Tachometer Twitch

I now own my first Jeep, an '89 XJ with approx 178,000 miles (XJ was towed behind a motor home, not sure of actual driven miles) and I am only the second owner. As I posted before, I corrected the high idle by tightening the manifold and adjusting the throttle plate (after spending over $150.00 in parts). So now I am on to fixing a minor (I hope) problem. The stock tachometer is erratic. When it does work, it is all over the map. I managed to clean most of the obvious grounds (another Renix issue I found), but no luck. As a side note, the cruise control is also non functional. The XJ is mechanically sound. I have some experience with older vehicles (my other ride is an '88 Mercury Grand Marquis) but this is my first Jeep (which I already LOVE). Could I be missing a ground or another connection not immediately obvious? She is a 4.0L auto with the Selec-trac X-fer case. Thanks.

mine does the same thing lol been doing it since i bought it 2 1/2 years ago

Vacuum issues can kill the cruise. I'll bet dollars to doughnuts the vacuum harnesses are the factory originals. About $18 ea @ rockauto.com, a little more expensive at Napa but when you factor in the shipping for rockauto.com the ones from Napa are less.

Check your vacuum reservoir, behind the front bumper, passenger side. The vacuum line that feeds the reservoir likes to break right around the battery. Relocating the reservoir into the engine bay is a good idea, and eliminates about 5' of vacuum line. I put mine under the coolant pressure bottle.

The Tach gets its engine speed signal from the negative side of the coil.

Did you get the grounds on the dipstick tube? ICM, ECU, and o2 all ground there.

I did hit the grounds you mentioned, but did not consider the vacuum angle. That will be next on my list as soon as it gets above 5 degrees here! You are correct about the vac harnesses. They appear to be OEM. The previous owner seemed to be thoughtful about obvious maintenence. I think they dumped the old girl because of the high idle problem. The EGR valve was shot. The metal vacuum connection crumbled when I removed it and the dashpot was disentrigated. The jeep seems to be a good vehicle despite her age. Thanks.

There is one big difference driving a Jeep than driving a Grand Marquis...the road noise!:doh: I really dont mind though. I really like this Jeep!arty:

The odometer is driven off the xfer case and should roll forward while flat-towing, so that 178k miles is probably correct. On the cruise, check the white switch near the top of the brake pedal. It tends to gum and cause no cruise and the tranny to not go into overdrive. It should also have a vacumn line attached to it, which bleeds the vacumn off the cruise when you hit the brake (kinda a backup in case the cruise malfunctions). The switch pops apart and can be cleaned up.

I agree on checking the vacumn lines for leaks. You might have just been masking the true cause for the high idle by adjusting the throttle plate.

The tach is driven from the ignition module that sits under the coil. Maybe check the connections on there.