ABS Light On & Limp Home On - Mercedes-Benz Forum
 
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post #1 of 3 (permalink) Old 04-19-2012, 05:48 AM Thread Starter
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Date registered: Dec 2010
Vehicle: Vito W202 C200
Location: Australia
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ABS Light On & Limp Home On

Hi All
I have a W202 C200 1996 build date. Right hand drive.

The other day the ABS light came on and the car told the electronic (722.6
model) gearbox to go home in Limp mode. (Reverse and 2nd gears only around 3000rmp at 40Kilometres per hour )

Reading up on the causes for ABS light to come on included the ABS
(wheel/diff) speed sensors (mine 2 front wheel and 1 rear differential),
brake light switch, Over Voltage Protection relay and/or inbuilt fuse (if
fitted on yr model, perhaps not my model), oil leak at gearbox on the ECU
cable from failed O ring seals causing cable to have fluid suck into it
and possibly creep all the way to the transmissions ECU box (mine under
left side front floor kick panel), brake light switch.

I also found the resistance of good speed sensors was around 1.1K ohms
from posts on the net.

So I thought it might be good to get the car scanned with the Star reader
to get a list of error codes. A 30mile drive at 40 Klm/hr and a 3 day wait
to get booked in to my local indie mechanic (nearest dealer is 100 klm
away)

While waiting, I looked into the speed sensors location and cabling.
My car only has the 3 sensors, 2 front and 1 rear

I found the connectors under the front wheel arches for the front wheels,
held in place by fine spring wire clips on the plug/socket, with a fine
screwdriver lift wire clip up in its slide and unplug the wheel sensor
from socket under front wheel arch

This allows a multimeter to read the resistance of the speed sensor
My front wheels were both around 1100 ohms (1.1k)

So I found the location and cabling route for the rear diff sensor, it is
on the right side towards the top of differential, cable goes through
floor plug into the boot toward the rear (lift the carpet and find a 400mm
x 100mm plastic cover screwed to a plastic tray.)

Here we can unplug the sensor and get a multi meter to read the resistance
of the sensor. This one seemed to read 160K ohms not 1K ohm

Symptoms of the failure were start car, ABS light stuck on. Limp Home mode on.
Drive for a bit (1 to 4 kilometres) and stop, wait a few seconds, restart
ABS light still on but drive 30/50 meters and light would go out.

Next was to measure the out of spec rear speed sensor resistance when
light on and light off

It seems that the sensor is responding to heat, when stone cold it reads
160K to 200K ohms and light is on

When light went out, at operating temp, the sensor resistance would come
down to around 20K ohms and the light and limp home mode were off.

Doing checks while sensor cooled showed the resistance would take about 5
to 10 minutes to get to 30K and 30 minutes to get to 60K and say 45
minutes to get to 80K-100K and over night at full cool the sensor could be
close to 180K-200K

So I have ordered a new diff sensor, hoping that it is the fix. It will be
here in a few days at around $100 the car scan was to be $80 ...

For a double check on my diagnosis I tried heating the faulty sensor untill the resistance dropped from 150K to about 40K ohms (as low as I could get it with the slimline electric heater under the car pointing at the diff), the fault would clear in a few 100 metres rather than he

I will post again when the part is fitted to see if it fixed things.
------------------------------------------------

Well I fitted the new diff sensor (noting the warnings in other posts to be very careful removing the sensor from diff as it can break off and require hours of surgery to fix!!) and drove 30metres and the light went out and problem solved.

Now light is off all the time, the new sensor was around 1.1Kohms

For some insight on what failed, I cut the canister off the sensor and guess what it was soaked in diff oil.

So a 16 year old sensor bathed in hot diff oil can have the seal on the canister fail and let oil seep in. It responds to heat or even the hot oil splashing over it, soaks what appears to be an inductive coil/magnet/sensor(sender) and there we have the classic failing part.

Hope this make someones life a bit simpler if the have ABS light stuck on and the fancy electronics on the gearbox telling them it is only safe to go 40 Klm/Hr !!!!

Cheers
Greg from 'down under'
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post #2 of 3 (permalink) Old 04-20-2012, 07:31 PM
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Date registered: Apr 2008
Vehicle: 1998 C43 AMG, 2006 E500 4MATIC Estate
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You're on the wrong board. Try the normal w202 board, as this is the w202 AMG board. Good luck.

1998 C43 AMG @196k miles
2006 E500 4MATIC Estate @126k (DD)
1992 190E 2.3 @162k (SOLD, then STOLEN, then TOTALED)
1984 190E 2.3 (SOLD)
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post #3 of 3 (permalink) Old 04-21-2012, 03:29 AM Thread Starter
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Date registered: Dec 2010
Vehicle: Vito W202 C200
Location: Australia
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(Thread Starter)
Move post to W202 forum

I noticed that just after I posted, and asked the moderator if he could move it over.

I dont see any way for me to move the post.. and dont want to create a duplicate ...

Hope the moderator can assist, though I have not had a reply from him in the past days

Cheers.



Quote:
Originally Posted by importfan21 View Post
You're on the wrong board. Try the normal w202 board, as this is the w202 AMG board. Good luck.
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