Tach not acurate

[Ptothej]

I searched a bit and didnt seem to see anyone this problem, so here it goes. As far as I knew when I got the truck it seemed acurate (fall '09). I did notice somewhere between then and now.. closer to now, that it read higher, but went back to where it was. Now it seems higher all the time, doing 90k/hr its reading between 1750 and 2k, where as when I got it it was running 100k/hr at 1750, (100k/hr present 2000rpm), has anyone had a similar problem? Or know how to fix it without just saying "f" it, and getting a new one?

[PToombs]

What brand tach, and how does it get it's reading? These little tidbits of info are important when we troubleshoot something like this.

[Ptothej]

Ahhhh sorry! When you think you've covered all the info! Factory! Factory tach.. and more info I just realized, I believe at idle the tach is pretty bang on, it seems almost the higher I go the worse it is? And I stand corrrected, at 100k/hr Im just a hair over 2k.. now I know this thing hasnt been reved as high as Ive reved it, not saying I rev the sthi out of it, (stock governor spring), but shes never had unrestricted exhaust, and pump tweaks.. original owner, but maybe its only been reved to a certain point in its life, and is getting sticky, or something from being pushed past whats its read for the past 18 years? Ideas?

[PToombs]

I think the next thing I do would be to check the wires to the crank sensor, then the air gap on the sensor. There is a plug at the front of the head that gets loose or corroded. The air gap to the balancer should be .050. Beyond that, I'm lost. Maybe check the grounds too.

[Ptothej]

Wow! Ive never known how the tach works on a diesel before, I appreciate that, thanks! Definetly gonna check it out.. I was thinking more gauge.. but perhaps it is more that side then not..hmmmmm

[Ptothej]

I adjusted the tach sensor up and down, and the tach never changed rpms from what it already read.. I guess its in the tach?

[PToombs]

Sounds like it. If you knew someone that had one you could swap and see if the problem stayed or swapped with the guage.

[BobS]

What year was this tach manufactured? Electronic devices have components that fail over time. 15 years life expectancy is about the average using components from back then and this applies to something new in the box.

[Ptothej]

Ptoombs, sounds like you have the best idea, BobS, it is a factory Dodge tach. Did I read, or hear properly that they are run through the ecu as well? Or straight from the sensor? Just trying to narrow it down here, thanks.