Thread: 2007 D3 SE A/C blower fan not adjusting

Originally Posted by hpal

Hi, I've got a D3 (with rear A/C as well) and this morning a funny thing happened on my way to work. A/C all working normally on fan setting 2 or so, pulled up at work and switched the car off and locked it while I went and signed on for work. Unlocked it and got back in and the blower fan for the a/c was on flat out ie max air flow. It seems the dial is either off (no air) or max air at any other setting (1,2,3,4,5 etc all max air flow. So its either off or flat out. Does anyone have any suggestions for me? thankyou.

Sounds like an issue with the blower speed control module. When off it’s power is interrupted by the blower control relay. When it’s on the module exerts pulse width modulation control over the motors negative supply. If the module is toast it’ll either not work or be jammed in flat out.

It’s D258 on the wiring diagram and the relay is in the engine bay junction box.

LAND ROVER LR3 DISCOVERY 3 / LR4 2005-2016 HEVAC BLOWER MOTOR RESISTOR LR031677 | eBay

Something like this could be the issue??

Yeah, that'd be it. It's described as a "resistor", but it's a little more complex. Schematic here :
Blower Motor Resistor.jpg

Found on this thread : How to fix the blower motor resistor module for $5 - Jaguar Forums - Jaguar Enthusiasts Forum

The two common failure modes would be the thermal fuse letting go causing no blower, or the MOSFET going short causing the blower to go flat out (sounds like yours).

If you were to put a multi-meter in diode check mode on the Green and Black/White pins you should get :
Meter Red -> Green Pin & Meter Black -> Black/White Pin : Open Circuit
Meter Red -> Black/White Pin & Meter Black -> Green Pin : ~0.6V forward voltage.

If the MOSFET is shorted you will see 0V for both tests and you'd see ~0 Ohms doing the same tests in resistance mode.

In resistance mode rather than diode check for the tests you should still see a very high (to infinite) resistance for the first test, but the second test will depend very much on what the test voltage is on your meter.

Anyway, it sounds like it's stuffed.

Thanks mate, I've got one coming from ebay, just a cheapie but will fit it and see how i go.

Originally Posted by BradC

.....The two common failure modes would be the thermal fuse letting go causing no blower, or the MOSFET going short causing the blower to go flat out (sounds like yours)........

Hi Brad,

Do you know how a MOSFET behaves if overheating? I suspect my MOSFET is overheating causing the a/c blower to run slow until it gets enough cooling from the a/c and then runs fine.

MOSFETs are highly non-linear. It would be unusual to have one fail in that manner. Usually it's either open or short, but I suppose it's possible.

Here's a bit of light reading on MOSFET thermal instability in a automotive heater fan environment : Semiconductor' '&' 'System' 'Solutions - Infineon Technologies

Re-checking the circuit if I were to suspect anything that was thermal I'd point the finger at the electrolytic capacitor. You could diagnose it by watching the DC voltage on the Gate of the MOSFET. The circuit is a basic PWM to DC converter, but the HVAC unit measures the output voltage to provide correction. No faults logged? Can you monitor the fan setpoint with a GAP ?