W124 heated seats
 

The 200TD I just bought is the only 124 I've ever seen in person with factory heated seats so I know nothing about it.

Hopefully someone on here knows about the system.

Both seats will go on low heat and both seats work. Your lower back gets nice and warm.

However neither seat will go on high.

Does anyone know what would cause that?

Never had a properly equipped 124 but I imagine there are separate fuses and relays for high. Try the tech (gasser) forum too since more US gasser than Diesel 124s came with heaters.

Sixto
87 300D

High/low mode selection is accomplished in the seat heater controller.
Same fuse runs the high or low temperature mode.

The relay looks to be set up such that in low mode, the back pad and keister pad are connected in series. In high mode, they are connected in parallel.

If you are getting heat out of both pads then chances are the problem might be in the relay or the switch since if both pads are making heat then they will be OK in high mode too.

When you choose high heat, does the "high" side switch LED light up? The relay tells that LED to light up to confirm the heat mode has been selected.

I wonder if there is an undervoltage protection mode similar to the rear defroster, that cuts out the heater power if the system voltage drops too low. You might be ok on low but the added demand on high might be tripping the relay protection.

On the defroster the LED will blink if this occurs. I suspect the seat heat system is similar.

drool.

The second LED will not light up. The relay is around $220 from Mercedes, I need to locate mine and track down a used one.


There is a yellow one that came in later cars and it seems the early cars like my 87 used a relay that is a 126 part number.

There is an early relay on eBay for $50, I just feel bad shot gunning $50 at it. I may have a spare OVP to try.

Will a 126 relay work? IIRC all US 500- and 560SELs have heated seats so 126 relays are common.

Sixto
87 300D

This is an 87 right?

If so then check the following at the seat relay:
drivers side
pin 2 - high LED
pin 3 - low LED
pin 4 - low switch
pin 5 - high switch

pass side
pin 7 - low LED
pin 9 - high LED
pin 11 - high switch
pin 14 - low switch

LED pins present 12 V to light the LED, check against ground
switch pins - ground the pin to activate the mode

It could be something as simple as a bad switch.

Early 87s had a separate relay for driver and passenger. These were located underneath the carpeting just forward of the front seats.

April 87s and later have one relay located under the back seat left side.

Both my 87 300D and my 94 E320 have heated seats....