I have recently had a problem with our 1991 180E. We have owned this car for 8 years and it has 276,600 kms on the clock. And although it has been the most reliable vehicle we have ever owned, it started to stall intermittently about 3 months ago . It was sometimes dangerous as it would silently just die as I slowed down to take a corner or stop at a light or roundabout. It seemed to be worse on a slow ramping down stop. However, I have read many forums including this one and have put the advice into practice and fixed the fault to my sheer relief and amazement (because my wife wanted me to take it to the stealership) . These forums are worth there weight in gold. After extensive testing and checking, (which I will go through later) it turned out to be the HALL EFFECT SENSOR, that's right that small little sensor at the back of the speedo instrument cluster. Picked up one from E-BAY from Germany for $30 and no more stalling!!!!
I had checked many components,
OVP relay - checked and no problems with bad solder joints,
air mass potentionmeter - replaced and adjusted to give 0.7 volts at pins 2 and 3,
ICV -cleaned with injector cleaner and applied 12 volts and snapped open OK,
throttle switch - checked with multimeter at both open and closed throttle and OK WOT/ Closed throttle switch checked OK,
Idle air hoses all replaced,
changed distributor cap and rotor, spark plug and leads.
I tested with a multimeter the resistance readings of the crank sensor L5 within specs at 950 ohms,
the engine coolant temperature sensor there are 2 both tested at 450 ohms cold.
I even checked the R 16 reference resistor which on our model is bridged - 0 ohms. I disconnected this to see what happened and the engine pinged like crazy. I finally found a post here, this is another fantastic forum about checking the diagnostic plug with a DVM and checking the DUTY CYCLE. This is LAMBDA mixture control from the ECU. When I did this I found out that my engine was not fluctuating like it should (slightly rich to slightly lean). It should read about 7.0 volts fluctuating to give around 50% duty cycle. This is done at pins 2+3 ( 2 is ground and 3 is duty cycle as voltage varying) on the small round plug near the Ignition control module on the passenger side wheel arch.
However my reading was a constant 5.5 volts not fluctuating - This non fluctuating is actually the fault the ECU gives out as a constant voltage. These faults range from 0% to 100%. When I used that voltage of 5.5 I found that my fault was in fact 60%. the equation is Duty Cycle = 1-(Vpin 3 / V Max) x 100. (V Max is battery voltage)
BINGO !!! this in the servicing manual states 60% is loss of the road speed signal in cars from 1988 onwards.
The road speed signal comes from the Hall Effect Sensor. Swapped this out and no more stalling, the revs at slowing down and stopping now don't fluctuate below 2 on my oil pressure gauge. The 180E has no tacho, but the oil pressure is directly related.
Now my wife will infact drive the merc again!!! Happy days