Rear Window Demister

  #1  
Old 12-29-2018, 04:55 PM
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Default Rear Window Demister

I have an old 1989 Volvo 240 DL. The rear window demister is not working at this time. When I push the button in, the associated fuse quickly blows. I pulled the dash switch and checked for continuity, and it appears to be working properly. I also visibly checked the rear window grid, and could not see anything unusual. Could anyone help me with this issue. Thanks!
 
  #2  
Old 01-01-2019, 04:09 PM
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if the grid was working perhaps removing cleaning replacing the connector may jolt it back to life ie. correct the short you have.
probably a shot in the dark..
 
  #3  
Old 01-02-2019, 04:21 PM
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Default Rear Demister

Originally Posted by silvermine View Post
if the grid was working perhaps removing cleaning replacing the connector may jolt it back to life ie. correct the short you have.
probably a shot in the dark..
I will try that when it warmer around here. I will let you know if it helps!
Thanks
Apex
 

Last edited by alex1366; 01-02-2019 at 10:21 PM. Reason: Spelling
  #4  
Old 01-02-2019, 04:29 PM
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the fuse blows because somewhere between the fuse and the defroster, the hot wires is shorted to ground

the power from fuse 12 goes to the switch via a blue-yellow wire, then to the time delay relay via a yellow-red wire
power from fuse 11 goes to to the timer relay via a blue wire, then on to the left side of the rear window via a yellow wire. this yellow wire goes down to the left floor below the fuse panel, through the tunnel under both doors, up and over the left rear wheel hump, then up to the window.

is this a wagon or sedan? on a wagon, the #1 place for the wire to fail is at the hinge...
 
  #5  
Old 01-02-2019, 10:12 PM
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Default Rear window demoster

This vehicle is a 4 door sedan. Thanks for the information!
 
  #6  
Old 01-02-2019, 10:24 PM
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Default Rear window demister

Does anyone know if this demister has a relay associated with it.
 
  #7  
Old 01-02-2019, 10:43 PM
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there is a timer relay, its behind the driver side dashboard, I believe just left of the steering column.
 
  #8  
Old 01-05-2019, 03:43 PM
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Default Rear Demister

I have done some more investigation on my demister and have observed the following:

The fuse pops even when the switch has been pulled (disconnected) from the associated wiring behind it. - Does that mean the short is located in wiring between the fuse and the switch?
I advised before that the fuse blew only when the button was pushed down - incorrect - It blows as soon as the key starter is turned,
I tried to wiggle and clean the two connections that attach to the grid with no effect.
The light in the switch does not even flicker when the fuse blows.

This is a real problem, as my wife will not use the car, because the rear window is always fogged up in the morning, and she can't see behind her! No one will even attempt to fix this, and an auto electrical shop charges $250 upfront to just look at it!

Thanks to all of you who have given me help thus far.
Perhaps these new details will help with the diagnosis.


Kind Regards,
Alex







 
  #9  
Old 01-05-2019, 05:39 PM
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which fuse is blowing, fuse 11 or fuse 12?

if its fuse 11, that goes to the time delay relay via a blue wire, so I'd be looking at that relay, and the wire from the fuse panel to that relay.

on the 240 fuse panel, to the right of each fuse is 3 spade lug connectors. the spade lug farthest to the right (and closest to the front of the car) is the 'input' to that fuse and goes to the left side fuse clip (the inputs to fuses 11,12,13 are connected together inside the fuse block). the middle and left-most spade lugs are the output of the fuse and connect to the right side fuse clip and go to whatever circuit(s) that fuse powers.

so, I would unplug the time delay relay and see if it still blows a fuse.

Ah, note, fuse 11 *also* goes to the automatic overdrive circuit, unless you have a 5-speed stick shift.
 
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Old 01-05-2019, 05:41 PM
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p.s. the time delay relay is 175 on this PDF, http://www.davebarton.com/pdf/relay_87_240.pdf

the originals are a white relay, with volvo PN 1324675... this is NOT a generic relay, its a timer module.
 

Last edited by pierce; 01-05-2019 at 06:08 PM.
  #11  
Old 01-07-2019, 11:57 AM
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Default Rear Window Demoster

Thanks for your input - I believe I narrowed the problem down to a faulty relay - It is not a typical relay you can purchase at Auto Zone! It goes for over $40 at Volvo - So I purchased a used one from eBay for $15 with S&H! If it does the trick - I will be very happy. I will let you folks what happens.

