Cruiser54, thanks for all the step-by-step diagnostices you've published on this site. I love the fact the repairs which have made the most difference in how our '89 XJ runs have been essentially free (I did the TPS adjustment this week and it made a huge difference).

Anytime I can find a simple test I can perform with a multi-meter to determine if a part is good, bad or simply out of adjustment I'm thrilled!

I'm planning on cleaning the grounds today and may just go buy a replacement for the engine-to-firewall ground before I get started.

 

I would like to read that instrument panel ground improvement when you write it.

 

First draft. I just did it. Please tell me if it all makes sense to you. I can update it if needed.
Improving the Instrument Panel Ground
The ground point for the complete instrument cluster on your XJ or MJ is located up under the driver’s side dash. If you lay on your back and look up under there with a flashlight, you will see a black wire attached to a shiny piece of metal almost directly above the hood release ****. The screw will have either a ¼" or 5/16" head on it.
This ground point is responsible for handling the ground circuit for the following items: Dome lamps, Seatbelt and key warning, trans comfort switch, wiper switch, headlamp switch and delay module, fog lamp switch, cargo lamp switch, all instrument panel grounds and illumination, power windows and door locks, cruise control dump valve, and a few more things.
The problem is that where the ground point is located does not have a good contact with the chassis where the ground should be. The solution is simple.
Make up a jumper wire with #10 gauge wire about 10" long. On one end, crimp on a ¼" round wire terminal. On the other end, crimp on a 3/8" round wire terminal.
Remove the screw from the existing ground wire and attach the small terminal of your jumper so that the original wire and your new jumper share the same attaching point.
Look above the driver’s side plastic kick panel just forward of the top of the hood release ****. You will see an 8mm stud there. Attach the large terminal end there with a washer and nut over it tightened securely.
**Special note for Comanche owners: Make your jumper wire 12" long and attach it on the driver’s side kick panel close to the fusebox on the 8mm stud.**

 

Good write up, thanks! I'll have to give this a go tomorrow.

 

Good write up, thanks! I'll have to give this a go tomorrow.

I want your thoughts after using the instructions. Thanks.

 

The ground to the alternator isn't strictly necessary - I find it really helps if you go with a high-output unit (over 150A or so) or if you have a "floating voltage" issue at a steady state (for instance, you're cruising on the freeway, and you note your system voltage wandering for no reason.)

Adding a ground from the battery to the chassis (min. 8AWG cable) almost invariably helps - bear in mind that the ECU is chassis-grounded, but there is no direct ground for the chassis to the battery (it's grounded via the engine block.)

Check, clean, and reterminate the wires at the primary engine block ground PRN (the cable attaches above and behind the distributor. The smaller wires there are sensor grounds.)

Check all chassis ground contact points (they're fairly easy to find, so just look for them) and reterminate/replace hardware/clean PRN. I also recommend that you clean the contact patch with the chassis down to bare metal and apply a corrosion inhibitor on reassembly. Replacement hardware should be CRES (for the chassis points - they're tapping screws) or brass/bronze (acceptable for machine threads,) and any OEM "black oxide" coated hardware should be replaced outright (the oxide layer isn't a very good conductor, and grounds need all the help they can get.) Even bright zinc-coated carbon steel is an improvement - I just happen to prefer CRES/brs/brz for longevity and corrosion resistance.