Continuous cool breezes

Jan. 1, 2020
So, you get a Mercedes C240 with this very common complaint: the cooling fan never turns off! Let’s say you are a little lazy and look up the problem in a repair database.

So, you get a Mercedes C240 with this very common complaint: the cooling fan never turns off! Let’s say you are a little lazy and look up the problem in a repair database. You might find a bunch of hits for PCMs and another bunch of hits for radiators fans. Now, let’s suppose this customer was at the dealer previously and already had both the radiator fan and PCM replaced in a space of less than a year. What do you do now?

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Well, this is exactly the car we had and we had to test, not guess. First, we made visual observations, noticing the vehicle had a "Coolant Visit Workshop--The Coolant is Too Hot" on the service information display on the cluster. Lucky for us, at this time we had a
known-good 2003 Mercedes C240 outside with a good fan and PCM to get good readings from in order to get good specifications for our problem car.

We diagnosed a bad fan module very quickly. You see, the radiator fan on this Mercedes has its own computer which receives a voltage signal from the PCM which determines how quickly the fan turns. We figured if we snipped the signal wire from the PCM to the radiator fan and the fan remained on, then the problem was internal to the fan’s computer and not the PCM.  Now, if the fan turned off or its speed changed, we would know the PCM is to blame. This is down and dirty diagnostics, but it works. We snipped the signal wire and the fan remained engaged at the same speed.

So, all we need is a fan right? Maybe not! How does a brand new fan from the dealer go bad so quickly? We had to make sure the PCM itself was not contributing to the problem, keeping the fan on too long and overheating its module.  That would require the right specifications and the right scan tool.

First, we repaired the signal wire we snipped. Then, we commanded the fan on 10 percent on both the bad car and the known good car. We measured the voltage readings coming out of the PCM signal wire to the radiator fan module:

Bad 01 C240:

Fan 10 percent 7.2V

Fan 50 percent 8.3V

Fan 90 percent 9.1V

Good 03 C240:

Fan 10 percent 1.8 V

Fan 50 percent 5.6 V

Fan 90 percent 9.2 V

Obviously, the PCM on the bad car was commanding the fan to run at about 75 percent even when it was supposed to be off.

We took a close look at the last piece of the puzzle, the “Coolant Hot” warning on the dashboard. We did some research and found that the PCM sends the instrument cluster information from the ECT (Engine Coolant Temperature) sensor. So, did we have a bad ECT or a bad PCM misinterpreting the ECT? On our scan tool, the ECT sensor’s Parameter Identifier (PID) appeared good.

It was at 35 degrees Celsius when cold and we had it warm up to 50 degrees Celsius with the engine running. Yet, the display told me to “visit the workshop the coolant was hot.” The only sensor on this system that can make the instrument cluster module say that is the ECT and the ECT is good, because the PCM is receiving accurate data.

Our little rule is that when the PCM contradicts itself, it is bad. Being that it has accurate ECT data, but an inaccurate interpretation and an inaccurate output to the radiator fan, we were confident in our diagnosis.  The PCM was bad, too.

But how would a PCM go bad so soon? We checked amperage on the solenoids that were controlled by the PCM, but none of them pulled above 200 mA. So, we networked at the WORLDPAC Discussion Board and a member there informed us that he had a similar case where the PCM went bad two weeks after the fan was replaced. He found out that Mercedes had issued a Technical Service Bulletin (TSB) advising that a wire harness has to be added between the fan and the PCM to prevent the fan from burning out the PCM. The bulletin number is P-B-20.40/33 and the part number for the harness is 209 540 56 35.

We gave our diagnostic results to the customer and advised him to go back to the dealer and ask them to warranty both the recently replaced fan assembly and the PCM. After much begging and pleading he was given both these parts and a new wiring harness, free of charge.  Thanks to the right tools (a capable scan tool and a pair of dykes), a logical process, a little networking with other technicians and proper training, this case ended up in the win column.

Last Chance for the Big Event
Mark your calendars for Sept. 29, which is when Technicians Service Training (TST) will hosts its first West Coast Big Event.

This one-day training event will be at the Ontario Convention Center in Ontario, Calif., from 8 a.m. to 5 p.m. Registration includes all handout materials, food and refreshments. G. “Jerry” Truglia and Dave Hobbs are scheduled to present.

Truglia’s “P0420s—Keeping the Light Off” covers diagnosing catalytic converter efficiency DTCs the smart way by utilizing a comprehensive diagnostic process that includes looking at fuel trim, oxygen sensors, PCM reflashing and much more. 

Hobbs’s “Network Diagnostics—CAN Data” covers everything you need to know to diagnose and repair network and communications problems (those pesky U-codes!) on domestic and foreign vehicles. Hobbs covers both pre-CAN and CAN.

For complete information, contact TST at [email protected] or call (845) 628-6928. Space is limited.

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