740 heated seats

  • Thread starter Thread starter Mungo \Two Sheds\ Toadfoot
  • Start date Start date
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Mungo \Two Sheds\ Toadfoot

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).
 
Peter K L Milnes said:
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.
Click to expand...

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).
 
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
Click to expand...


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,
 
Peter said:
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.
Click to expand...


Yeah I've seen those, I thought we were talking about the standard
single temperature heaters.
 
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).
 
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).
Click to expand...
The "relay" is really a timer/thermostat and comes in two flavors, one
for cloth seats and one for leather.

Bob
 
Peter said:
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.
Click to expand...

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
 
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