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Testing wheel speed sensors... gearbox in limp mode again.

14K views 14 replies 6 participants last post by  Armchair 
#1 · (Edited)
I've got the dreaded gearbox limp mode problem again.

The symptoms were the same as last time. The ABS ETC lights came on with a bong and the cruise control didn't work. This time, however, I was in Wales not Germany so drove straight home without turning off the engine which meant the gearbox didn't drop into to limp mode until I got back and parked up.

I've used my tuning box thingy to scan for codes but it didn't find any.

Just tried to check the rear wheel speed sensors. I assumed that it would simply be a case of checking the resistance but that doesn't seem to work. The left hand sensor does have a measurable resistance but it only flashes up on my digital multimeter for an instant and then the reading returns to 1. It does this no matter how often I try and the resistance isn't always the same. The right hand sensor just shows 1 (out of range?) all the time.

Does anybody have any advice?


It's starting to make a bit more sense as the 300C appears to have active wheel speed sensors, not the self generating Hall effect type.
 
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#2 ·
Maybe unrelated and don't think it causes limp mode but I had an issue with the front ABS sensors versus the muck on the rear of the hub which was obstructing 'the view' of the sensor. I removed the ABS sensors, cleaned 'the eye' cleaned off the line of sight area on the back of the hub and all was good again. In my instance it was the fronts which seems to be a fairly common problem. Another associated issue can be the torque of the nearside rear hub nut, which can affect the ABS side of things. As said may be unrelated as I don't think either causes limp mode but apart from this symptoms are similar and don't necessarily show themselves as error codes.
 
#3 ·
After cleaning the sensor, did the lights on the dash switch off by them self or did it need a code reader?
 
#4 · (Edited)
Cheers Tim.

Based on my similar experience in des Deutschen Vaterland, I suspect that only the rear sensors give the limp mode problem as, apparently, there is no seperate gearbox output speed sensor on a CRD so the system relies on the combined input of the RH and LH wheel speed sensors.

The odd thing is that, in Germany, it dropped ito limp mode the day after the lights came on and I used the car several times in between. In this case, it was as soon as I turned the engine off when I got home.

I've rather rashly ordered a new RHR speed sensor so we will see if it makes any difference. I've also ordered a more sophisticated code reader than the emotion unit I've been using to date. It's a near 50 mile round trip to the nearest dealer for a scan, and that's a long way in first gear.
 
#7 ·
TBH not so sure it was the tone rings I cleaned (maybe I don't know). If you locate the ABS sensor, look at its viewing point of the hub, then thoroughly clean that area revolving the wheel as you do it, it cleans / de greases the muck preventing the sensors line of sight being obscured and causing the ABS / Traction prob's. I just removed the wheel allowing hub movement (not removal)
 
#8 ·
I had exactly the same syptoms, with the additional issue of the sat nav being confused. I ended up changing the rear nearside tone ring and sensor. All is well now 2 years on.
 
#9 ·
No problems with the sat nav or the speedo, As far as I recall I didn't last time either.

I was hoping that I would have received the code scanner I ordered by this weekend but it looks like it will be Monday at the earliest now. It looks like I will be driving the Land Rover for a few days.
 
#11 ·
Just ordered a pair of reluctor rings for mine....o/s ring no signal on test gear or osciloscope with new sensor but wiring test is fine.
Once the bearing/hub is removed can the tone ring be changed through the hub hole in the carrier without removing the half shaft fully....or must the shaft be removed as well ?
 
#13 ·
I wonder how is that possible, generally I thought the lot would need to come out. Any chance pointing to the links, it should be useful to know, bar the axle.
 
#15 · (Edited)
It's fixed ::smile::

I took the wheel off to get a better idea of what is involved in replacing the tone ring. While I was at it, I thought I may as well see if I could see the ring down the sensor hole, expecting to see a rusty mess. What I saw was an uncorroded silver ring partially covered in greasy crud so I thought I'd have a go at cleaning it.

After fruitlessly hunting for a suitable stiff brush I eventually used a bit of rope. I cut the end off so it was clean and square, soaked it in turps, pushed it down the sensor hole and rotated the ring one full turn. I then removed the rope cut the dirty end off, soaked in turps and repeated about a dozen times. The final turn was with clean dry rope to wipe the residual turps off.

I cleared the codes and took it for a test drive fully expecting to hear a bong and the lights come on again. it was fine by the end of the lane I live on so I drove it to the next village and back.

Everything works as it should, no warning lights and more than one gear.

Let's see how long it lasts.
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