I put new LED bulbs in all the instruments in 87 XJ6 Series III and it made a big difference in all the gauges except for the tach. Somehow the tach guage bulb only shines half a bright as the rest no matter what bulb I put in it.

I was thinking there might be a grounding problem with the gauge but wondered if this is a common issue with some of the gauges. The Volts gauge also shows no reading but lights up just fine. Not sure if the problem is related.

 

you might have a tachometer that still has a plastic dome inside the bulb cavity, and maybe that's why. One way to confirm is to remove both the speedo and tach and look inside the bulb cavity to make sure they are not different.

another check is to remove the bulbs from both gauges and ground them to see if there's any difference in bulb output.

 

You may not have noticed but the wood instrument panel has a full sheet metal backing. Tack welded to this backing are two grounding tabs. Make sure the ground wires haven't been knocked of the tabs.

The tabs are not easy to locate with the instruments in place. From memory....

If your sitting in the driver's seat looking forward one tab would be near the 7:00 o'clock postion of the tach and the other would be at near the 5:00 o'clock on the speedo. Hopefully these coordinates will help you locate them!

Cheers

DD