91/318i/ seats don't heat up

1991 318i convertible in Canada. My seats were heating in spring, don't work anymore!
i have a manual and the fuse section has no mention of fuses for my heated seats (strange)Can anyone give me a tip on where to start my troubleshooting? It can.t be my switches cause neither of the two front seats heat up low or high.

Originally Posted by stewcats

1991 318i convertible in Canada. My seats were heating in spring, don't work anymore!
i have a manual and the fuse section has no mention of fuses for my heated seats (strange)Can anyone give me a tip on where to start my troubleshooting? It can.t be my switches cause neither of the two front seats heat up low or high.

Try fuse 16 (15 A)

These seats get their power from the Unloader Relay K5.

This relay should be energized at all times except while the starter motor is engaged and turning.

Check that relay K5 is energised....you can remove it and try shorting out the normally open contact and see if you seats heat up.

K5 and K7 are the unloader relays which drop power to certain things such as heated seats, power seats, power windows, power rear-view mirrors, air-con clutch etc. so as provide maximum power to the starter when starting up.

The starter solenoid should have a Black/Yellow wire going onto it....this is for starting...do not touch!

The solenoid should also have a Black/Green wire going to it (could be White/Brown)....this is the wire that goes to the bottom side of the K5/K7 relay coils. The top side of these K5/K7 relay coils goes direct to +12 volts when ignition is ON.

The Black/Green wire goes onto the rotor of the starter motor....it effectively joins inside the solenoid to the large lug on the solenoid that feeds power into the starter motor via a flat flexible cable.

When the starter is at rest....the Black/Green wire grounds the bottom side of the K5/K7 relays by going through the rotor winding to ground....so the relays energise.

When you start the motor...the solenoid closes a large contact inside itself which supplies +12 volts to the starter motor so at this moment the Black/Green wire also has +12 volts on it.

The K5/K7 relays now have +12 volts on both sides of their coils so they drop out....dropping power to the power-drawing items listed above.

So you need to see if you K5/K7 relays are functioning correctly.

Or you could just replace the fuse 16 I mentioned in which case this theory of operation lesson was all in vain.

thanks Billwill but it seems that your theory of operation lesson was not all in vain after all. Fuse 16 seems to be in good health, so ill be looking into the K5/7 relays, and will be tracing the system from the power source thru the switches to the source if required (hope not)

wouldnt you be able to ohm out the heater coils to see if there isnt a break in the wire somewhere?

Originally Posted by downhiller

wouldnt you be able to ohm out the heater coils to see if there isnt a break in the wire somewhere?

Yes....this would be one of the first things I would do but the fact that both seats are not heating makes it unlikely that both elements are burnt out.

Best to look for the voltage right near the source...which is the fuse and the unloader relay normally open contact and then trace through to the switches and then onward toward the heating elements themselves.

Not rocket science but a bit time consuming and if you do not know the theory of how the unloader relays work.......you could be scratching your head quite a bit.