[V8] more speedo drivel

dsaad at icehouse.net dsaad at icehouse.net
Thu Feb 5 16:31:38 EST 2004

So my speedo got worser and worser...
It started out a few months ago as very intermittant. Anybody following the 
story will remember I tore apart and rebuilt my dash - thinking that I nailed 
it, then the connector at the sender really seemed to be the problem. Well, it 
finally all but stopped working so I went back in yet again to investigate. It 
turns out that the sender has in fact gone bad. This was obvious all along if I 
had only followed my own testing advice.
The manual gives a procedure to test which shows a 0 to 12 volt transition on 
the signal line to the speedo. 
Mine only switched from 0 to 9 volts when it was working in the car, and went 
to 9 volts steady state when it was not working.
Every time I had the sender out, I tried to bench test it but never got it to 
work. It always did work when I put it back in the car, which is why I figured 
it was a connection problem. Last night I pulled the sender out, intending to 
get a new one. 
Just for fun, I put a pull-up resistor from the sig out pin to +12v and guess 
what? It now works fine on the bench. So - since I have more time than money, I 
put a pull-up resistor in the car and re-installed the sender. It had been 
totally dead for days before I did this and now it is working fine again. Time 
will tell if I get to buy a new sender or not.

As a side note, the other hall sender in this car is on the right distributor. 
Some of you have had that unit go bad too. It is easy to try this pull up trick 
on that one too. Who knows - it may just be a cheap and permanent fix.

For anyone who wants to try this on their intermittant speedo or distributor 
sender, I tried both a 1K and a 400 ohm resistor. Both worked. I went with the 
400 ohm 1 watt for a little extra drive current (around 30mA).


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