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?

__________________
68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

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.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

drool.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

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.

__________________
68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

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.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

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.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

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