I need to say, before everyone else does, that I totally bungled troubleshooting on this problem.

When I pushed the "Roof" switch and got no deflection on the ammeter, I thought "Ah, no power getting to the motor". After checking the fuse and the switch, I concluded that something was amiss with the wiring between the switch and the motor. (At this point, R. Conan Doyle was probably turning about 100 RPM in his grave.)

Of course, what I failed to consider was the common failure mode of carbon-brush DC motors: OPEN! As Poirot might say, "the little grey cells were not working". So when I applied DC directly to the motor (which should have been step 1 in the troubleshooting), sure enough, nothing.

Dis-assembly of the motor revealed that the brushes were not jammed and were in fact in pretty good shape, but there is a component in series with the green wire that was essentially open (several K Ohms resistance), which turned out to be the problem.

I'm thinking that this component is a thermal-overload switch, that opens when it heats up due to a jammed motor or to someone holding the switch down when the sunroof is at a travel limit. It is swagged in, so it doesn't seem like is a "one-shot" -- that is, I believe it is intended to reset once cool and not to be replaced once open. However, the motor has the dreaded "L-word" (Lucas) on it, so anything is possible I guess. But that would be shoddy engineering even for Lucas.

So I replaced the motor with another, and I now have a working sunroof. As part of my penance for being such a dough-head, I'll investigate the (alleged) thermal-switch to see if I can find a replacement (it has a part number on it) and post the results here.

I'll also post the pin-out of the roof switch, for I could find no information in the 16V Bentley on the roof circuitry. Although the 8V Bentley has a diagram for the roof wiring, when applied to the 16V it deserves a Pulitzer prize for fiction as it isn't even close.

Thanks to all who helped.

Jeff Rininger



posted by 108.252.12...

• Sunroof: Everything Good, No Voltage at Motor WTF?, Jeff Rininger, Tue, 27 Jan 2015 14:55:19
  • Re: Sunroof: Everything Good (brain fade alert!), Jeff Rininger, Thu, 29 Jan 2015 16:53:45 <-- Viewing This Message
  • Main ground behind radiator?, Used to be Chaz, Tue, 27 Jan 2015 19:34:43
    • Jumper past switch, see what happens., RayF, Wed, 28 Jan 2015 03:17:28
  • Re: Sunroof: Everything Good, No Voltage at Motor WTF?, tweetiedancer, Tue, 27 Jan 2015 15:32:33

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