Speedometer

I am struggling with a speedometer problem in a 91' Dynasty. I won't bore
you with full details (which have already been posted), but I am seeing the
signal (I assume this is a square wave) sent to the speedometer (as measured
on back of instrument panel), increase in voltage when the high beams are
on.

What I am looking for is an explanation of the speed sensor circuit. Two
wires go to the transaxle. I assume one is a positive, and the other is a
signal wire (carrying a square wave pulse). What voltage should the square
wave be? Does this signal go straight to the ECM, and is it processed and
sent to the Speedometer? Or does the signal go straight to the speedometer?
What voltage should be the peak of the square wave voltage I see on the back
of the speedometer? I am seeing 3.2 to 3.8 volts with high beams off, and
over 5 volts with high beams on.

I am also curious if the high beam indicator light circuit shares anything
in common with the speed sensor circuit (except for the ground on the back
of the instrument panel)? Alternatively, does the high beam headlights
share any circuit in common with the speedometer? Are their circuit
diagrams on line for these cars?

Thank you,
Chris

 

Sorry - I thought my first post didn't get through - retyped it all for
nothing!

Chris

 

"C. Bailey" wrote

Fix the problem with the indicator light.
Follow the little copper trace to the ground point.
Find where it is broken.
Use an ohm meter and a sharp pin to poke through the coating if necessary.
Or use a 10x or better magnifier to look for the crack. (a jewelers loop is
good)
Test the traces parallel to the indicator light trace, where you find the
break.
One or more of these are likely broken as well.
This will fix your speedometer

To fix break. Remove coating on either side of crack to expose copper.
Tin exposed copper (flow some solder on it)
Tin end of some copper bell wire (18 gauge)
Place wire across crack and heat till the solder flows around it.
Cut off excess wire.
Use small iron so you don't burn the board.