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speedometer inop fixed 900 1993

Hello to all. Finally found the problem with my 940 Speedo.
Many thanks to Art for providing me with so much needed
information. Also to Spook and others who had input. Both the
sensor in the differential and the wiring leading to the speedo
were ok. The sensor reading was measured at 1.35k ohms (provided for
future reference). Removal of the speedometer assembly was straightforward.
The FAQ regarding removal of this assembly was invaluable. A number of
screws to remove the backplate and to expose the speedometer itself.
A close examination of the circuit board showed that melting/leakage had
occurred on the solder side of the board and some of the copper trace
had been dissolved. All the electrolytic Capacitors were ok, no indication
whatsoever of leakage. I measured all capacitors and all were within
capacitance spec's. This does not account for possible DC leakage due
to the ageing process of the dielectric. I did not change any Capacitors.
Regarding the board, the copper trace disappeared when under and next to
diode ZD1. I had to lift the diode in-order to clean the board of whatever
had melted onto it. Denatured alcohol did a fair job.
Continuity checks affirmed that an open had occurred.
A jumper wire soldered in parallel with the broken trace took
care of bringing the speedo back to life. Just my opinion of what may have happened;
I believe the blackened coating of whatever was on that portion of the
board may have come from the heat generated by diode ZD1 and/or D1. Either the plastic, or the coating of the diode melted onto that area of the board. The chemical contained in the coating probably caustic to copper. The slow process of eating away at the trace area began. It may have taken years to eat away at the copper trace which does not have any thickness to speak of. Art had sent me an example of a 93 speedometer board which had been repaired and contained pictures of where the boards copper traces were suppose to be. A great help.
Re-installing was a snap, and the test drive confirmed the speedo "back on line".
I want to mention some advice regarding the various caps on the 93-94
speedometer boards. These caps can be changed out without removing the speedometer circuit board from the speedometer. The caps that should be
swapped out are on or near the outside corners of the board and are within reach of either your fingers or needle nose pliers. I will change mine out later in the year, time permitting .Use temp rated 105c, not the 85c.
Somehow I hope this short experience helps someone. pic uploaded.







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New speedometer inop [900][1993]
posted by  Dom's940  on Sat Apr 13 16:28 CST 2019 >

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