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After sitting in storage for 5 years I got my 92 formula back. Noticed the speedometer stopped working. It worked briefly for one drive after I noticed it was dead. Then went dead again for what’s been a few weeks. I’m using an SGI-5 Dakota (T56 swap).. It’s getting an input signal. So the issue is between the SGI unit and the speedometer. I took out the speedometer and took some resistance measurements on the board (see photo). Anything look off? I’m no expert here. Next I will diagnose between the ECU, although I need to research a bit more what I’m looking for.
Last edited by 92-Formula; 08-08-2022 at 04:42 PM.
#1, measuring resistors "In Circuit" can lead to false (lower) readings due to the possibility of measuring other components in the circuit that may be in parallel with that resistor. If the resistor reads as indicated on it's color bands, or perhaps slightly higher, it's a good reading. If it reads lower than it's indicated value, since it could be another component skewing the result, it's customary to unsolder one lead of the resistor off of the circuit board and measure it again.
#2, the device you have listed as 50K is actually a diode, and needs to be checked both forward and reversed. With the probes on it one way the reading will be higher, with the probes the opposite way it'll read lower.
#1, measuring resistors "In Circuit" can lead to false (lower) readings due to the possibility of measuring other components in the circuit that may be in parallel with that resistor. If the resistor reads as indicated on it's color bands, or perhaps slightly higher, it's a good reading. If it reads lower than it's indicated value, since it could be another component skewing the result, it's customary to unsolder one lead of the resistor off of the circuit board and measure it again.
#2, the device you have listed as 50K is actually a diode, and needs to be checked both forward and reversed. With the probes on it one way the reading will be higher, with the probes the opposite way it'll read lower.
thanks for this info, very helpful. I did notice some readings very constant, others ramping up. I figured it was due to interactions with other components like capacitors etc. if there are any other tips to diagnose this board, please let me know! I’m beginning to think it may be easier to just hunt down another board.
I see 4 diodes and 4 electrolytic capacitors. I would check all 4 diodes to see if the forward and reverse Ohm reading is different, it's supposed to be (providing there isn't any interaction with other circuit components) . If the Ohm check indicates the diodes may be good, I'd replace all four of those electrolytic capacitors, since they are the single highest failure rate item on the board. I restore antique tube radios as a hobby and electrolytic capacitors are the #1 item that fails. I've repaired many automotive circuit boards by replacing the caps, this thread was from when installing new caps got my delay wipers working again;
PS, I know it would be nice to be able to trace it down to the defective part before replacing anything, but in this case there really isn't any practical way to test the two chips on the board, with the caps being the #1 failure rate item and being as cheap as they are, it's worth a shot to change them . Ya got way better chances with this gamble VS playing Mega Millions....
Last edited by OrangeBird; 08-09-2022 at 12:59 PM.
08-08-2022, 04:24 PM
