So on my 1990 240 DL my overdrive stopped working a few months ago because the fuse went out for it. I changed the fuse and it started working again. A couple days ago the fuse went out again. Every time i put a new one in it blows when i turn the key or shortly after. What is the problem? Relay or solenoid maybe?

 

Electrical problems are a nightmare. But, it could be a simple as an intermittent short in the wiring. The first (and cheapest) thing I'd do is chase the wiring as much as I could to see if the insulation has frayed off and occassionally grounds/shorts out. The wire could now be in constant contact with the body, causing "instablow" fuses.

I hope I'm right as this is the cheapest problem.

The problem, of course, is that you'll need some sort of wiring diagram to follow these wires. Could be 1 inch from the fuse block to the trans itself, or anywhere in between. Where it passes through sheet metal are the likeliest culprits, however.