1982 245 GL no instrument cluster back lighting working.

HI all, I have a 1982 245 GL. Has the big tach as well as small clock and 3 bar oil pressure gauge. Issue is that randomly one night none of my instrument cluster back lights illuminated when turning the headlight switch on.

When this occurred my tach would drop to zero and quit working. Turn the headlight switch off tach works again. Only the instrument cluster cluster back lights do not work yet my oil pressure gauge and all of my center stack back lights work as well. Didn't plug the clock back in thinking that the clock harness was shorting out. Checked my fuses and all are okay as well.

Any suggestions you all? Thanks.

Gonna be pulling the instrument cluster an checking it out.

The cluster has two bulbs in sockets to light up the cluster. They are replacable. Ithink the generic number for the bulb is 168.

Might be a good time to go over the connections and make sure the contact pins are in good condition and not spread open making marginal connection.

Well, I did forget to mention you all that I did put new bulbs in and still the instrument cluster lights were inoperative. Just doesnt make sense that when I turn the headlight switch on my tach is inoperable.

First thing to check with circuit weirdness is the fuse box and make sure everything is clean and good connections. the cluster light brightness is controlled by the brightness knob above the headlight switch. It is good to ohm out the potentiometer on those and see if it's open. You can also connect the wires together to bypass it for a full brightness dash illumination test.

The tach problem may be a connection that is marginal. Maybe the pin connections on the tach are a bit dirty or loose?

Could also be a fried circuit board. I think Ethan might have a spare cluster that you could try.

@Dave, apparently, the rheostat works. Dash lights work, it's just the cluster that has no lighting. Tach works when the lights are off, but when they're turned on, it zeroes out. I had this problem once on an '84 245GL. Clock harness had somehow fried itself and was causing the tach to zero out when the lights were turned on. Eventually, the car started idling funny, temp and fuel gauges started pegging, and smoke began pouring out from behind the cluster. Went to a yard, grabbed an extra cluster circuit board, swapped it, and then built a new clock harness. Clock never worked afterward, due to the burnt contacts on the back of the tach.

Ah, I see said the blind man as he took up his hammer and saw. Probalby in the cluster as you surmise.