Lately I have been noticing that the tach on my XJ has been acting weird.

when I start it up, it pops up to where I expect, then if I rev it up while in neutral or even with the clutch it, the tach zooms up like it should

BUT when I am driving it doesnt read where I would expect, its always low, and seems slow to rise, and while driving a steady speed it starts to drop.

and after driving a while it will barely read over 1200 RPM while I am in 4th gear doing ~65mph, and I know that is 100% wrong.

Anybody have any ideas what is going on?

I would assume the problem is the tach, as the shift up light is working as it normally does (judging by engine sounds)

 

nobody has any ideas?

Does anybody know what the Tach on these Jeeps runs off of? just the Distributor?

I have a sensor mounted on teh bellhousing, is that a crank position sensor?

 

the sensor on the bel housing is the CPS I am not sure about the factory tach as I have all aftermarket gauges. but I will pull out my FSM here in a few and let you know what I find

 

The sensor attached to the bell housing on the passenger side is the Crank Position Sensor (CPS).
When the tach starts acting up; does the engine hesitate or stumble??
Several years ago I the tach on my '98 was acting up and discovered the problem was the Cam Position Sensor (CKP) going bad... The CKP is located inside the distributor and slides over the distributor shaft.

 

No. No problems at all. it keeps on ticking fine. That is why I originally thought it was just a problem with the cluster, because as of saturday afternoon the tach is acting normal.

its weird, its as if the little motor that operates the tach gets weak.

like it will pop up with short quick engine revs, but when driving a distance, the rpms will go up to where they need to be, then slowly droop down, usually settling just over 1100rpm, yet the Jeep it self will be happily going down the highway at 55-60

 

Might be a possibe reason you might got bigger gear's in ur jeep.. if you do or not know.. bigger gears 4.56, 4.10's would do something like that maybe.
might just be ur tach is going out too, i dont know :]

 

I've never taken a tach apart but it could like there a gear that drives the needle and it could be missing a couple tooths an when you rev up quick it uses momentum to push the needle up higher and then it drops again and when you slow down it reengages <-- kind of a stream of consciousness but it happened to me with a servo I had in a robotics competition once couldn't figure it out so we just started changing thing and when we got back we saw that it was stripped beyond belief

Then again I heard it's a mechanical tac

Sent using my solar powered shoe

 

I dont think its a mechanical tach or speedo for that matter.

Judging by the way its acting, I think it might be something electrical inside the cluster. maybe a loose ground or something.

Im 100% I have 3.07s

What Im saying is the tach, when Im driving at a constant speed. will "wiggle" and sometimes drop rpms

 

I dont know what the heck is going on.


But its been acting 100% normal today and yesterday afternoon. weird