Tachometer Failing, 98 Gen 2.5

Salonika

Monterror Pilot
My tach has started to act up. It finally started to freeze entirely at zero for 30 seconds to a couple minutes. It still works most of the time, but every few times I drive it I have this happen. I did some searching already with Google and found a few threads on the wire and this forum but nothing useful.

Anyone know where to start? My first thought was if the RPM signal is used by the ECM for anything important, or is it only to run the tach for the drivers information? Truck runs fine when it fails so I’m thinking it only serves a visual purpose. I might hook up my scanner and see if when the gauge is dead, does the ECM still report engine RPM. Then I’d know it isn’t the sensor.
 
My tach has started to act up. It finally started to freeze entirely at zero for 30 seconds to a couple minutes. It still works most of the time, but every few times I drive it I have this happen. I did some searching already with Google and found a few threads on the wire and this forum but nothing useful.

Anyone know where to start? My first thought was if the RPM signal is used by the ECM for anything important, or is it only to run the tach for the drivers information? Truck runs fine when it fails so I’m thinking it only serves a visual purpose. I might hook up my scanner and see if when the gauge is dead, does the ECM still report engine RPM. Then I’d know it isn’t the sensor.
Dude, mine just started doing the same, and around the same time.
Just found the part number- MR390916
But good golly, at a glance we're talking $250+

Is it anything that can be re-soldered on the original? Any wires that can be replaced? Or am I just going to have to eat it...
 

SD_Montero

Observer
I took mine aprt trying to find a tiny loose solder point or connection, just made it worse. Does anyone know if I can just put an aftermarket tach gauge in using the same signal?
 

Salonika

Monterror Pilot
Level-1 diagnosis done.....I gave the instrument cluster housing a hard tap with my hand the last time it died during a drive and the tach came back to life, temporarily. At least now I know the problem is in the IC and not a sensor issue!
 

SD_Montero

Observer
Dude, mine just started doing the same, and around the same time.
Just found the part number- MR390916
But good golly, at a glance we're talking $250+

Is it anything that can be re-soldered on the original? Any wires that can be replaced? Or am I just going to have to eat it...
i have this same problem, took the thing apart from the dash and there was nothing notable or obvious to fix. all the soldered connections were fine.
 
Damaged solder joints may not be obvious. I've seen some computer boards that have been fixed by .....
- Removing the board from the device.
- Removing all the plastic parts from the board.
- Preheating an oven to between 350 and 450 degrees. This depends on the board.
- Place the board on metal stand offs.
- Place the board in the oven for between 5 and 8 minutes. Again this depends on the board.
- Remove the board from the oven and allow it to cool.
- Reattach all the plastic parts to the board.
- Reinstall the board in the device.

I don't know if this process will work for one of the dash boards. Perhaps someone with a junkyard board could give it a try.
 

blantonator

New member
Mine recently failed (stuck at 2.2k rpms). I took it apart cleaned it up, including the connections, blew it out with compressed air and it works again. I'm an electrical engineer by trade and all the parts looked fine, if it fails again, I'll try to reflow like Mark mentioned above. However I'd suggest using a heat gun as its a bit simpler and less likely to damage anything.
 
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