Volvo 850 - Mirrors not heating + Chairs

[BeerDeer]

Hi everyone,

Suddenly, my two side mirrors aren’t heating. It’s -10°C, and they’re not warming up at all. The rear windshield heater works fine, and the ice clears, but the side mirrors stay cold, even though they adjust properly. I’ve checked the fuses, and they’re not blown.

Additionally, the seats stopped working. The left seat button doesn’t turn on at all, while the right seat turns on but doesn’t heat up.

Does anyone know what could be causing these issues?

[scot850]

It is strange that the mirrors are working but not the heating element. Also the rear windshield heater is working as it usually works on the same circuit as the heated mirrors. It could be a broken wire in the door harness. You need to look and see if the circuit needs both mirror heater circuits to work. The harnesses like to break where it enters the door or the piece between there and the A-pillar plug.

As to why the seats are not working I'm not sure. The 850 seat heater likes to break the wires where they attach to the heater element under the seat cushion near the middle of the base.

For the passenger set, check it is plugged in and pop the switch out and check it is not just the bulb has blown in the switch.

Neil.

[BeerDeer]

It is strange that the mirrors are working but not the heating element. Also the rear windshield heater is working as it usually works on the same circuit as the heated mirrors. It could be a broken wire in the door harness. You need to look and see if the circuit needs both mirror heater circuits to work. The harnesses like to break where it enters the door or the piece between there and the A-pillar plug.

As to why the seats are not working I'm not sure. The 850 seat heater likes to break the wires where they attach to the heater element under the seat cushion near the middle of the base.

For the passenger set, check it is plugged in and pop the switch out and check it is not just the bulb has blown in the switch.

Neil.

Hi,

The rear windshield heater works, but the side mirrors aren’t heating anymore. They were working before, but now they’re not. The only change I made was upgrading the radio. Is it possible that both door wires could break at the same time?

[scot850]

Unlikely. Just as unlikely as both mirror elements to fail at the same time. There must be something in the wiring or switch that is unique to the door mirror circuit or they both need to work for both to work. You could test the element for continuity on the mirror elements to confirm they are not open circuit.

Then you need the wiring diagram and either check it if you are able or have someone who understands them to look it over for you for possible places they have common wiring. Also worth checking if they maybe operate on a relay with the heated seats possibly?

I am not electrical in my background so I often have to ask for circuit diagram help.

Neil.

[matthew1]

The only change I made was upgrading the radio.

Can you back out the radio upgrade? Revert?

With software and programming background, this would be my initial thought. This is exactly the first thing I would try if this was software, in other words.

If you changed the radio wiring harness I think this would not therefore be the “lowest-hanging fruit,” and would be 2nd, 3rd, or fourth.

[BEJinFbk]

I think that you have several different issues.

The heated seats are on their own circuits
and not a part of the mirror heaters at all.

The mirror heaters and the rear glass heater
both get power from the timed relay 208,
shown as 2/44 in the attached diagram.

It looks like the rear glass is fed from contact 87
of the 208 relay, and the mirror heaters and the
indicator light in the dash switch get their power
from terminal 87B.

When you push the dash button, 3/8, does
the indicator light in that button come one?


.

.

[BEJinFbk]

And here are the heated seated diagrams:
.

.

[BeerDeer]

I checked the mirrors using a multimeter, and it appears that both have failed. They are receiving power but do not warm up.

[scot850]

strange they both failed at the same time. But at least you have a reason for them not working. It is hard when both fail at the same time. I had an issue with both low headlight beams on a friends car going out. Spent a while figuring it out to find both bulbs were failed!

Good Luck!

Neil.

[BeerDeer]

Ah I forgot!

I wanted to share another issue I’ve encountered. Every time I replace the fuse for the driver's (left) seat and start the seat warmer, the fuse blows immediately. This leads me to believe the unit might be damaged or have a short.

On the passenger (right) side, the seat warmer stays on, but it doesn’t generate any heat.

I’d appreciate your advice on diagnosing and resolving these issues.