Not only does the tach not work, but the odometer still doesn't work. Has to be an electrical problem.

 

As you can see, the main connector doesn't have pins, it just has contacts that touch the conductive strips on the film. I cleaned all of the contacts on the plug and the film with Q-tips soaked with electrical cleaner. Still nothing. Looks like the odo traces come off the speedo traces, and the tach comes directly from the plug. The outside upper and lower main plug contacts are the tach, and with the engine running I get a constant 3.3v no matter how fast the engine is reving. The inside upper and lower contacts go to the speedo, and read 12.9v constant. Doesn't look like any cold solders, can't find any wire breaks, and no burnt spots on the inside components. BTW the lower left solders are the tach connection, the middle solders the odo connection, and the right is speedo. I checked continuity between the solders and contact strips of the receptacle, and it seems good.

I'm baffled... Anyone have any ideas?

 

Welp, I took it apart again and upon closer examination found a burned resistor on the tach board. I might be able to repair this (have done electronic repairs since I was a kid) but I may look around for an old cluster to salvage (if any are left) and piece one good one together .At worst, if I can't find the parts, I'll just put it back as it was, but I can spend some time searching since it's a second vehicle. Not sure if this is why the odo isn't working with the new gears or not. At least if I get an old cluster that otherwise works, but has broken odo gears I can pop this one in it

 

The tach V of 3.3V regardless of RPM is most definitely bogus. Without replacing the resistor, can't say what it should be other than the following"

The tach is getting a signal from either the crank (CKP?) or input shaft speed sensor ? which comes through as a series of pulses as the sensor is magnetic and senses each time the shaft rotates and brings a magnetic piece in proximity, generating a current in the sensor coil.

The frequency of the signal from the sensor (pulses) is then converted to a DC voltage.
The voltage is sent to the needle which has a coil attached to it, which in turn has a spring that resists it's movement.
All of the above are in a N S magnetic gap
When the RPMs are non zero, the V from the converted freq signal flows into the coil within the magnetic gap. The flow of current through the coil generates its own magnetic field that then interacts w the N S fixed gap magnetic field. The spring is there for a restoring force and to dampen the movement so it does not display a jumpy movement.

You may want to look into the health of whatever sensor is feeding the tach. On an ODBII car you'd get a leg up with the code. UBD1 -- nope.
In fact you may want to check this before replacing the resistor just in case the sensor went bad and torched the resistor --though I doubt anyone would design a circuit of this type with such a simple catastrophic failure mode.

The steady V reading may be indicative of the sensor feeding it putting out a bogus signal/unchanging signal.

Seeing the chip on there, I'm guessing that is a Frequency to Voltage converter, and the resistors and capacitors set the gain aka what the v range is for a given range in freq.
It may turn out that that resistor's absence is preventing the output of the Freq to V converter from changing.

The fact that the needle doesn't move from 0 rpm with >0 V is not necessarily a bad sign -- withiout knowing how it is supposed to behave, there may be a dead zone at the bottom of the needle sweep range to ensure there is no bouncing of the needle when the RPMs head toward 0 and the incoming freq signals get messy, annd.....you get the point.


SAK215 PULSE SHAPER CIRCUIT FOR REVOLUTION COUNTERS

 

Thank you for the reply and information Derf! I just cleaned up the resistor and board, and am thinking it may still be good, but the gunk was shorting to that jumper next to it. Without removing it I get a ~30K ohm reading, but can't read the colors to know it's value. I may put it back together tomorrow and see what happens, but may hit that upper leg with a soldering iron on the back side. I've seen that gunk happen when there was a cold solder in a guitar amp, and just cleaning it then resoldering it fixed it. BTW all the removing/replacing of the odo has slightly cracked the clear plastic screw mounts, but I can rebuild them with 2-part plastic epoxy I have. A quick search shows this year cluster may not be available, so I'll see if I can save this one.

Also was wondering if the odo being bad for a while may have stressed other electronic components... maybe causing overheating? And if the crankshaft position sensor was bad, would the engine even run?

 

Wow! That cleaned up nicely. I don't really know ****e about electrical work, but would THINK you could at least do a continuity test on that resistor. I seem to recall that each stripe has an Ohm value, but I sure don't remember what they are. That was over 40 years, thousands of beers and two brain surgeries ago. I just re-learned how to tie my shoes last week!!! LOL

 

I tested it for ohms and it's 30K or so. May not be the original value, but maybe enough to work. Actually one leg should be removed from the board for a true reading, but small boards like this are easy to screw up with an old guy using a soldering iron. I had a Ham license when I was a kid, still have a Tech license, but am not active with it. Used to know a lot about electronics but have forgotten most. I just can't make out all the colors. I'm hoping that it was just shorting out to that bare jumper next to it. Have epoxied the odo mounts that cracked and need to let them set.then will see if I can remount it all and see what happens. I'll be an expert in dissembling and reassembling the cluster when this is all over

 

Here's what I'm doing to fix the two odo mounts. First pic shows the bottom one missing a piece, the upper on just cracked a bit. I cut pieces from a rubber smoothie straw I had that just fits around the mounts, filled it with 2-part clear JB Weld "plastic weld". Let it set an hour, it shrunk in, so I topped it off and will let this harden for a while. Then drill new holes and put some lithium grease on the screws when I install the odo. Hope it works!

 

THE TACH WORKS!!!

Odometer is trying, but it might have something hanging up the dials, or maybe something sticky is hanging them up. I know the stepper motor is pretty weak. Goes about 2/10, then stops. Only able to register one mile more driving 5 miles. Guess I'll take it apart again, but there's PROGRESS!

 

ODOMETER WORKS!!!

The rebuilding of the odo mounts with epoxy didn't work, so I removed them, drilled through the IP faceplate, and put nylon screws in. Used a couple nuts on the backside as spacers, but turns out it was too close to the faceplate and the gears were slightly rubbing, causing the hesitation. Now I have a 1/4" nylon spacer and thin nylon washer, and it works!

BTW the nylon screw heads on the faceplate don't show when it's all together. The front IP lens assembly covers them.

Still need to get some new bulbs before closing her up, but am SO happy to have it all working again

Just to give an idea what I did with the nylon screws...




EDIT to add: When the new gears were first installed and the odometer still didn't work, it was because I didn't have the stepper motor assembly fully secured to the odometer (two screws), so the motor was not engaging with the new gears. I forgot to mention that before, and it's important. When I secured it, then reinstalled the complete odo assembly to the faceplate, is when the odo mounts on the faceplate finally cracked, leading me to those repairs.