big66440 Posted October 19, 2014 Share Posted October 19, 2014 I installed a full gauge cluster today in place of my idiot lights , my tachometer reads 1500 RPM at idle even when the engine is at operating temperature and when I shut the engine off the tach goes just below 1000 RPM and very slowly goes down to zero I have already done a search on google and here in the forum and cannot find info, can you guys give me some advice? Thank you 1989 comanche 4.0 AW4 2WD Link to comment Share on other sites More sharing options...
AeroNautical Posted October 20, 2014 Share Posted October 20, 2014 Did the vehicle you pulled the cluster out of also have a 4.0? If not, the tach needs to be adjusted. There is a write up somewhere around here on how to do it, I'll try and find it. Link to comment Share on other sites More sharing options...
Eagle Posted October 20, 2014 Share Posted October 20, 2014 Did the vehicle you pulled the cluster out of also have a 4.0? If not, the tach needs to be adjusted. ^^^ This. Think of it this way: All the tachometer really does is count ignition pulses. For a 6-cylinder, it takes 3 pulses for one revolution. For a 4-cylinder, it only takes 2 pulses for one revolution. So at 1000 RPM, the 6-cylinder engine generates 3000 pulses. The 4-cylinder engine only generates 2000 pules at 1000 RPM. Put a 4-cylinder tach into a 6-cylinder vehicle that idles at 1000 RPM, and the tach will report those 3000 pulses as 1500 RPM all week long. Link to comment Share on other sites More sharing options...
big66440 Posted October 20, 2014 Author Share Posted October 20, 2014 Just got a hold of the guy I bought it from he said it was a 4.0, what you guys are saying makes complete sense, do you guys know how to recalibrate it? If it's something I can do I don't mind getting my hands dirty. Link to comment Share on other sites More sharing options...
big66440 Posted October 20, 2014 Author Share Posted October 20, 2014 Do you guys think cleaning it with electronics cleaner will help? Link to comment Share on other sites More sharing options...
Eagle Posted October 20, 2014 Share Posted October 20, 2014 Do you guys think cleaning it with electronics cleaner will help? No. Link to comment Share on other sites More sharing options...
big66440 Posted October 20, 2014 Author Share Posted October 20, 2014 What about the tach not returning to zero when the engine is off, do you guys with full clusters have this issue as well? is it normal? Link to comment Share on other sites More sharing options...
Eagle Posted October 21, 2014 Share Posted October 21, 2014 What about the tach not returning to zero when the engine is off, do you guys with full clusters have this issue as well? is it normal? Your opening post said the tach does return to zero. Which is it? Yes, or no? Link to comment Share on other sites More sharing options...
big66440 Posted October 21, 2014 Author Share Posted October 21, 2014 Does not return to zero, sorry. Link to comment Share on other sites More sharing options...
jimoshel Posted October 21, 2014 Share Posted October 21, 2014 On the '86 thru '90 ( I think, maybe '91) there is a resistor in the circuit. This resistor has to be changed to calibrate the tach for 4 or 6 cylinder engines. In 1990 they went to a variable resister and it can be adjusted between 4 and 6 (and V8)engines. Link to comment Share on other sites More sharing options...
big66440 Posted October 21, 2014 Author Share Posted October 21, 2014 Thank you, I've been researching the issue myself and found a thread on NAXJA with a similar issue and the guy posting the thread mentioned exactly what you said, he mentioned his 89 cluster had an adjustment screw I'm going to remove my tach and see if I can see it and if I don't I might take the tach to a shop and ask them to check the calibration on it. Link to comment Share on other sites More sharing options...
Eagle Posted October 21, 2014 Share Posted October 21, 2014 It's not a screw, it's a potentiometer. Link to comment Share on other sites More sharing options...
big66440 Posted October 21, 2014 Author Share Posted October 21, 2014 thanks Link to comment Share on other sites More sharing options...
jimoshel Posted October 21, 2014 Share Posted October 21, 2014 You can check the tach for operation, accuracy yourself. All you need is a 12 Volt transformer. Hook the two input leads from the tach to the 12 volt terminals on the transformer. 60 cps = 3600 RPM. Link to comment Share on other sites More sharing options...
Eagle Posted October 21, 2014 Share Posted October 21, 2014 You can check the tach for operation, accuracy yourself. All you need is a 12 Volt transformer. Hook the two input leads from the tach to the 12 volt terminals on the transformer. 60 cps = 3600 RPM. Cool -- I never knew that. Of course, that just confirms operation, it doesn't address calibration. (Or does it?) Link to comment Share on other sites More sharing options...
jimoshel Posted October 21, 2014 Share Posted October 21, 2014 You're not going to find a more accurate signal source than the power line coming into the home. The voltage may fluctuate but never the frequency. Link to comment Share on other sites More sharing options...
Eagle Posted October 22, 2014 Share Posted October 22, 2014 You're not going to find a more accurate signal source than the power line coming into the home. The voltage may fluctuate but never the frequency. You are correct. But, does 60 cycles (Hertz) per second translate directly into something we can use to assess the calibration of a tachometer? IMHO, not directly. The power is 60 cycles per second. As you note, this is 3600 cycles per minute. But ... 3600 cycles does NOT equal 3600 RPM. A 6-cylinder engine fires 3 pulses (cycles) per revolution, so 3600 cycles ==> 1200 RPM. A 4-cylinder engine only fires 2 pulses per revolution, so 3600 cycles ==> 1800 RPM. Link to comment Share on other sites More sharing options...
jimoshel Posted October 22, 2014 Share Posted October 22, 2014 Hey hombre,,,You are of course correct. I just got in a hurry and didn't go deep into details. :thumbsup: Link to comment Share on other sites More sharing options...
Eagle Posted October 22, 2014 Share Posted October 22, 2014 Hey, we got there in the end. So we can check calibration ... just not directly. What kind of transformer is needed? Any old thing that puts out 12 volts? Link to comment Share on other sites More sharing options...
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