Tachometer / Clock Not Working When Instrument Lights On

[hamiltonian]

Hello there. I was hoping to get some advice on the potential cause of the following:

I have a 1984 240 DL. One of the previous owners installed a tachometer in place of the large clock. Several months ago, the instrument panel lights for the gauges went out, one by one. I thought it was simply blown bulbs, and didn't replace them right away. Around the same time, the tachometer and clock suddenly died. However, if I turned the rheostat all the way down (so that the lights on the dash received no power), the tachometer and clock would work.

As a check, I bypassed the rheostat, and the tachometer and clock no longer functioned. However, I discovered that if I held the light switch just above the off position, but not on the "parking light" position, the tachometer and clock start to work again. In this position, the console lights (and the daytime running lights) are both off.

So, it appears that the tachometer and clock will only work when no power is being fed to the instrument/console lights and running lights.

Another possibly related issue: When the light switch is in the off position, my daytime running lights do not glow evenly. The RH light is much brighter than the LH light. (see picture). However, when I turn the light switch to the parking light or headlight position, the brightness of the lights evens out.

link to picture: https://drive.google.com/file/d/0BzZ...ew?usp=sharing

[Clan]

Originally Posted by hamiltonian

Hello there. I was hoping to get some advice on the potential cause of the following:

I have a 1984 240 DL. One of the previous owners installed a tachometer in place of the large clock. Several months ago, the instrument panel lights for the gauges went out, one by one. I thought it was simply blown bulbs, and didn't replace them right away. Around the same time, the tachometer and clock suddenly died. However, if I turned the rheostat all the way down (so that the lights on the dash received no power), the tachometer and clock would work.

As a check, I bypassed the rheostat, and the tachometer and clock no longer functioned. However, I discovered that if I held the light switch just above the off position, but not on the "parking light" position, the tachometer and clock start to work again. In this position, the console lights (and the daytime running lights) are both off.

So, it appears that the tachometer and clock will only work when no power is being fed to the instrument/console lights and running lights.

Another possibly related issue: When the light switch is in the off position, my daytime running lights do not glow evenly. The RH light is much brighter than the LH light. (see picture). However, when I turn the light switch to the parking light or headlight position, the brightness of the lights evens out.

link to picture: https://drive.google.com/file/d/0BzZ...ew?usp=sharing

I think you will find it is a faulty earth wire in the three pin plug which fits near the tachometer .

[hamiltonian]

Thank you. Is this the plug that fits into the back of the tachometer? I will examine that.

[Clan]

Originally Posted by hamiltonian

Thank you. Is this the plug that fits into the back of the tachometer? I will examine that.

yes , the black wire should eventually go to earth , I fixed many of these back when the 240 was current and for a few years after . .

[hamiltonian]

Hmm - the three way plug into the back of the tachometer appears to be connected properly and have no loose wires. Is the ground for this plug inside the instrument cluster or the tachometer itself? I've checked slot #10 on the circular plug on the back of the instrument panel, and it is firmly attached. Also checked the main ground for the instrument panel near the gear selector. But no change.

[Clan]

Originally Posted by hamiltonian

Hmm - the three way plug into the back of the tachometer appears to be connected properly and have no loose wires. Is the ground for this plug inside the instrument cluster or the tachometer itself? I've checked slot #10 on the circular plug on the back of the instrument panel, and it is firmly attached. Also checked the main ground for the instrument panel near the gear selector. But no change.

if you ground the black wire it should work , I cannot remember if it grounds through the circuit board and multi plug or takes a direct route .