Woodchuck Posted January 22, 2014 Report Share Posted January 22, 2014 Hello, i can't seam to get tach to read, I have resoldered the board everything else works. Analog tach, oil, and amp. Digital mph, odometer,and fuel. I am sure there is a write up but am having trouble finding it. does anyone have any ideas? thanks Quote Link to comment Share on other sites More sharing options...
55trucker Posted January 22, 2014 Report Share Posted January 22, 2014 (edited) Unfortunately GM's method of diagnosing the tach is brutal on the out of warranty vehicle owner, their method is to disconnect the harness behind the cluster (that feeds the tach from the DIS), then plug in a hand held cluster tester that is powered from a regular wall outlet (120v), if the tach responds to the testers input (1300rpm @54mph) then check the wiring going to the tach from the DIS, if the tach does NOT respond then replace the entire cluster. (you really didn't want be told that...did you) attached is the schematic at the DIS so you can see the feed wire to the tach Edited January 22, 2014 by 55trucker Quote Link to comment Share on other sites More sharing options...
Woodchuck Posted January 22, 2014 Author Report Share Posted January 22, 2014 What if I changed the little stepper motor? Quote Link to comment Share on other sites More sharing options...
Imp558 Posted January 22, 2014 Report Share Posted January 22, 2014 Check continuity on the wire from the dis to the tach? Quote Link to comment Share on other sites More sharing options...
55trucker Posted January 23, 2014 Report Share Posted January 23, 2014 (edited) What if I changed the little stepper motor? That's possible, they can be found in the aftermarket, that would be done last if the gauge is at fault & there was no wiring problem found between the tach & the DIS Check continuity on the wire from the dis to the tach? That's right, check the wiring to the DIS to see if there is a problem right there (one would have to backprobe that 121 circuit to read the pulse on a meter), ohm the entire circuit to the tach to see if there is an open along the length. Edited January 23, 2014 by 55trucker Quote Link to comment Share on other sites More sharing options...
Woodchuck Posted January 23, 2014 Author Report Share Posted January 23, 2014 Thanks I will try that first Quote Link to comment Share on other sites More sharing options...
Woodchuck Posted January 23, 2014 Author Report Share Posted January 23, 2014 If the dis system isn't working would that make the tach faulty? Quote Link to comment Share on other sites More sharing options...
55trucker Posted January 23, 2014 Report Share Posted January 23, 2014 ^ sure, its an electronic item, and given where the silly engineers at GM placed it where it is exposed to heat and other elements it could happen. Me personally I would check the harness right at the DIS (and as I said backprobe that terminal to see if it is good), then the harness to the instrument cluster & and finally the tach itself, if in the end the tach is to blame, you need to find another stepper motor that is the same as THAT gauge .....(who knows..perhaps it is a common motor to ALL the GM tachs of that era that had a 90 degree sweep to them). Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted January 23, 2014 Report Share Posted January 23, 2014 there were no stepper motors used on any 1G W-body cluster.... all magnetic units. stepper motors didn't come into play until 2G. sounds like the half-digi cluster, which i've never had to play with before, but the 89-93 UB3 units are notorious for the speedo and tach for going way out of calibration. the calibration resistor(which is a laser cut unit) had been at-fault every time i've come across it. the 88-89 half-digi MAY be the same way. EDIT: i lied, the odometer is a stepper motor on all of them that i've come across, but that is it AFAIK. Quote Link to comment Share on other sites More sharing options...
55trucker Posted January 23, 2014 Report Share Posted January 23, 2014 ^I see, I've not had the (un)pleasure of having to pull out my cluster to see what the backside reveals, I have pulled a few others (GM) from the same era to see that there is a yellow module fastened to the circuit board right over the tach, I'm assuming that it is a pulse modulator (motor) that evens out the signal to the tach. Quote Link to comment Share on other sites More sharing options...
Woodchuck Posted January 23, 2014 Author Report Share Posted January 23, 2014 can that calibration resister be fixed? were is it located on the board? Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted January 23, 2014 Report Share Posted January 23, 2014 I'm assuming that it is a pulse modulator (motor) that evens out the signal to the tach. AFAIK, variable voltage drives the needle, but i've never checked to see if it was from a switched source. next time i have a cluster hooked up to my bench, i'll have to remember to check for it. IIRC, the tach is a 3-post needle driver as well, vs 4 for the speedo. can that calibration resister be fixed? were is it located on the board? assuming it is setup anywhere near the same as the 89-93 UB3, near the tach needle, you should find a ~12 pin white IC with probably 2 sections of black rectangles on it. that would be the calibration resistor. here is a relatively low-res pic i took of the 89-93 UB3 a while ago, shows both of them that it uses(one for tach, one for speedo). http://imgur.com/JK4niRy you can ignore most of the text, just focus on the white blocks. you'll notice a large section and a small section on them. the large section on the 89-93 UB3 controls the gain for the speedo/tach needle. i remove the connections the PCB has to the resistor on those two pins and calibrate it using a 25-turn 500K potentiometer and using a spare PCM to generate the tach/speedo signal. Quote Link to comment Share on other sites More sharing options...
Woodchuck Posted January 23, 2014 Author Report Share Posted January 23, 2014 I can tell it is getting some juice to it, as it moves a little when I turn vehicle on and off. Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted January 23, 2014 Report Share Posted January 23, 2014 other than moving from being powered on, does the guage move at all in accordance to engine speed? even if incredibly inaccurate? if i had to guess, i would say that the 14 pin IC next to the calibration resistor is a frequency to voltage converter, but with no industry-standard markings on it, it's hard to tell who it actually came from and an exact model, should it need replaced. if the calibration resistor being bypassed doesn't fix the issue, i would look there next. Quote Link to comment Share on other sites More sharing options...
Woodchuck Posted January 23, 2014 Author Report Share Posted January 23, 2014 No no movement with engine, just sits there flicks a little when turned on and off. Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted January 23, 2014 Report Share Posted January 23, 2014 that is how it would act if the signal from the DIS wasn't making it to the cluster... i would make sure that is happening first before potentially borking something on the board. the easiest way i can describe that doesn't require any other external equipment is to swap the signal pin on the cluster's connector to feed the 4K PPM MPH circuit into the tach. if the tach starts working using the MPH signal, then the cluster is at fault. alternatively, plugging the DIS tach circuit into the MPH feed will show engine speed as MPH. that would let you know if the DIS signal is making it all the way through the harness. Quote Link to comment Share on other sites More sharing options...
Woodchuck Posted January 24, 2014 Author Report Share Posted January 24, 2014 I unplugged from the dis and plugged it back in and it started to work. weird! Thanks everyone for the helpful info Quote Link to comment Share on other sites More sharing options...
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