Thread: ASR malfunction
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Old 04-06-2003, 11:47 AM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I just had an interesting repair of an ASR problem on a 92 500SEL.

It probably doesn't apply here but it will go to the issue of why the engine management system is involved (along with quite a few others).

One of the interesting things about this repair was that the car owner was another shop owner/technicain. Because of this we worked the problem out together. The main symptom was that the ASR light would come on just after the tach dropped to zero.

A scan of systems found a code in the EA/CC/ISC module for no TN (engine speed signal) from the base module. The problem was intermittant so we started by breaking out the speed signal. We did it first at the instrument cluster and found that sure enough the TN was leaving when the tach left. We looked at the crank sensor signal but knew it had to be alright as the engine continued to run. We then looked at the TN signal from the ignition controller to the LH controller. We monitored the signal at the tach and from the ignition (should be the same) on two channels of my scope. When the event occured the signal at the tach left but the one from the ignition to LH continued.

We were then faced with breaking out the connection from the LH to the base module. Tough but we got it. The signal also maintained at this point. Here is where working on another techs car was a benefit as the problem was intermittant and often required monitoring for 20 minutes or more (I set up - he monitored). From the wiring diagram we then knew that the base module had to be the culprit (sort of what the fault code had said). A used base module did the trick.

The point here is that the ASR uses throttle control and timing control (along with brake control) to keep traction in control. All of the controllers are involved. A lack of CAN communications along with numerous other factors are possible faults that can set this off.

One thing that comes to mind is the throttle safety switch. This switch is located on the throttle linkage and is designed to monitor the continuity of contact between the throttle and the gas pedal. One can set this off by moving the throttle at the motor with the engine running. because the gas pedal didn't move the system recognizes the disconnect.

During cuise control, this activity has to happen (the disconnect of throttle to gas pedal). The circuit that is monitored is bridged by the cruise function. This might be a point to check on. (I am not sure how - but my first stop would be the on-board diagnostics).
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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