-2001 appear to have the exact same connector. The part number is 996.606.406.00. The sensor itself would not work without some modification (if they even have compatible electrical characteristics), but the plug looks identical. So, you could find a used one (eBay or elsewhere) with a good connector and splice it into the 928 sensor. Used 996/Boxster wheel speed sensors are anywhere from $15 to $30 on eBay. Compared with the $170 to $210 cost for a brand new sensor for the 928.

Replacing the plug is not the route that I'm going, but it might make sense for some situations.

Hopefully, my measurements (1k ohm resistance, ~20mv when spun by hand) will be helpful to someone else.

And, as always, beware the old and internally failing wires on our cars that can cause intermittent failures.
User avatar
By Crumpler
#50452
Great write up as usual.
I stick to my initial response that it’s not fair that we now have to worry about the continuity of each wire on our cars. I have enough to fix already :banghead:
User avatar
By worf
#50509
Crumpler wrote: Tue Jan 12, 2021 7:25 pm I stick to my initial response that it’s not fair that we now have to worry about the continuity of each wire on our cars. I have enough to fix already :banghead:
Life's not fair. I get to worry about every wire on ~50 928s.

@Shifted didn't read your entire treatise yet. But, after 30+ years, all(*) of the ABS harnesses from each wheel well up to and including the body harness are all one hard bend from breaking internally. To date, all but one ABS problem I've encountered were due to intermittent inside-the-harness or at-the-plug-connector issues. I've not once ever found a bad sender or bad sender pigtail harness.

(*) The '87-89 wheel well harnesses are particularly bad. A material change was made towards the end of MY'89 and those harnesses are much much better. However, the white (brown now) plastic connectors are the same crap material.

There is a 'dude' who's slowly but surely developing the bits and pieces to make new ABS harnesses. Roger and others resell his bits. But, he's only about 1/2 there.
User avatar
By Shifted
#50568
Crumpler wrote: Tue Jan 12, 2021 7:25 pm Great write up as usual.
I stick to my initial response that it’s not fair that we now have to worry about the continuity of each wire on our cars. I have enough to fix already :banghead:
 
I know, it's a really impressive and advanced vehicle for its time, but the penalty is that keeping it running properly involves a lot of rewiring in the long term!

Thankfully, mine was rebuilt from the body up shortly before I bought it. Which has brought a LOT of problems, but also a lot of new parts and wiring. A mixed bag, but overall it's been a good thing.
Honda ACTY (Kei Truck)

I want one. God knows what I'd actually use it fo[…]

I believe Chris connected with 928 specialists s[…]

The 928 Photo Thread

https://i.imgur.com/biMOPsy.jpg https://i.imgur[…]