Does anyone know if seat heaters go through a relay? I push the switch, it lights up but no heat. Thanks

 

they do get the power from the accessory relay, but if that relay is dead, you wouldn't get any lights on the heater switches... the heaters have thermostats in the base of the seats, they only come on if its cold enough.

using a 1992 740/940, at the front of the outside rail of each seat, there's a 2 pin connector, white-black pin 1 is seat heater power, and black pin 2 is ground.

assuming you have power at that connector, you'll need to unbolt the seat and flip it over on a tarp or pad to sort out the wiring. odds are the thermostat thing is fried.

seat connector pin 1 (white-black) goes to thermostat pin 4, and thermostat pin 3 is red to the first bank of heater elements (the bottom and seat-back heaters are partly in series, and partly in parallel). you could jumper thermostat pin 3 to 4, and see if they work. if so you need a new thermostat.

 

Great! Thanks, I'll check that out.

 

Ya know, now that I think about it, every time I tried to turn them on it was a warm day. So maybe they do work, it's just not cold enough to activate them. I wonder what the thermostat limit is?

 

Pierce I know this is an old thread but I'm trying to catch up on old projects. My 940 has a relay, no thermostat(external). I've got power to the connector to the relay and the relay clicks when power is applied. I don't know how to check the other components of the relay but I want to try your suggestion and jump power to the heating element. Is 12v straight from the battery too much? And how long before I fry it, if it works? Strangely, both seats don't work and the passenger relay clicks but driver's doesn't. Are they individually wired? Thanks

 

afaik, the only relay in the seat heater circuit is the accessory relay, which powers all sorts of miscellaneous things, there's no separate relay on each side, so I'm not sure what you're referring to.

the thermostat is a little round thing attached to the springs under the lower seat cushion, and pressed up against the bottom of the seat, you can only see it if you remove the seat from the car entirely (or, ok, maybe if you raise the seat as high as it can go, and get under there with a flashlight and inspection mirror).

12V from a battery to the heater plug by the seat rails should be fine, thats what it gets.

 

According to Brickborad cloth has thermostats and leather has relays. Here's a couple of photos of what I have. The Blk n Wht power lead from the seat switch goes to a connecter where the relay attaches and out of that go a big red wire(power) and small blk(ground) to the heater grid. Strangely, I have power to the relay, on and off, controlled by the switch, and I have continuity on the grid(big red to small blk). With continuity I should have heat when I jump 12v to it. If it works then the relay must be shot even though it clicks. Don't know yet about the driver's side.

 

Well, now I'm really baffled. I have continuity between the heater plug grnd(small black wire) and the chassis. I also have continuity between the heater plug grnd and power(big red wire) so it's a complete circuit. I unplugged it and jumped 12v to big red straight from the battery(with a test light, which lit up). And no heat? If it's a complete circuit why can't it heat up? Any ideas?

 

btw, I forgot about those relay modules under the seats... I was thinking relays in the main panel etc. That 'relay' is in fact labeled 'overheating protector', and is a thermostat+relay..

the seats get theirr power from Fuse 23, which is a 25A (!) fuse, suggesting they probably use like 10 amps each.

on that thermostat/relay, pin 1 is black/ground, pin 4 is white-black power-from-car, and pin 3 is red, power-to-heaters. red goes to a 2-pin connector "D"(left) or "E" (right seat), and from there it heads to the seat bottom. black/ground goes several places. the seat back heater is plugged into another 2-pin connector "L" (left) or "I" (right).

anyways, if you find that connector D/E under the seat with the black wires on pin 2, and red wires on pin 1, you can jump power directly to it, and the heaters should get hot regardless of the over-temp protector circuit. also at that connector D/E, the 'seat' side should have relatively low ohms resistance when its unplugged from the thermostat/relay. Since the seats can draw like 10 amps each, I'd guess around 1 ohm resistance, which isn't far from a dead short.

there's a total of three actual heat elements. the seat bottom has two, the backrest has one. the backrest is in SERIES with one of the bottoms, go figger, then that combination is in parallel with the other one of the bottoms.

 

Sounds like the heater elements in the seat are broken--usually this is the cause for no heat.

 

Typical seat heater failure is described nicely here. It's written for the 240 but I'm guessing that the 940s aren't a whole lot different.

http://cleanflametrap.com/seatHeater.htm

 

Thanks, good info.

 

Can't be, I have continuity from one end of the grid to the other.

 

did you measure that at the connectors to the actual grids, or at the connector to the seat that goes to the 'relay' overtemp thing ? I'm asking because that overtemp thing probably has some resistance between ground and power, which it needs to power its internal stuff like the relay coil... but this would probably be a few 100 ohms whereas the heater elements are like 1-2 ohms which is hard to measure and looks like a dead short to most meters.

 

Sorry it took a while but I wanted to test everything right. I now have both heaters working. They're warm but not really hot. Probably the "over heat" protection. The relays we're ok(although one day I had to put them in the freezer to get the thermostat to kick in) and the heating grid was ok. I did test continuity at each pad - top and bottom separately. And I did jump power and they both created heat. On the passenger side all I can guess is that there wasn't a solid connection at the connecting plugs? On the driver's side it was a different story. 2 wires had been yanked out of the relay connector. How, I don't know. Maybe the power moves of the seat had tangled the wires and pulled them out. Anyways, did the same tests, hooked it back up and it works too. A side benefit was my memory function worked intermittently, now it works all the time. Also, even though the directional switches worked manually the "backwards" switch for the bottom would only move 1 notch at a time. Now it keeps moving until I release it. Don't know what fixed that? Anyways, I'm happy to have the seats working. Thanks again for your help.

 

ah, cool that its all good now.

I've never bothered to fix the seat heaters on my 740, 'cold' is 45F around here, I can tolerate it. Actually, on the drivers side, I 'stole' the seat heater power to run the 940 power seat I replaced my original seats with (740 wasn't wired for power seats)...

 

I live in roughly the same neighborhood and 45f is a little too chilly for me. I guess I'm thin skinned. What's an added bonus is I forgot what great heaters Volvos have. Very toasty. I was wondering about eventually pulling my seats to re-skin them. I heard that when you reinstall them the memory has to be reprogrammed so it knows what position it's in. Did you have to do that?

 

my power seat (drivers only) doesn't even have working memory

 

In a way that's good. Less to remember. Ha!

 

Forgot to ask: Any cure for creaky, groaning seats? Will silicon spray short anything out?