So my seat heaters in my shiny new 940 work...sort of. the initial burst of warming is real but the cycling from the relay/control unit is spotty at best.

After an exhaustive search it appears that the control unit/relay is the culprit. A hint to fix or a hack is made but I never found the fix. Not willing to shell out for a new relay any time soon so I will try the passenger to driver swap f no one has a hint.

Please keep in mind that these are the heaters that use the actual relay under the seat and not the thermistor (sic?) button in the seat.

"Please keep in mind that these are the heaters that use the actual relay under the seat and not the thermistor (sic?) button in the seat."

Looking at the '93 Greenbook diagrams (and component locations), the only thing shown under the seats is a relay-looking item called an "Overheating Protector". It appears to have an internal heat-sensing element that actuates an integral on/off voltage switch.

The actual relay that supplies voltage to the console switches is in the relay rack (front-left as you face the relays). If that relay fails there will be no heat at all on either side.

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Yes but the "Overheating Protector" is what cycles the voltage according to all the research done here. The wires to not reach X temp and then throttle back. they reach X temp and are then "pulsed" (with a really long pulse width) to maintain the temp.

When they go bad, they do what mine is doing and initially warm up then fail to do so until the switch is seat heater switch cycled off for a period of time and then turned back on.

Naturally this is all from searching the archives but it a) makes sense (varistor would be large and relatively unreliable for the wattage involved) and b) matches my symptoms.

Yeah swapping passenger to driver might work, in my case neither worked. Tried re-soldering the thing with no success, I may have another issue, but since daughter mostly drives the car, decided she can "tough it" out without the bun warmer....