Hey all;

It just started getting cold(ish) here in the South! I flicked on the driver's seat heater ('90 560 SEL); the two red lights on the switch came on, but I didn't notice any heat from the seat whatsoever. The other seat heaters work.

I looked at the connections in the front underside of the seat, and all appear to be in good order; the seat adjusts in all directions without hesitation. There are no blown fuses.

Can anyone point me in the right direction to any other areas/things I can check to make the seat heat up?

Thanks in advance.

__________________
1990 MB 560 SEL
1985 MB 300D (r.i.p)
1999 Audi A4

Assuming that the seat heaters where working properly before, first verify that the fuses are in good working order rather than just “not blown”, remove the fuse verify it's intact and has clean connections at both ends; then if neither the seat bottom element nor the seat back element is functioning that would suggest that the seat heater control relay is probably the problem as it controls both heater elements. If it was only one element not functioning that would eliminate the relay as the potential problem.


But the first thing to try is to swap the switches around that way you can be certain it is not a faulty switch that is causing the problem, even though the LED illuminates the switch contacts could be dirty or malfunctioning in some way. Make sure the passenger's seat heater works, then swap the switches to verify that the driver's switch operates the “known good”passenger's seat heater and that the “known good” passenger switch doesn't effect the non-functioning problem driver's seat heater system.


I'm not certain between the 124 chassis and the 126 chassis the exact component locations but basically each heater element has two wires that connect to the control relay via a two pole electrical connector that is located under the seat usually secured with the black Benz tie wraps to the seat springs and the seat frame. You can get to them most easily by moving the seat forward and raising the seat and tilting it upwards in the rear, then you can get a better look underneath. Find the heater element connectors, unplug them, test each element for continuity to establish that the element's resistance wire is not broken and creating and open circuit situation. If you have an open circuit, the heater element would need replacement, it can be repaired but it is difficult to solder nichrome wire with conventional solder and flux.. If the heater elements exhibit continuity, then the control relay is most likely the problem.


You can test the control relay output by testing for voltage at the other side of the mating plug that the heater element connects to, either a voltage meter or you could fabricate a visual test device from plug that would mate, a couple lengths of wire and a 12 volt bulb connected between them. Plug in that test device in place of the heater element connector and position the bulb where you can see it, activate the heater switch and observe the bulb, if the bulb lights up the control relay is at least providing voltage to the elements, test both control relay heater element connectors.


The seat heater switches control the amount and duration of the voltage the control relay provides to the elements. I'm not sure of the values off the top of my head but there should be some mention in the Owner's manual and a more detailed description in the Body & Chassis and Electrical Troubleshooting Manual.


If the control relay is not providing voltage to illuminate the test device bulb or is not providing it long enough the control relay would need to be replaced or repaired. The control relay for that late generation 126 is located under the front seat, under the carpet, clipped to the steel cross support that the transverses the floor pan where the seat front bots to. I order to access it is probably easiest to remove the four 10 mm bolts that secure the seat and the single 10 mm bolt that secures the seat belt rail, slide the rail forward, unplug the seat frame electrical connectors at the front underside of the seat where they plug into the seat frame memory control module. This will allow you to lift the front of the seat and tilt it back far enough to rest against the rear seat allowing you to get to the underside of the seat to disconnect and unsecure the wires and connectors to the heater elements, and with that done you can feel around just behind the transverse support for the slit openings (some 124 cars have the seat heater element connections in this location) in the carpet that allow you to get to the seat heater control relays. Some earlier 126 cars had the control relay under the the front carpet sections, so you should probably check there for your 126 before you start yanking the seat out!

wow! awesome and detailed post; I appreciate you taking the time to post this. I'll work this out this weekend and report back with the results.

__________________
1990 MB 560 SEL
1985 MB 300D (r.i.p)
1999 Audi A4