Na8 glass rear window demister fault?

[revhead]

I noticed when I purchased the car that the demister wasn't working. I thought a simple fuse or dodgy connector would be the fix. But no.

The light on the dash illuminates when the demister switch is flicked to the on position. I checked the fuses under the dash (could not find one marked "demister" but I checked them all and they were fine) and they are ok. Checked voltage with my multimeter on the window and got nothing. window side of switch, still nothing. Checked other side of switch, nothing. Pulled wires out of switch, and only got a negligible 2 - 2.5mV.

Short of pulling all the carpet and seats out and stripping off wiring insulation to look for faults, are there any suggestions?

Find it strange that the dash light illuminates but no voltage reaches the window and all fuses under dash where still intact. Type of connector is a two wire kind with the wires running into the parcel shelf and behind the passanger (left side) seat.

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[MXMatt]

being new to MX5's I have been reading everything I can on little "issues " like this - for when i fitted a hard top to my NA6 - try this write up to show you a couple of other possibilities and location of the wiring (in the boot of all places) - it may just help to double check???
M@

viewtopic.php?f=57&t=34566&hilit=demister&start=15

[taminga16]

If is possible remove the dash switch and check/ clean the contacts, they may be carboned up.
Greg.

[revhead]

Thanks for that. If the dash switch was carbon coated it wouldn't be tripping the dash light to illuminate would it? I am just trying to avoid excess disassembly. Inread somewhere that some mx5's have a 'top down' switch which disables the demister if the top is folded down. I had a look but I can't seem to find such a thing on my na8. Maybe that was for the nb and nc models.

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[aka_juffa]

Remembering that the NA series did not have a glass rear window as standard yours will either be a retro fit from a NB, or an aftermarket from a company such as Robbins.
I have an NB soft top fitted to my NA8. It has a simple spring loaded pressure switch to disable the demister. The switch is attached to part of roof frame. If you follow the wiring from the demister to the connection behind the passenger seat you should find the switch.

J

[revhead]

Ah so it (the switch) will be attached to the folding frame and not to the car body?
Damn, I will have to feel around the frame for the switch. It does look like a retro fit but the wiring seems to be spliced into the main harness (big bundle of wires with the small glass wiring running into it? Might be hard to trace the wire when it's in the bundle.

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[taminga16]

My apologies, I read your description as refering to the cut-out switch (behind the passenger seat you should find the switch).
Keep us informed.
Greg.
P.S. I chased a faulty brake lamp on my BMW motorcycle once, tested the switch (showed as carrying current) three times before realising that there was enough to trip the meter but not enough to power the lamp. The current draw for your demister would be pretty high.

[revhead]

Had a quick look in the car in a dark garage, couldn't find or feel a "top down" switch on the soft top frame. After uni exams I will have to pull the carpet up and dig around not going to be fun.

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[taminga16]

Have you checked the demister for continuity, it may have been dead shorting.
Greg.

[revhead]

could you explain that please, if you don't mind of course?

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[taminga16]

Mate, I don't mind at all.
I am running on memory here, in my NC everything is sealed. The window demister should have two terminal connectors. Remove the cable from both of them (careful not to break or tear them off the glass) you are only testing the element on the glass.
Set your multi -meter accordingly, ie; in the highest ohm range and connect one probe to each terminal on the screen and watch for a reading. If the circuit is OK the needle will move to zero, if it is broken it will not.
Hope that this is of help.
Greg.
The demister works because the thin wire is of high resistance and when a current is applied it gets hot and incidently the reverse occurs with thicker wire which is why we use same for speaker cabling. Less loss.

If you need to chase a break in the cable you can use the same method by attaching a jumper wire to one end of the cable, ie; from the dash switch to one probe and the the other probe to the cable end and so on but remember, It is important to isolate the circuit that you are testing.

[revhead]

Ah ok, I see thank you for that! I will give it a shot.

I did something similar in regard to checking the actual demister element for voltage (when the switch was on the "on" position) though. Got nothing. I wasn't getting any voltage on the wiring side of things either (glass disconnected). Am I correct in saying that I would expect roughly 12 volts to be going to the demister filament? Or much less?

If all else fails, I will disconnect the rear glass from the circuit, and I will add a length of speaker wire to where I find the factory wiring leaving the dash to the rear glass and connect it directly to the glass circuit points (by passing all the factory wiring). If it starts to work, then I guess I need to hunt for a dodgy splice/twist job done by the previous owner.

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[taminga16]

You should expect 12 volts. Re-read the last paragraph and use the speaker wire in the method that I have explained, disconnect the terminals from the dash switch and the window element and so on.
Keep us informed.
Greg.

[revhead]

Will do, thank you. I will spend a good couple of hours on Thursday after my last exam. Determined to sort this out!
Cheers.

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[aka_juffa]

FYI. The switch is located right behind the passenger side seat belt tower. It sits right above the rain rail drain. There is a single wire from the switch to the demister wire loom.

J.