250 Dimmer Switch rebuild

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T-Spoon
Posts: 5
Joined: Sun Aug 22, 2021 11:29 pm

250 Dimmer Switch rebuild

Post by T-Spoon »

The instrument lights in my 250 were not working and it turned out to be the dimmer switch. Not sure if it was an unmolested example but the design seemed to be asking for trouble with very little travel in the stationary wiper that makes a connection to the moving wiper. With a little axial motion or free-play in the shaft, an open circuit would put the lights out. Also, the travel limit was provided by the moving wiper inviting damage from too much force on the switch.

I looked at available replacements and was not excited about spending $650 - 900 for one. So I spent $2,000 of my own time redesigning it and fabricating 3 new parts to improve it......
250 Dimmer Switch.jpg

After the rebuild it works smoothly and has firm positive stop at both ends of travel.

As an aside, I find it amusing that Ferrari decided to include a dimmer for instrument lights in the first place. I can't recall ever adjusting the instrument lights on any car I have ever owned.
Tyler Schilling

1963 250 GTE rebodied as SWB Cal Spyder
John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: 250 Dimmer Switch rebuild

Post by John Vardanian »

Please tell us what you did and how. These have a tendency to come unraveled in worst case and in best case they just get carboned up. Thanks.

john
PF Coupe
T-Spoon
Posts: 5
Joined: Sun Aug 22, 2021 11:29 pm

Re: 250 Dimmer Switch rebuild

Post by T-Spoon »

John,

Here is a summary of what I did. I designed and fabricated 3 new parts; two wipers and a block to hold the moving wiper. I modified two original parts; the shaft of the switch and the snout of the switch body to accommodate the shorter effective length of the modified shaft.
250 Dimmer.PNG
Ferrari 250 dimmer mods 2.PNG
Ferrari 250 dimmer mods 3.PNG
The wipers are made from .020 thick beryllium copper because it has good electrical properties and crazy high yield strength and hardness. This makes for a durable, long lived part. If I did it again I would most likely use .010 thick material. It would be more forgiving in terms of adjustment and still provide plenty of contact pressure.

The wiper block with keying to the shaft provides a nice positive stop at both ends of travel and prevents the wiper from becoming the end stop (one of the problems with my unmodified dimmer).

I trimmed back the snout of the switch body to compensate for the shaft fitting further up into the assembly to engage the slots in the wiper block. This restored the original end float of the shaft to body fit.

Finally, I am a little puzzled by the build up of carbon being a failure mode as you mentioned. I think that would imply sparking during operation. Not sure why that would happen if the wiper remains in contact with the resistance wire windings.

Thanks,

Tyler
Tyler Schilling

1963 250 GTE rebodied as SWB Cal Spyder
John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: 250 Dimmer Switch rebuild

Post by John Vardanian »

That's pretty brilliant, Tyler. You are right, the problem is not carbon but it's just contamination. This switch doesn't get much use so it collects dust, and then when you turn it you get dead spots. I gather that yours overcomes this problem by applying greater "wiping" pressure against the coil and scrapes the contact surface clean.

john
PF Coupe
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