Am I correct in thinking that if the switches illuminate then it's the elements which are at fault, rather then the relay? Si - Hoping I'm wrong!
The problem lies in the thermostats, the elements, the inter element connections, or the earthing (grounding) which is by the parking brake. The relay is for both seat heating and power windows. The seat heating fuse is 23 (1991). If you have dual output heaters for thermostats read overheating safety cut-outs (a different beast to a thermostat) which serve the same purpose. All the best, Peter. 700/900/90 Register Keeper, Volvo Owners Club (UK).
Thanks Peter. I've discovered that my 740 has a relay under the driver's seat, I didn't check the passenger's but I assume that's got one too. I understand the seats *and* the backrests have heating elements but none of the four are working. Is it likely that all four have failed? Si
The "relays" are the overheat cut-outs (thermal switches). The power feed is on the White/Black wire, the Red wire is the heater side of the cut-out and the Black wire is the earthy side of the heater pads. The red wire feeds the seat pads, one half being directly earthed, the other half is earthed via the back pad. The relay (left on first relay row) feeds power to the switches and the power windows. The ignition feed for the relay is routed via fuse 9, fuse 8 feeds power windows and fuse 23 feeds the seat heaters. Your circuitry is of the Dual output type. It is more likely that the common earth has become detached from it's bolt or the bolt has become loose, but if this were the case the switch lamps would not light up. It is possible that the two earth wires from the heaters have become detached (probably by the two 2-pin connectors under each seat). This fault would leave the switch lamps operable. All the best, Peter. 700/900/90 Register Keeper, Volvo Owners Club (UK).
The seat and backrest elements are wired in series, mine both have open circuit in the seat element, unfortunately it's not an easily replaceable component as the 240 seat heaters are, it's molded into the foam. I've been thinking about finding a set of elements from ratty Saab 900 seats as the seat heaters in my 900 are very toasty, better than the Volvo ones.
Sorry James, but there are Volvos with dual output heaters where the back pad is paralleled with one half of the seat pad. This necessitates the "relay" (overload cut-out) rather than the thermostats. All the best, Peter. 700/900/90 Register Keeper,
It seems that the OP has the dual output type as he refers to a "relay" under each seat. Glad to see that you are still very much "on the ball" James. You have given some pretty good answers recently. All the best, Peter. 700/900/90 Register Keeper, Volvo Owners Club (UK).
The "relay" is really a timer/thermostat and comes in two flavors, one for cloth seats and one for leather. Bob
That sounds correct. I found a broken black (earth?) wire under the passenger seat and repaired it but no joy. I did hear two relays clicking on and off, one a couple of seconds after the other, about 20 seconds between each pair of clicks, while I was waiting to see if the seats would heat up but I couldn't tell where the clicks were coming from. I'm going to take the seats out and have a proper look soon. Si
