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Old 11-21-2018, 02:52 PM
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Join Date: Mar 2012
Location: Central New York
Posts: 5
ABS Wiring Problem

Hello all! So I'm gonna be as succinct as possible here.

I bought a 2003 ML320 this year. When I got it, it had all the usual ETS BAS ESP lights on constantly and I figured it was the brake light or something. After replacing leaky brake lines, I noticed that the BAS was trying to halt the whole car after first putting it in drive and trying to take off. So I asked the seller and apparently it was a bad wheel speed sensor. I replaced one and yielded no change. I had it scanned and it gave me nothing except for "faulty speed sensor 1." So I bought a Foxwell scanner with Benz software loaded (amazing inexpensive device btw) and used it to chart the sensor readings. Turns out I replaced the wrong sensor. One of the rear sensors is reading normally some of the time and matches the other ones, but then it'll periodically spike from 25mph to 50-90mph for a split second and trigger the ABS and cause it to halt (screenshot coming). It's only giving me bad sensor codes btw, no other codes. I replaced the bad sensor and there's no change.

So I was driving it for a while like that by going slow for like 10 seconds after start-up waiting for it to kick on the light and then drive "normally." Then I noticed there's a direct correlation to the temperature outside. If it was a hot day, it'd give me little trouble after start up. If it was below like 60F it'd start halting the car to a stop and wouldn't kick off like normal. Then below 50F it'd do that and then start beeping with the brake light on, trigger transmission limp mode ("safety mode") and stick in 3rd gear. Eventually after driving a bit and restarting, it'd drive ok again. With this New York winter cold, I've been obligated to drive it gently without the ABS fuse in and override the shifter just to get to work (that also triggers limp). It's only been wanting to drive in limp mode lately.

So my thoughts are that there's at least the sensor wire that's bad. So I'll have to look for bad connections on the harness. (I've read the wiring likes to fray near the pump) The fuse being out contributes to it. But I can't figure out why the hell the temperature would cause it to behave differently. Unless there's a separate fluid thing going on with the transmission. Maybe low pressure? (already did the fluid level the right way) Once it heats up, it quits going limp. Code reader shows me nothing about that except for "transmission safety mode: Yes" after limp is on. I'm thinking once I get rid of the root issue, the wire, it should be able to run fine with the fuse in etc.

I've checked pretty much everything else like lights and switches etc. Any other insight from you guys? Any wiring charts floating around that could show me how the wires are routed?

Thanks!
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Old 11-21-2018, 03:17 PM
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Join Date: Mar 2012
Location: Central New York
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Here's the graph. The tips of the peaks aren't the same speed as the other sensor line. It's showing them on different scales since the spikes are reading at different speeds.
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ABS Wiring Problem-img_20180909_191131.jpg  
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