Tech Talk - MGF Heater Fan
Heater fan resistor burn out anyone?
Our MGF's heater fan only worked on the two highest of four speeds.
A Google search on the topic suggested that two of the heater resistors had burnt out.
There are a number of sites which suggest various ways of replacing the resistors; but only if your MGF does NOT have air con fitted.
If you do have air con the evaporator box is in the way and the resistors are impossible to access unless the air con is degassed, removed and then recharged afterwards - an expensive exercise.
A chap in Jersey called Willyphixitt has designed and sold a PWM (pulse width modulated) speed controller to avoid having to access the resistors. This just fits in a convenient location and is tapped into the existing wiring. See link here
The schematic diagram is available for those wanting to build their own controller.
I was about to do this when I found a readymade PWM controller on Trademe which offers some advantages over Willyphixitt's design and is cheaper than buying the components for the build it yourself design.
Look for seller 'safensecure' on Trademe.co.nz
The PWM controller is listed as 15 Amp speed controller for DC motors PWM and sells for $16.80.
To make it work on your MGF there is a little surgery to the wiring required
By Richard Whitehead
Fan Speed Control - MGF
Heater fan speed 1 had stopped working on my F. I referred to Richard Whitehead's excellent article describing his DIY repair. Having completed the modification I am adding some additional notes which I hope will be helpful to anybody tackling the job in the future.
Sure enough, the PWM identified by Richard was readily available on Trade Me and the additional circuit items from Jaycar. My completed unit is shown in the photographs. There are 8 connections to be made to the internal circuitry and I identified these as A, B, C, D, E, F, G and J. These correspond to identical lettering that I have added to Richard's circuit diagram - see attached.
On Richard's advice I subsequently drilled several ventilation holes in my PWM box cover.
To insert the unit into the existing electrics it is necessary to gain access to, and remove, the 6-contact plug behind the Fan Speed Selector Knob in the centre console. Remove the radio and the passenger side panel holding the foot well light. The connector can be seen through the slot at the bottom of the radio aperture. With a suitable long screwdriver, press down the nylon locking clip on the top of the connector and push it out.
Viewed from the rear, the removed connector looks like the attached diagram, and I have annotated it with the idents A, B, C, D, E and F.
E and F remain intact but B, C and D should be cut about 75mm from the connector. Cut off A at the connector as this contact will no longer be used. Bare a short section of both E and F to enable additional wires to be soldered in.
A, B, C and D which disappear under the dashboard are now redundant.
I mounted my PWM box as suggested by Richard and connected B, C and D from it to the short wires B, C and D now sticking out of the connector.
A wire soldered into wire F at the 6 way connector joined it to F in the PWM box.
A wire soldered into wire E (common) at the 6 way connector joined it to E in the PWM box.
Moving to the driver's side, identify the two wires going to the heater fan motor. This is just to the left of the clutch pedal and the wires are grey/black and light green/purple. Cut both about 65mm from the motor or at least where you can get at them to strip the ends of insulation.
Connect the light green/purple wire from the motor to G on the PWM box.
Connect the slate/black wire from the motor to H on the PWM box.
Connect the light green/purple wire from the loom to J on the PWM box.
The slate/black wire from the loom is no longer used.
I soldered all the joints and insulated them with heat shrink sleeving. Crimp connectors can obviously be used but I never seem to find them too successful.
Peter Fleming
18 June 2021
