The original 3 speed fans on diesel and V6 cars normally stop working because the earthing motor brush (fed by black wire) wears out. This is because it is used solely for both the low and middle speeds as well as for the fast speed.
This modification is to use all 4 motor brushes to equalize the wear on the brushes and make them last a great deal longer.
When all the brushes are used the motor runs on the fast speed. This is done by joining together the Red & Grey motor leads and supplying 12volts to the junction, and joining the Black & Purple motor leads together and connecting them to the earth of the car.
The slow speed is achieved by using a limiting resistor in series with 12 volt supply to the Red/Grey motor leads.
Wiring Changes for Modification.
To stop corrosion I decided to make all the wiring changes inside the control box.
Pull the Black (and small black) and the Purple from the left hand (fast speed) relay. Connect the Black and Purple together leaving the small black still plugged into the circuit board.
Pull the Red lead from the right hand (slow speed) relay. Cut the terminal off and join the Red lead to the Grey lead (which goes to the middle relay medium speed)
Use the terminal which was cut from the Red lead, and solder it to a suitable length of new wire. Plug this terminal back into the slow relay where the Red had been.
Take another length of new wire and connect it to the junction of the Red and Grey wires.
Drill 2 small holes in the lower corners of the control box and push the wires through. These new white wires will go to each end of a new 0.47ohm 100Watt resistor.
To finish off inside the control box wrap some insulation tape around the the Black/Purple connection and tuck it down the side of the box. The fast relay is now redundant.
Insulate the Red/Grey/White with tape as required, and seal the 2 drilled holes on the inside (I used a dab of UHU glue for this).
The slow relay is still for the slow speed. Current goes via the white lead through the resistor and back to the Red/Grey motor leads, thus limiting the current through the motor.
The medium relay is now for the fast speed and applies the full 12volts to the motor.
Resistor Assembly
You need to buy a 0.47ohm 100watt resistor ( the gold one) You can get these from the traders on the forum or on Ebay. (or a green one from Hong Kong)
Obtain a small piece of sheet aluminium, cut and shape to the required size and drill holes to match it to the resistor and the preformed holes on the fan housing.
Mount this resistor on the fan housing and solder the two new wires from the control box to either end. These joints need to be sealed with glue lined heat shrink sleeving (you can find this on Ebay)
That completes the modification. If you do not know how to get the fan assembly off the car or don't know how to get the fan blades off etc. look in the “How to” section, somebody will have done it.
The Motor
The object of this modification is to make the 4 wire, 3 speed motor more reliable.
It may be useful therefore to check inside the motor and replace any worn out motor brushes prior to doing the modification. There is a “How To” on the forum showing how this is done.
Good Luck, Pete.
P.S. Thank you "Arctic" for supplying the missing leads and plugs, you may notice their absence in the last picture.
Disclaimer:
You are responsible for any work or modifications carried out on your car and you undertake any such work at your own risk.
The 75 and ZT Owners Club nor the original author of this How-To can be held liable for anything that may happen as a result
of you following this How-To.