Reinstalled instrument cluster - tach now acting weird

Hello All,

I've been cleaning out the interior of my truck - lots of dust and grime from when my dad was in construction.  So, I took most of the dash out and cleaned everything up and repainted the gauges while I was at it (and installed LED lights for the instrument cluster)

I just plugged the cluster in to make sure all the bulbs were working, and mostly success with one strange issue.  When I turn the key to "on" (not start), the tach pegs all the way to the right above 6000 RPM.  I haven't tried to start it up fully yet because obviously something is amiss.  Any ideas?  I'm afraid the PCB on the back has a problem somewhere but I'm not sure where to start.  Seems like a grounding issue.

Thanks!

That's a strange one.  And it might be a grounding issue.  Check the ground that goes to the firewall to the left of the radio.  It is supposed to ground almost everything in the cab.

Also, check to see that the traces on the circuit film are clean where the connector plugs in.  You can use a pencil eraser to clean them - gently.

And, the nuts that hold the tach in are to connect to the circuit film, so you can take them off one at a time and clean the trace below them.

It's really easy to test the wiring in the cluster connector for proper KOEO voltages.  If they're correct, then the tach is just dying.

Gary Lewis wrote

That's a strange one.  And it might be a grounding issue.  Check the ground that goes to the firewall to the left of the radio.  It is supposed to ground almost everything in the cab.

Also, check to see that the traces on the circuit film are clean where the connector plugs in.  You can use a pencil eraser to clean them - gently.

And, the nuts that hold the tach in are to connect to the circuit film, so you can take them off one at a time and clean the trace below them.

I just did the LED bulbs in my cluster and before putting the bulb sockets back in I hit them with eraser.
It did turn the eraser black but did not look like it did anything to the copper.
Then I would say it did not because I had to wiggle the sockets to make better contact for them to work.

If there is a next time I will be using 600 grit sand paper as I know they will be clean then.
I wonder if that cluster plug is not making a good connection?
Then again he had to remove the gauge nuts so maybe that is the issue?
Dave ----

Thanks guys.  Just a little more detail - I never actually took the tach out to paint it, I just masked it off when it was installed.  That being said, I took it out and cleaned the contacts and nuts, and still no luck.  All other lights/leds/gauges are working.

It jus bounces right to 6k RPM when the ignition is turned to on.  It was working fine before I took it out... I guess I may have damaged the PCB itself.  I guess the next step is taking an Ohm meter to to the PCB and seeing if anything odd pops out?  I'll look for that ground wire too, thanks Gary.

Hi Steve,

I measured the C208A connector, and Pin 1 (supply voltage) and Pin 13 (tach return I think?) are both registering ~12.6 volts.  Seems like everything is as it should be.  I think that means the connector is OK?  Are there any other pins I should be checking?

Or is there a way I can test the tach by itself?

The tach is just like any other 12V device on the truck - apply power, return, & some signals; and it should work.  It doesn't matter if it's inside the cluster, outside the cluster, or outside its own round housing.  So if it's out of the cluster, use jumper wires to power it up withOUT either signal terminal connected.  If it still jumps to 6K, you could try re-stabbing the needle, but it's probably just worn out.

If you decide it is the tachometer itself, you may want to see if it can be repaired.  RCCInnovations (F-series tachs towards the bottom of the page)  has done great work for some of us, and he may even have a replacement if it can't be fixed after diagnosis.

 

Thanks for the tips everyone, I got it worked out tonight and it's acting normally again.  It was a bad ground after all...

Excellent!