Dear Board,

Some time ago, I got the notion to install a tach (from a 85 GLT) in my 86 240 DL. Simple enough I thought, put everything back and tach worked great! I turn on the head lights and the tach immediately dies, both turn signal indicators start glowing and the high beam on indicator starts glowing. Shut off the head lights and function to the tach is restored, turn signals and high beam indicator no longer glow. Fearing I shorted something out, or power was somehow shorting through the tach, I yanked the tach and stuck the clock back in. A few weeks later when I had more time, I went back to this, again with the same results. Except this time, I left the the lights on, and after a few minutes smoke started pouring out the back of the instrument cluster. Oops! Turned the car off, pulled the negative, tore into the instrument cluster and found a diode? on the back of the speedometer circuit board completely cooked. I pick up another instrument cluster from the local Volvo shop, install the tach and everything was fine. Hmmm...the other day, I was back in the instrument cluster yanking the temperature compensation board thinking that was the reason for the jumpy temp needle (turns out the sending unit was bad), put everything together and its fine. Driving along the next night, all of a sudden, I loose all instument cluster lights, the tach dies and the highbeam indicator as well as both turn signal indicators are lite bright! What is going on here? I have plugged all the wires in their correct places, including the single yellow/red wire to the 1/4 terminal in the lower right corner (below fuel gauge), and the white wire with red stripe to the pick up on the tach. I even went as far as to cut out the pegs on the circuit board feeding the tach and brought power and ground to the tach by means other then the circuit board and this this is still acting up. What did I knock loose, short out, or otherwise screw up when installing the tach/IPD temp board bypass? Help! Thanks!