Re: 740 heated seats
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
All the best, Peter.
700/900/90 Register Keeper,
Volvo Owners Club (UK).
"Mungo "Two Sheds" Toadfoot" <email@example.com> wrote in message
> "Peter K L Milnes" <Peter.K.L.Milnes@btinternet.com> wrote in message
>> 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.
> 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?