Heater Fan Motor Transistor - More

[Uncle Tom Cobley]

A couple of weeks ago my heater fan motor developed what is apparently not an uncommon fault, it ran continuously at full power at all positions of the switch, even when it should have been turned off. This was very uncomfortable when first driving the car with the fan motor running at maximum speed blowing fresh, but very cold air.

I was able to find the likely fault quickly and easily from the various postings on this forum, it was the heater blower motor transistor, Volvo part no. 30864189.

Following the guidance on this sight I was able to remove the unit quite easily. The metal bar which supports the glove box and which restricts the ability to remove the motor transistor unit is a soft material and I was easily able to bend part of the lip on the bar with a pair of pliers. Then by slightly ‘springing’ the bar the unit came out. Although not absolutely essential the job was made much easier by removing the glove box, just two screws. Interestingly, for several days I used the car with the part removed and found the heater passed virtually no air into the car.

Removing the top screw which retains the motor transistor unit was also simple using what is described as a ‘right angle offset ratchet screwdriver’, essentially a very small ¼” socket ratchet which accepts screwdriver bits, a very handy little tool for inaccessible screws. I can see one now on a very popular auction site for less than £3 (at this price I wouldn’t speak to the quality but I’m sure quite suitable for occasional use).

The cost of these Volvo transistor units is now approx. £45 and as suggested on this forum I thought I’d try to simply replace the actual power transistor on what is for the most part a very large heat sink. As others have done I went to Maplins and purchased what is described as a Bipolar NPN Power Transistor, the Maplin's part no. is N03AJ and the cost was £5.39; if you don’t have any you will also need a small tube of heat transfer grease (Maplin's part no. A29GF, £1.99). This Maplin's Power Transistor had MJ802 marked on it, exactly the same number as the part on the Volvo heater blower motor transistor.

I found replacing this power transistor was the most difficult and time-consuming part. After cutting the crimped wire connectors, unscrewing and replacing the power transistor I found the wires were not long enough to be able to solder to the new power transistor and so had to solder in a short connecting wire. The wires then needed to be insulated (I used heat shrink tubing). All this was very slow and ‘tricky’, not only because access to the solder joints was very restricted due to the fins on the heat sink but also in the restricted area the heat from the solder gun shrank the end of the heat shrink tubing. I did complete the job but it was time consuming and is not the neatest, and although this part is not visible when fitted it still left me less than fully satisfied with my work. I would recommend any others attempting this job to try to cut the power transistor wires as far back as possible but as I have already noted, this is not easy because of the very restricted space.

The reward for my work came when I refitted the unit (the offset ratchet screwdriver was particularly useful here) and found that it was now working as it should, with all the numerous fan speeds and it would switch off. I had the further satisfaction of having saved some £40 (or much more if I’d taken it to a garage to have it fixed).

[stephend]

Great write-up! I did the same job on mine a year or so ago, and your description is spot-on.

[adamamam]

Fab post!

I just did this today; took about 30 minutes including toilet break.

Money saved, job done!

Thanks!!



***Just a tip; don't cut off the crimp connectors; grip them (in turn) with pliers and PULL off the transistor. The space allowed by the lack of the transistor wires/tails will now allow for the crimp connector to come off, leaving you with enough wire to play with when reconnecting.***

[DXMachina]

Just always remember to use an NPN Power transistor of equivalent power. MJ802 is the original type used in the genuine Volvo module and is still available.

2N3055 transistors (more widely available) look identical and have the same pin arrangement but are only rated for half the power, and will burn out