Skip to content

BMW E38 7-series Heater and A/C Blower Motor Inoperative, 740i, 740il, 750il

November 24, 2010

Q
1998
750iL
My heat/AC fan does not work. I unhooked the battery and let sit overnight because I was told it had worked the day before I purchased it so I thought it might be a computer error. Obviously that did not work or I wouldn’t be writing. My next step is the relay which I understand is just behind the radio. Ameridata says the whole dash has to come out to get the radio out but a mechanic told me no, just the radio will come out. How do I do that? It is cold up here!!! The heated seats and steering wheel help but my toes are frozen!!

A
There are various possible reasons for the blower motor not working. These include; fuses, relay, blower final stage (resistor unit), the control panel and the blower motor itself.

In diagnosing why the motor is not working, we would typically just access the motor and test for power on the input wires and/or apply power directly to the motor. However, the motor on the E38 chassis (7-series 94-01) can only be accessed after a somewhat major disassembly of the dashboard. Therefore, we must attempt to do some initial diagnosis without actually accessing the motor.

* Fuses – The blower motor is powered by a series of fuses, a relay and the final stage unit (and, of course, the
control panel)
* Fuse # F100 in the High-Amp fuse panel (located in right side of trunk, ahead of battery)
* Fuse # F113 in the passenger side floor panel (under right-front seat)
* Fuse # F81 in the engine compartment E-Box (right-rear of engine compartment)
* Relay # K4 in relay panel forward of the glovebox
* Final Stage Unit is mounted to the motor itself

One of the easier diagnostic steps would be to check for power out of the blower relay #K4. Access the relay (via the details in the Bentley repair manual) and look for the yellow/green wire that comes from terminal #87 in the relay socket. When you have the system set so that the blower motor should be running, there should be 12-volts between this wire and ground. If there is, all of the fuses and the relay are OK. The fault is likely at the blower motor or the final stage unit. If there is no power, check for 12-volts in the relay socket (relay removed) at terminal #30, with the ignition on (blower controls do not need to be turned on). If there is power, check for 12-volts at terminal # 86 (with relay removed) with the blower controls turned on (and ignition on). If there is power, replace the relay. If there is no power, you have a problem with the output from the control panel. If there was no power at terminal #30, check fuse #81.

If things are still not clear after these tests, let us know what the test results were and we can continue from there. Note that all of this information comes from the Bentley repair manual for the E38 chassis, from the heat & A/C sections and the wiring diagrams. The manual would assist in developing a proper diagnostic track as well as detail item locations and the access and repair procedures.

Bentley repair manuals:
BMW

Blower Motors:
BMW

Blower resistors and final stage units:
BMW

Blower relay:
BMW

Tags → , , , ,

Categories → 7 Series 95-01 - E38

Comments are closed.