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Here is the posting I was refering to. The 7xx info starts in the second paragraph. This info, along with the FAQ, really helped me w/my 89 745t seats, but unfortunately, I need new elements! And at $60 x 4, I won't be getting them soon! Dan
Seat Heater relay location & testing 900
posted by Dave Stevens on Fri Apr 16 02:31 UTC 2004
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See the 700/900 FAQ. Here's some additional info culled from my own notes that may help you.
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Volvo 140/240 and 700/900 Seat Heater Problems
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The heater grids used in the 140/240's cloth, vinyl and leather seats were a heater grid wire held in a plastic frame behind a thin layer of foam. They are controlled by the heater switch and a button thermostat in the lower seat cushion plastic frame. Some early seat heaters were non-switched, always going on at the set temperature -there was a weight sensing switch under the passenger seat between the springs (in addition to the seat belt warning weight switch). There is no relay with any of these seat heaters.
The heater grids used in the 700/900's employ a heater wire woven into an elastic fabric pad. Non-leather seats and early leather seats are controlled by the heater switch and a thermostat in the lower seat heater grid. The thermostats turn on at 14degC/57degF and off at 26degC/79degF. Although there is a master relay ahead of these, they are generally referred to as non-relay controlled seats. The newer leather seat heaters are controlled by the heater switch and a special relay control module, one clipped under each front seat, marked "LEATHER" on the case. These modules initially sense air temperature under the seat to determine heating is to be enabled and, if so, the initial heating time and a warmth maintenance duty cycle. Testing indicates that warmth maintenance continues indefinitely, unaffected by temperature, so you should remember to switch them off when you no longer need warmth. Switching them off for a minute or so then back on will start the full heating cycle again. There are earlier style relay modules which are black, supplied by two fused circuits -one from the heater switch and one directly from the fuse panel. There are newer style relay modules which are green, supplied by a single circuit through the heater switch alone. The relay modules are functionally identical, but not interchangeable. The newer style relay modules are used with "dual output" heater grids where the lower seat cushion grid has two heating coils to quickly toast your buns. The non-relayed and early relay module controlled seats have "normal output" seat cushion grids.
The backrest and lower seat grids are wired in series so a break in one affects the other (with a possible exception for the dual heated seats). Opening the grid connector(s) under the seat and performing a basic continuity check can isolate problems with the heater grids. Remember that for non-relay controlled heater grids the temperature must be low enough to trip the thermostat for testing (hint: check the lower seat cushion after putting it in a freezer for 15 minutes). Replacement heater grids are available and are not that difficult to install.
To check the heater grids in leather seats with relay modules:
o Unplug the relay connector (make sure the other connectors are snug).
o For the older system with the black relay there should be four wires going into the connector: white/black, red/black, black, red.
o For the newer system with the green relay there should be three wires: white/black, black, red.
o Check for continuity between the red and black pins -zero ohms means a defective grid.
There are many failure modes for heated seats:
a) No power. Always start by checking the fuse (or fuses). Other items (such as power windows) may be on the same circuit. If one seat heater works it is not fuses. If the light in the heater switch turns on then power is likely getting to the heater grid or relay (except in the case of the early relay module normal output seats where the relay gets heater grid power from a separately fused curcuit).
b) Seat heater grid is damaged -either the backrest or lower seat grid. Heater wire breaks at the thermostat or the connector on the edge of the fabric grids can often be repaired. Broken wires elsewhere are more difficult to repair and repairs may not last -it's safest to replace these. Burnt grids that cannot properly support heater wiring must be replaced.
c) Seat too hot or has hot spots. Wires in the grid that are shorted to nearby wires will produce hot spots and/or extra hot seats -this can be dangerous. Also, in the older style plastic grids, if the thermostat becomes dislodged from its socket it will not properly sense seat temperature.
d) The leather seat heater relay module is dead. Try switching in the relay module from another working seat. Otherwise, put your hand on the relay then turn on the heater switch. If you feel/hear an initial click then the thermostat is alive and likely functioning. During cool weather there will be a further click after 5-10 minutes at the end of the initial heating cycle then clicks every minute or so during the warmth maintenance cycle. If the clicks indicate something like a heating cycle of about a minute or less plus a warmth cycle of 2-5 seconds every 30-60 seconds then this may be caused by a no-load situation from a faulty grid. For a dead relay, opening the relay and re-flowing the solder at the major connections and any overheated/burnt areas may fix it.
e) Supply wiring is damaged, a connector has been pulled open or a wire has been partially pulled out of a connector. Visual inspection and diagnosis with a multimeter will isolate these. If a problem is indicated then the seat heater switch connector and wiring in the tunnel console should be checked.
f) For leather seats, the initial heat cycle timing in the relay module can degrade so that the seats do not become hot enough. This is caused by a degraded response by the internal thermostat -actually it's a thermistor (thermal resistor). A relay module modification has been developed that can alleviate this.
Failures in the early plastic seat heaters are often broken wires at the thermostat button. Failures in the fabric grids are often broken wires and shorts due to the grid getting poked or folded and broken wires at the plastic edge connector. Failures of leather seat heaters with relay modules are often broken grid wires, but can also be relay problems due to extensive usage and age.
--
Dave -not to be confused with a real expert
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