Macraja Posted April 25, 2016 Report Share Posted April 25, 2016 Hi I have 1984 Datsun 720, Z24, 4WD. My OEM tach quit working. I replaced resister with New OEM. At the three prong plug ( that plugs into the back of OEM tach)I have 12V in one leg when ignition on. I plugged tachometer in other two legs and it does register RPM. Any suggestion please. What I meat to say is that when disconnected oem tach ( three prong plug) # 1 wire from resister, # 2 wire from ignition when hot and I assume # 3 wire is ground,I hooked up external Tach/ dwell meter from Sears to # 1 and # 3 Tach/dwell meter did work., 1 Quote Link to comment
datzenmike Posted April 26, 2016 Report Share Posted April 26, 2016 Hi I have 1984 Datsun 720, Z24, 4WD. My OEM tach quit working. I replaced resister with New OEM. At the three prong plug ( that plugs into the back of OEM tach)I have 12V in one leg when ignition on. I plugged tachometer in other two legs and it does register RPM. Any suggestion please. So..... you're saying that it works???? 1 Quote Link to comment
Lockleaf Posted April 26, 2016 Report Share Posted April 26, 2016 Your explanation doesn't make a whole ton of sense. Try explaining it again so we can offer better help 1 Quote Link to comment
84720FourWheel Posted April 28, 2016 Report Share Posted April 28, 2016 The OEM one doesn't work, but the Sears one works when plugged in with the OEM harness? 1 Quote Link to comment
Draker Posted April 28, 2016 Report Share Posted April 28, 2016 Sounds like a dead gauge? 1 Quote Link to comment
datzenmike Posted April 28, 2016 Report Share Posted April 28, 2016 What I meat to say is that when disconnected oem tach ( three prong plug) # 1 wire from resister, # 2 wire from ignition when hot and I assume # 3 wire is ground,I hooked up external Tach/ dwell meter from Sears to # 1 and # 3 Tach/dwell meter did work., Take the factory one out and have a look at it. Not much you can do but blow the dust off of it. Does it move freely? Look for a broken or loose solder track on the printed circuit. 1 Quote Link to comment
Macraja Posted May 10, 2016 Author Report Share Posted May 10, 2016 I did take gauge out. Blow dust out with compressed air at 5 PSI. Looked at solder joints. Needle moves freely. Nothing visible crack. Reinstalled gauge . same issue it did not work. Installed another OEM tach. This one does not work. No needle movement when truck started. 1 Quote Link to comment
Draker Posted May 10, 2016 Report Share Posted May 10, 2016 Hook up your meter to the wiring harness, do you get a reading? If two tachs didn't work, it's probably the wiring. 2 Quote Link to comment
Macraja Posted May 13, 2016 Author Report Share Posted May 13, 2016 I diid hook meter. I do get 12 v in # 2 wire when ignition is on with # 3 ground, I also get signal in # 1 wire from resister. 1 Quote Link to comment
datzenmike Posted May 14, 2016 Report Share Posted May 14, 2016 I did take gauge out. Blow dust out with compressed air at 5 PSI. Looked at solder joints. Needle moves freely. Nothing visible crack. Reinstalled gauge . same issue it did not work. Installed another OEM tach. This one does not work. No needle movement when truck started. Seems to me that it's very unlikely that you would happen to have two tachs that do not work. Check that both coils are firing. Got spark???? 1 Quote Link to comment
Charlie69 Posted May 21, 2016 Report Share Posted May 21, 2016 Do not forget the resistor under the hood by the Brake vacuum booster. From Datzenmike's bucket Quote Link to comment
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