Re: [alfa] Speedo Electrical Gremlin Found!

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Kenneth Casper wrote:

> Hi All,
>
> I posted a note about a month ago asking for some help diagnosing an
> electrical glitch that caused the fuse controlling my speedo to blow when I
> reached speeds of 75-80 mph. I got a fair number of individual responses from
> many of you with some great suggestions. I checked many of the items you
> suggested and finally found what I believe to be the source of my woes over
> the weekend.
>
> I crawled under the car and found a pair of exposed spade lugs just dangling
> high up on the side of the bell housing on the tranny. After investigating, it
> looks like the previous owner had had the housing off at one point and failed
> to hook up the backup lights. I checked, and I have no backup lights. The way
> the lugs were just dangling, I thought, "Man, I'll bet that is my problem!" I
> knew the backup lights were part of the circuit in question. I could see no
> way of hooking these up without removing the bell housing. So I put some
> electrical tape over each wire, taped them together, then secured them to the
> frame.  I guess I'll be without backup lights until I do some tranny work.
>
> Afterwards I took the Spider out for a spin. I ran a pretty long stretch in
> excess of 80 mph, and the speedo held strong! No shorting. I drove long enough
> that in the past the fuse would have blown, so I am very hopeful I have found
> the problem. I guess at speed, the wires were being pushed up by the wind to
> short out against the bell housing or something else.
>
> Next up: my tach, which some days reads correctly and other days reads 1-1.5K
> too low. I've read the "how to" on adjusting a lazy tach, but mine seems to be
> more than lazy as some days it reads fine. If any of you have any good tips on
> this one, drop me an email!
>
> Thanks again for all your input and help!
>
> Ken Casper
>
> 93 Yellow Spider Veloce (with a working speedo)
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>  
Well the most common problem in there would be a broken 
potentiometer(variable resistor). Most instruments
are calibrated with one and then never moved again. The problem is that 
because of the daily vibrations in your
car the graphite pickup "gnaws" through the resistive surface and that 
shows like either wrong resistance or
completely loses contact. So my bet would replacing a 
potentiometer(trimer) in the instrumental panel.
Before doing that find the right one and try if turning it affects the 
tach in any way.
Calibrating the new one should not be so hard, you just need a precise 
rpm gauge(most of mechanics have them),
that works by counting ignition cycles (induced electricity from an 
ignition cable).

Regards,
Ales Golob
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