RE: Latest Alfa Owner and Spider Speedos...

[Jon Pike <jhpike@domain.elided>]

>OK guys, what's MY deal?  I also have a '93 with speedo
>problems.  Typical behavior has been that it's been reading
>low (also about 15 mph), but depending on weather conditions,
>it'll run from very low (winter) to about accurate - summer
>behavior was either 10-15 mph low or accurate.
>
>Now, about a month ago, it basically quit, and the odo (which
>has been OK all along) quit as well.  However, while the odo
>has stayed dead, the speedo has given me a day or two of
>working (reading accurately) during the course of this "dead"
>month.  So the new behavior is that the odo is dead and the
>speedo is dead 95% of the time, and fine the other 5%.
>
Aha..  was going to say that your adjustment pot,  or some other part 
that determines the cal has turned into a thermistor..  changing with 
temperature. (very rare.. but happens)  

But, the rest of your symptoms sound more like a fairly common cold (or 
cracked) solder joint.  Can you get it to come back by banging on the 
console?

Interesting to hear from Peter,  about the pot value (or other parts) 
drifting and needing correction.  The console on a sunny day with the 
top up and windows close is probably as bad as the engine compartment 
for those components,  and they might not have been specced for that 
kind of temp range..

The short time I've been on the Digest,  I've heard of more than a few 
bad solder joints in the electronic consoles.  Solder is a really lousy 
materiel for mechanical strength,  especially with a lot of flexing and 
such.  It's not ment to hold things together,  just make a mettalic 
electrical connection.  But,  with cowl shake,  and general car 
vibrations, as well as thermal cycling,  a PC board  is having more 
asked of it that way, then normal.  So, we'll see more of this problem 
than in other kinds of products.

Not to say,  that a good joint can't last the life of the car..  but 
marginal ones won't.   Miltary and aircraft electronics are typically 
dipped with a heavy varnish like coating,  gluing the parts down so they 
can't vibrate and eventually crack the joints..  Of course,  this is a 
hassle and expensive,  and probably dosen't make sense for a car..

The bright side is,  once you find the joint,  resoldering it takes a 
matter of seconds,  and it's ready for 10 more years...  I'd check all 
joints,  see if any need retouching due to these problems or corrosion.. 
 then when putting the board back in place.. make sure it's mounted 
well,  and can't vibrate eaisliy..  maybe even shim up some rubber 
washers or blocks to help prevent it..  (have never been inside one of 
these,  so I don't know how they are mounted)

Hope it helps..

Jon  - coming from years of dealing with solder joints and PC boards in 
the electronics biz...
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