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Old 07-03-2010, 08:29 AM
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540iman 540iman is offline
resident, old fart
Location: N.W. Indiana
 
Join Date: Sep 2008
Posts: 1,504
Mein Auto: 2002 540i sport
ABS Trifecta lights- how I would approach

As some of you may know, I have been contributing a fair amount of my personal experiences with BlueBee on how to approach the whole situation of ABS issues. Well, I just got the 3 lights of death back again (I call them the trifecta). History, replaced my module a little over a year ago and still had a bad RF sensor as determined by diode test. All has been well since replacing my module with new. Tried the re-build route and just lost time and got frustrated.

Car had been up on blocks for last 6 weeks as I did major rust repair and then sprayed the car. Got it back on the road again and travelled to and from work twice (about 100 miles total)and trifecta lights came on. WTF??!! New module (relatively) and I think car has maybe two new sensors RF and LR. Well, hooked up Carsoft and Carsoft says my LR speed sensor CABLE is bad. How it distinguished between the cable and the sensor I don't know. I will assume it saw the circuit as "open"- could be the opposite too and it saw a dead short. However, when sensor is bad, they will often read open both ways.

I checked with my VOM in diode position at the Module end of cable (connector). Tested between 13-29 iirc and showed 50 ohms in both directions. Checked a different speed sensor and it read a correct "open" in one direction and 1.707V in the other direction so I knew I really did have a sensor problem. I unplugged the sensor at the LR wheel and measured at the end of the speed sensor pigtail and guess what...it read the same...basically a "soft" short (50 ohms) in both directions at the sensor. I have one on order.This is in my estimation the correct way to try and separate the sensor from a module issue. Had that sensor read open in one direction and 1.7V in the other, I would be seeing if the module was still under warranty. Some have said they don't have a meter with a diode check position or that their meter gives different readings from my Fluke 85. In my opinion, it does not matter. Test all four sensors and it does not matter whether you use the resistance setting (use mega ohms setting) or a diode setting, don't sweat getting readings identical to mine.Look for one senor of the four that reads DIFFERENT from the others and then double and triple check that your paper clips or whatever you are using to insert into the connector are absolutely making good connection. Also, you must test each sensor twice. Once with positive lead of your meter and black or negative lead from you VOM oriented one direction and then reverse the meter leads to the same two pins. You want to check the sensor forward biased and reverse biased. If the readings are the same, your sensor is toast. Three of my four sensors checked fine. One was a bastard child! Hope this helps someone. Nothing here is new. You will find all this same information in BlueBee's comprehensive post. I just walked you through how my mind thinks on this issue.

Last edited by 540iman; 07-03-2010 at 08:31 AM.
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