1990 240 wagon rear defogger sticks on

edelmatt · Post by **edelmatt** » 09 Jun 2014, 17:17

When I first press the brake pedal after starting, the lamp out indicator comes on, and stays on, flickering. It also turns on the rear defogger switch lamp (and also presumably the defogger element). I'm not sure how to start debugging this. I think whatever the electrical issue is, it is causing the battery to drain when the car is off. I've taken to disconnecting the (-) battery connector when I'm not using it.

Any ideas? Much appreciated!

Post by **jimmy57** » 09 Jun 2014, 20:59

that sounds like a ground problem. Has any part of the dash been apart lately?

edelmatt · Post by **edelmatt** » 09 Jun 2014, 22:23

No, but the dash has a lot of electric issues. I've had the headlamps cut out intermittently, though that hasn't happened in a long time. I've had another issue where I've had to jiggle the ignition tumbler to get the ABS light to turn off. That also hasn't recurred in a long while. I'm off from work this month and I've contemplated having the dash apart. Not sure if there might be a faulty switch, or more than one, or some bare wires grounding out in there.

The top rear hatch wire harnesses were also both compromised and I've replaced them both. Though there are other wires running to the hatch that are not in either harness. I have theorized that perhaps one of these other wires may have frayed and is grounding out.

I was hoping the 'pressing the brake pedal' trigger might sound familiar to someone.

Thanks for the reply.

93Regina · Post by **93Regina** » 10 Jun 2014, 06:47

edelmatt wrote:dash has a lot of electric issues.

Save and look at this wiring diagram.

I doubt if brake pedal switch and rear window defogger have wires in common.

edelmatt · Post by **edelmatt** » 10 Jun 2014, 08:45

Thanks 93regina. The blown up diagram is helpful. I've got the Bentley manual but this portion is tiny. If I could find that fuse box (from what I've read it's in the rear engine compartment) I'd remove the defrost fuse and see if this works around the problem for now.

EDIT: OK I see that the fuse in position 11 (per Bentley) controls the rear window defroster, but also apparently the transmission overdrive. So it seems best to not remove that.

I just took the car for a spin, and while the bulb out indicator only comes on now when I press the brake pedal (as expected), the defroster still comes on. I'm not sure if it came on when I pressed the brake pedal as I was looking at the road. I'll replace the broken bulb(s) and try again.

edelmatt · Post by **edelmatt** » 10 Jun 2014, 17:42

After further observation, I have the following theory:
the ground path from the tailgate to the main chassis is broken

When I press the brake pedal, the ground current from the center brake light travels THROUGH the defroster element, and that current is enough to trigger the defroster relay, turning on the defroster.

Once the defroster is on, the ground current from the defroster flows through the center brake light through the brake light circuit. Since the right brake light is out, the lamp out indicator stays lit, but barely. As the amount of voltage going across the brake lamps is much less due to the drop across the defroster.

I haven't probed around with a voltmeter, due to lack of an assistant. I'm lucky to have any time at all in the garage, what with two small kids. Anyway, does this seem plausible?

93Regina · Post by **93Regina** » 10 Jun 2014, 20:52

edelmatt wrote:brake

Remove fuse seven, and see if brake lights work

edelmatt · Post by **edelmatt** » 11 Jun 2014, 08:20

I made a direct connection between the tailgate ground point and the right rear light ground point (on the main chassis). Once I did this, the behavior stopped. When I removed the ground cable, the behavior comes back. I think this verifies my theory. I believe the two black wires in each hinge (which are both severed from use) are meant to carry ground back to the chassis. I'll have to re-wire those when I get the tailgate off.

So, on to the next issue. The right brake light doesn't light. And, if the running lights are on, pressing the brake pedal doesn't turn on the left brake light, but does dip the running light back there. Will search for another thread on this, else start a new thread.

Consider this one SOLVED.

Edit: full disclosure: I posted this question on justanswer.com and got an answer from a Volvo tech that led me in this direction. He suggested the ground path from the brake lights was interrupted and forcing current through the defogger.