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Another sunroof problem

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My sunroof gives no indication of life. It worked fine after I replaced the switch but then gave up on Sunday afternoon. 

The fuse is fine, as are the connections into the fuse board.

The switch is fine. It shows the same volts as between either switch terminal (2 or 4) and earth terminal (3) as it does between the power input terminal (1) and earth.

I have disconnected the motor and tested the cables coming in from the switch. When the switch is activated, they show the same volts as the test did on the switch, so there is power on the motor-side of the connector plug.

Having posted a wanted ad last night for a replacement motor, I realised I should test the motor directly with a spare battery, which I have done and it worked. So the motor is fine.

So why is it that when the motor is connected to the switch cables it is dead as a dodo?

Could it be because the voltage is low at 11.1v (when checked at the disconnected power in and earth connectors to the roof switch: 11.05v at the motor cables)? If so, why could the voltage be so low when the battery itself is showing 12.5v? 

What else do I need to check?

Many thanks in advance.



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If you checked the voltage with a meter, remember that the meter only draws milliamps.  if a wire or a ground is bad, it may well show a voltage on your meter but as soon as you load the circuit (with the sunroof motor),  it cannot pass the required current.   

You are losing a volt or so between the battery and the switch so I would start by check this wiring, connections etc.  With the battery disconnected, check the resistance between the battery negative cable and the earth connection at the switch - should be near to 0 ohms.   Do the same between battery + and the power in. 

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Talking of motors, my sunroof motor stopped working , bought a used spare but didn't work either annoyingly . I need to test the motors .. is it  a positive and neg coming from motor  or is the motor body ground ? I had a quick go last year but gave up .. must try harder  

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I think the motor works on reversed polarity, with earth from the switch. Test by connecting the motor wires to the battery terminals one way, then reverse it by connecting to the opposite terminals. 

I bought a replacement motor from Douglas Valley before working out how to bench test the existing one in situe. Haven't opened the box yet but was sold as bench tested and working. I can pass on at cost if it helps? DM me if so. Otherwise I'll return it next week.

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  • 3 months later...

Closing this one off. Can't quite believe that it's taken this long to get back into the garage to start testing...

Followed your advice, @Jonny Hart, and after the first test on the battery negative terminal to ground (at the switch terminal) found 0.2 ohms (which I thought "close to zero") thought I'd just check the fuse again and there it is blown. Replaced it and the roof works again! This is obviously  the desired outcome and a huge relief but I am completely baffled as to what has gone on. Ho hum. No doubt it will be fine until deciding not to close again when it starts raining...

For completeness, I also tested the battery positive terminal to fuse board input and found 2.54 k-ohms both sides of the (new) fuse (fuse 14 in Bentley; 1st no 5 on the CR fuseboard). Does this sound right?

Edited by Dyaque
Added positive result.
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