TheBMan Posted May 7, 2023 Share Posted May 7, 2023 I thought I would post the steps I took to troubleshoot and diagnose the issue with my inoperative fuel gauge (stuck on 1/2 tank). Using the factory service manual and wiring diagrams along with a couple of YouTube videos I found, I was able to run some tests to confirm that the gauge itself is faulty. Hopefully this will help someone else down the line. I disconnected the sending unit ground wire that sends a variable signal to the gauge. Grounding this wire should have made the gauge go to empty. It did not. I removed the dash pad and I was able to test that I had power to the cluster (pink wire per the wiring diagram) and a weak ground signal via the tan wire from the sending unit (weak because it's varied by the up and down arm on the float/sending unit in the tank). It tested this at the connector directly going into the cluster. I removed the instrument cluster and tested the gauge directly at the posts on the back of the gauge. Power and ground caused it to go to empty proving that the gauge works. Next I followed the circuit board pathways and connected alligator clips at the end of the circuit board for power and ground. The gauge did not move. This proved that there is a break in the circuit board. *Note that someone has attempted some sort of shoddy repair on one of the bulb connectors. 🥲 I'm replacing the housing, circuit board, and lens. While I have it out, I'll replace the clock, which also didn't work. 6 Quote Link to comment Share on other sites More sharing options...
Searsman Posted May 7, 2023 Share Posted May 7, 2023 Good procedure. Yal , had problems with mine when I first got the car. Left turn signal indicator would not flash, fuel gauge everywhere. . Had to replace the PC board also. It also fixed some bulbs that wouldn’t light up.. 3 Quote Link to comment Share on other sites More sharing options...
Searsman Posted May 7, 2023 Share Posted May 7, 2023 I might mention also that I was quite surprised the ribbon technology had been introduced into the auto mfg industry in the early 70s. I was an electronic repairman at that time and ribbon technology wasn’t in tvs until a little later on. In the early 70s we were dealing with tubes and wires. Then ribbons began replacing wires and transistors replacing tubes. The ribbons don’t have to completely open in continuity to not work. Heat caused resistance changes in the material and less data or current could flow.. GM was introducing some high technology at the time , especially in the Oldsmobile. 3 Quote Link to comment Share on other sites More sharing options...
Blake350 Posted June 30, 2023 Share Posted June 30, 2023 B man great info on what could be a very pesky problem...maybe that belongs in the tech archives once you've completed? 1 Quote Link to comment Share on other sites More sharing options...
Blake350 Posted June 30, 2023 Share Posted June 30, 2023 On 5/7/2023 at 2:06 PM, Searsman said: I might mention also that I was quite surprised the ribbon technology had been introduced into the auto mfg industry in the early 70s. I was an electronic repairman at that time and ribbon technology wasn’t in tvs until a little later on. In the early 70s we were dealing with tubes and wires. Then ribbons began replacing wires and transistors replacing tubes. The ribbons don’t have to completely open in continuity to not work. Heat caused resistance changes in the material and less data or current could flow.. GM was introducing some high technology at the time , especially in the Oldsmobile. YES! when I first heard the term "printed circuit board" I said what in as 1970 auto?? but like you said its more of a flexible flat ribbon type set of conductors? No "ACTIVE" components on there or even "PASSIVE" ones? just conductive paths? should it really be called a printed "circuit" board in the abscence of components?? hope mine don't go bad in any case...doesn't look like fun in there!! Brian 2 Quote Link to comment Share on other sites More sharing options...
cbolt Posted July 1, 2023 Share Posted July 1, 2023 I am scratching my head at the pic of the jumper wire used to replace the faulty run. If you have it down to that point why didn't the person just swap the printed circuit board while they had it apart? 2 Quote Link to comment Share on other sites More sharing options...
TheBMan Posted July 2, 2023 Author Share Posted July 2, 2023 On 6/30/2023 at 7:40 PM, Blake350 said: B man great info on what could be a very pesky problem...maybe that belongs in the tech archives once you've completed? Good idea. I think one of the moderators has to do that. Quote Link to comment Share on other sites More sharing options...
TheBMan Posted July 2, 2023 Author Share Posted July 2, 2023 On 7/1/2023 at 6:33 AM, cbolt said: I am scratching my head at the pic of the jumper wire used to replace the faulty run. If you have it down to that point why didn't the person just swap the printed circuit board while they had it apart? Right?! I had that exact same thought! Why go all that way and not spend $50 for the circuit board? Quote Link to comment Share on other sites More sharing options...
TheBMan Posted July 2, 2023 Author Share Posted July 2, 2023 I should have updated this after the repairs were complete, but the diagnosis and repair was all correct and the fuel gauge works properly now. I also replaced all of the bulbs. I had to fiddle with some of the connectors to get them all to work. Thankfully I had the foresight to check them before putting everything back together. 2 Quote Link to comment Share on other sites More sharing options...
Blake350 Posted July 2, 2023 Share Posted July 2, 2023 so the culprit was the ribbon board? guage was ok? sorry if i missed it but I think you replaced several items while it was apart..which was wise... Quote Link to comment Share on other sites More sharing options...
TheBMan Posted July 5, 2023 Author Share Posted July 5, 2023 On 7/2/2023 at 10:35 AM, Blake350 said: so the culprit was the ribbon board? guage was ok? sorry if i missed it but I think you replaced several items while it was apart..which was wise... Yes, faulty circuit board. Quote Link to comment Share on other sites More sharing options...
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