To recap... this is a 1997 E420 (210.072; M119), produced 11/97; presently @ 90K miles. Vehicle is accident-free and has had no electrical system modifications. The original complaint was elevated engine cooling temperatures, esp. with the A/C engaged. The elevated coolant temp was found to be the result of an inoperative aux. fan.


Presently, it is known that:

1. The vehicle IS equipped with pulse module N65 (210 545 03 32), which is housed in the engine compartment fuse box. This module encompasses traction systems, HCS, ATA and AAC.

2. There is NO frame rail mounted engine/climate control electric cooling fan control module on this vehicle. There is no signal pick-up and activation module (SAM) located adjacent to the ABS pump (left front).

3. The pulse module supplies current directly to the aux fan motor via a dedicated harness that connects to the module at pin group D. Harness continuity has been verified.

4. The aux fan runs when directly supplied with current.

5. Relevant fuses have been checked, cleaned and continuity confirmed.

7. Aux fan activation criteria (engine coolant temp > 95 deg C or refrigerant pressure > 14 bar) are present and have been confirmed using both an HHT and reading of actual values via A/C pushbutton control module [PBCM; N22].

8. DTC check using an HHT revealed no fault codes.

9. DTC check via the A/C PBCM revealed no fault codes.

10. Aux fan activation via the HHT was unsuccessful.

11. Reading the actual values via the A/C pushbutton control module [N22] revealed the presence of control current for the aux. fan. Control currrent could be followed from 0 to 2 mA up to 10 mA, depending on presence of activation criteria.

12. Inspection of current output at pin group D on the pulse module (group D provides current to the aux fan motor) revealed no current when activation criteria were absent. With activation criteria present, control current could be seen on the A/C pushbutton control module (e.g., 2 mA) but no voltage was found at pins D1/D2, except for a brief spike of approx 8.6 volts, after which the voltage returns to 0. This cycle repeats every 6 seconds as long as the activation criteria are present.

13. Installation of a new pulse module resulted only in the momentary activation (<1 sec) of the aux fan at low speed. No further fan activation has occurred. Both new and used modules presently provide the same pattern of results.

14. According to WIS, the aux fan motor on this specific model is supplied curent solely by the pulse module, which in turn receives its command signal from the A/C PBCM. Sensors external to the PBCM gather data regarding engine coolant temperature (ECT) and refrigerant pressure.... ECT is passed first to the CFI control module, then, via CAN, to the instrument cluster, and finally, via serial interface K1, to the PBCM. Refigerant pressure is fed directly to the PBCM.


SO....

In light of the above, it would appear that the new pulse module was either defective, failed immediately after installation, or has been damaged by the vehicle. Inspection of the original pulse module's circuit board revealed no gross defects.


Does the pushbutton control module actually make the decision that fan activation criteria have been met and thus sends control current (2 mA - 10 mA) to the pulse module?

Or does the PBCM merely pass on the ECT and refrigerant pressure data and lets the pulse module make the decision to activate the fans?

Given the above listed findings, is it safe to install yet another new pulse module or are additional diagnostics warranted to rule possible damaged by the vehicle?

If additional diagnostics are required, what should be checked?


TIA!