Alex
 
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Old 01-14-2019, 08:18 PM
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Hi folks,
I tried installing a new (used) relay and the fuse still blows! I tried pulling the relay and the fuse still blows. This car is an automatic, but I have replaced the overdrive solinoid with one of those by pass covers sold by IPD, so the fuse should not be affected by the unused old solenoid. I have disconnected and cleaned all associated spades going to the fuse 11, and cleaned the two connectors at the grid. I tied off and separated the old wires that attached to the the old solenoid under the car. Perhaps the two wires are touching each other now - so I guess I'll go under the car again, and take a look see tomorrow, Would those two wire touching cause this problem. Thanks!
 

Last edited by alex1366; 01-14-2019 at 08:22 PM.
  #13  
Old 01-14-2019, 09:19 PM
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so let me confirm, its fuse 11 thats blowing?

fuse 11 has 2 blue-black wires coming off it. one goes to the rear window defroster circuit, the other goes to the OD relay pin 15, and from there a grey wire goes to the OD button on the shifter. if that grey wire was to short to ground, it would blow the fuse.

the wire from the solenoid (should only be one wire, the solenoid is grounded to the tranmission via its bolts) goes to a 3 pin connector thats just forward of the shifter, this connector has 2 grey wires and a white wire (the two grey wires are the ones to the OD pushbutton on the shifter column). if you unplug that connector, see if fuse 11 blows.
 
  #14  
Old 01-15-2019, 04:39 PM
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Yes, it is definitely fuse 11 (30 amp) that that continues to blow..
Hi - I just plain cut the 2 grey wires right under the shifter cover (the overdrive button will never be used again) and the fuse still blows. At this point, can I assume that the overdrive wiring has nothing to do with the rear demister problem. What a frustrating problem this has become!! Thanks everyone for hanging in there trying to help me with this!!!
Regards,
Alex
 
  #15  
Old 01-15-2019, 05:08 PM
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ok, there should be two blue-black wires coming off the fuse panel right next to fuse 11. unplug both of these, keep them isolated, and use a multimeter to see which one is 0 ohms to ground, tape up the one thats shorted, and plug the other one back into the fuse panel, and replace the fuse and it should not blow. now try your rear window defroster...

again, on the fuse panel, each fuse has 1 input connection, thats the spade farthest to the right, and it has two output connections, thats the spade in the middle row and the one farthest to the left (closest to the fuse). the input connection is wired inside the fuse block to the left fuse spring, and the two outputs are wired to the right fuse spring. the inputs to fuse 11, 12, 13 are also wired together inside the fuse block.
 
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Old 01-15-2019, 06:05 PM
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I will try that tomorrow and advise of results. Thanks!!
 
  #17  
Old 01-16-2019, 06:03 PM
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I attempted to do the test you suggested. The right side of the fuse has (3) blue wires. I used my multi-meter switched to ohms and connected to a good ground and each of the disconnected wires, and did not get any readings on any of them! Should this test be conducted with the key turned to accessory, or off position? Sorry, I'm a little ignorant with electricity! Am I doing something wrong here, and what should the ohms reading be? Thanks, Alex
 
  #18  
Old 01-16-2019, 06:54 PM
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3 blue wires? the rear/rightmost of the 3 spade lugs is the power input, and everything shows that as a blue-yellow wire connected to fuse 12 (fuses 11-12-13 inputs are tied together). a blue wire on that rightmost spade at 11 would be an unfused output of ignition switched power, possibly added aftermarket?

the middle and leftmost/closest spade should each have a blue wire, one going to the rear window defroster timer/relay, and the other to the automatic overdrive control relay.

all ohm meter tests have to be done on cold, unpowered circuits. if you touch the two meter leads together, you should get 0 ohms, open them, it should show OVL or 9999 or go blank or something, indicating no connection.
 
  #19  
Old 01-17-2019, 05:16 PM
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Default Rear window demiste

Originally Posted by pierce View Post
3 blue wires? the rear/rightmost of the 3 spade lugs is the power input, and everything shows that as a blue-yellow wire connected to fuse 12 (fuses 11-12-13 inputs are tied together). a blue wire on that rightmost spade at 11 would be an unfused output of ignition switched power, possibly added aftermarket?

the middle and leftmost/closest spade should each have a blue wire, one going to the rear window defroster timer/relay, and the other to the automatic overdrive control relay.

all ohm meter tests have to be done on cold, unpowered circuits. if you touch the two meter leads together, you should get 0 ohms, open them, it should show OVL or 9999 or go blank or something, indicating no connection.
Thanks for the explanation - I think I was doing it correctly - but Iíll give it another shot!
 
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