I drive a Mercedes 500E, 4matic, 2005 model. It has 120K miles, dealership maintained. I just got it serviced by the dealer. As usual, they reset the service messsages. Ever since it came back from service I find that the electronic system every now and then slips into message mode of ESP malfunction, drive to the workshop, ABS light comes on and the steering wheel seems to lose power assist. The same thing happened 10K miles earlier on the previous service as well when the Auto messaging system was reset after service. The same lights came and went away. It appears to me the individual system like brakes and steering are functioning fine, the car drives fine, but something in the software of the ECU (engine control unit) is malfunctioning. Any help?
Yeah, take it back to your dealer and show them your post. Those kinds of software gremlins can usually only be addressed by the dealer.
You might have a wheel speed sensor problem. The advice to take it back to the dealership for a look-see is a good one. A sensor could have come loose from its connector is all.
I agree. I am in touch with dealer.
A wheel speed sensor problem - would that come on when I am driving??? I have seen the ABS/ESP light come on when I am in idling. Its intermittent. For instance it came on last morning. In the afternoon I started the car - the light went away - I drove over 100 miles at 80 - 100 MPH and it did not come on and I returned home and the light never came on.
The four wheel speed sensors – there is one for each wheel – measure how fast the wheels are rotating as you drive. The computers use that information for the abs and sometimes stability and traction control. How the computer determines there’s a problem and if/when it turns on the warning light(s) varies car to car.
Thanks. Right now the ESP message and ABS lights are off. I have not seen it come on. I am waiting for it to come on before I take it to the dealer. I also noticed when the ESP message and ABS lights came on there was a message “Battery low some conventional functions will be disabled”. Would anyone know if the AUX battery or main battery powers the Mercedes messaging system. The AUX battery was changed recently and main battery was changed 2 years ago and its performing fine.
I also see that the car when stationary and idling is shaking. Is it due to misfire , bad tuning or what? Any reasons for car to shake when stationary and idling.
Most common reason is a misfire. Faulty engine mounts next in line probably.
dealer diagnosed it to airmass sensor and replacement costs $781. I said go ahead.
Interestingly they also find that the code for coolant thermostat is showing fault but the engine light is on. The dealer thinks the light will come on soon. I dont wish to spend money on something that is not a problem yet. What do you guys think? Thats another 600.00 for pre-caution. For that matter we can go change a lot of parts on cars. What is your opinion? Leave it alone till light comes on or becomes a problem. Comments?
the OE bosch MAF is $128 at rockauto.
Should have made an appointment with Rock Auto to have that diagnosed and repaired.
the RA bosch part is probably remaned while the dealer part is new. if dealer says the bill is 80%part/20% labor than we can run with that.
What is the Bosch MAF part? Is that the air flow sensor or is it the coolant thermostat
I don’t know how it works on your e500, but as a reference point if I disconnect the coolant temp sensor on my Corolla the CEL turns on immediately, next time I start the car. That sensor has never been faulty on the Corolla in the sense that it doesn’t read the correct temperature, or is intermittent. If either of those happened the CEL might not turn on immediately; the computer might wait to see if the problem is ongoing or not. A scan tool would tell the shop what the coolant temp sensor is saying the coolant temperature is. On a warm & running engine, it should be saying something in the 190-210 deg F range. If it was showing out of that range and an independent temperature measurement showed the coolant really was in that range, then that would need to be addressed immediately. The engine computer just won’t be run the engine well without an accurate coolant temp sensor input.
I think it is sort of unusual to replace a MAF sensor when there’s a known problem with the coolant temp sensor. Those two sensors are inter-related as they both provide inputs to the air/fuel calculation. So if one was bad, it might appear that the other was faulty too, when in fact it wasn’t.
The replace-this-replace-that approach worked pretty well on pre-computerized carbureted cars. 1960-80’s. Make a guess, replace the part. Eventually it would usually work, often on the first educated guess for the experienced car repairer. Since the advent of computerized electronic fuel injection, not so much; it is very easy to run out of money before running out of guesses.
Bosch is a German parts manufacturer. MAF is the acronym for “mass airflow sensor”. The coolant temperature sensor acronym is CTS.
Thanks for this post . What u say makes sense that coolant T fault could go away with MAF replacement. I asked the dealer to check error code after replacing MAF. It was still showing faulty. So I asked him to change it. I agree with you on this change this change that philosophy of repair.
Dealer also tells me that the sensors aren’t related. Is that wrong?
It appears to me its best that I run the car for sometime before I replace the coolant T. I have so far not had any temp issue in the car. No over heating etc. Perhaps after running the car with the New MAF the coolant sensor code will reset.What do you think?
They are independent in the sense they are two different parts located in two different places. But they are related in the sense that both are primary inputs to the computer’s fuel injection formula. The objective of that formula is to set the proper air to fuel ratio so the engine performs properly. If the engine isn’t performing correctly due to a fault in one of those sensors, there’s no way for the computer to know for certain which sensor is wrong and which is right.
As an example of how this could be confusing, there’s a test called a “fuel trim” which tells the mechanic whether or not the air/fuel parameter is set correctly. If it isn’t set correctly, it could be due to a faulty MAF, or to a faulty CTS. It’s easier to test the CTS, so that’s where the mechanic would probably start.
The fault code is usually P0128, it indicates that the engine isn’t reaching operating temperature or is taking too long to reach operating temperature. Erasing the fault code doesn’t make the problem go away, it may take several drives for the computer to recognize the problem again.