Roger, you are a f*@#ing genius.

I was having an intermittent problem with my 1992 Volvo 240 DL not switching into 4th gear. I paid $100 for a transmission flush + $30 for a kick-down cable adjustment which made it work again for a couple of weeks, but one day, after work (which was a warm sunny day) it failed again. When I called the mechanic to report the problem happening again, he said that the it must be the transmission and that it needed to be replaced with a rebuilt one to the tune of $775.

(Here's a note: Adjusting the kick-down cable isn't going to do anything if it isn't stuck (springs back freely), it isn't hanging slack, and it engages when you step on the accelerator all the way (you can feel it when you step on the gas pedal at the very end of the stroke - it's a "click" feeling, not a sound). All the kick-down cable does is bring the car down into third gear as a "passing gear" to get past other cars on the highway when you stomp on the gas. If it feels / seems to be working correctly, it's probably not that.)

So, I figured I may as well try Roger's fix before I replaced the damn transmission anyway.

First, I wanted to eliminate all the other possible problems. I checked the voltages on the relay switch behind the glovebox with a voltmeter in accordance with this other post on another forum: http://www.thevolvoforums.com/volvo-240/8789-1992-240-no-fourth-gear.html I didn't get any big voltage drops when I switched the overdrive on and off so the switch seemed ok with these readings:

Pin 31 - Ground
Pin 15 - 11.89V when on; 11.81V when off
Pin 87a - 1mV when on (the arrow lit up on the dash); 11.81V when off
Pin 87 - 0.9V when on; 11.81V when off
Pin 86 - 0v when on; 0V when off (the meter would jump when I hit the switch, but would settle back to 0 after a couple of seconds)

Then, I checked the arrow light on the dash and it turned on and off every time without fail.

Finally, I lifted the car and checked the wire coming from the shifter and going to the solenoid - it looked in decent shape.

The next thing I did was get under the car and got my head close to the solenoid and had my wife push the overdrive switch a few times and listened. The solenoid was definitely clicking, but it was much louder when the overdrive would engage (the arrow would light up on the dash). Since my problem was intermittent, it could have been working this time and not others, but I had a suspicion (because of the difference in the volume of the clicks) that the solenoid wasn't opening fully.

I decided to pull the sucker out and have a closer look. I had to buy a seriously angled box wrench to get at the solenoid bolts - which I found at the local auto parts for $7.99 - and went to work. I bought some brake cleaner and a wire brush for $4 total and did my best to clean out the area before I pulled out the solenoid.

NOTE: The cap on the solenoid DOES NOT COME OFF, so don't try to pry it off with a screwdriver. The cable is part of the assembly and has to come out with the solenoid.

After disconnecting the connector on the other side of the transmission and removing the two cable clips I carefully pulled the thing out being careful to not get any gunk in the holes. The o-rings stayed stuck on the transmission, so I carefully(!) pulled them both out, too.

I took some alligator clips and connected the chassis of the solenoid to the negative on the battery and connected the other clip to the solenoid wire. I tapped this clip on the positive terminal of the battery and I could hear/see the little piston inside the solenoid moving. It seemed to work ok, but I am so fed up with this problem that I decided to move ahead with Roger's fix.

Here is a picture of the original solenoid (no alterations): http://www.omargarciaportfolio.com/Solenoid/SolenoidUntouched.JPG

I used a Dremel with a metal burring tip to "connect" the holes and the result looked like this (after cleaning with brake cleaner, wire brush, compressed air and a paper towel):
http://www.omargarciaportfolio.com/Solenoid/SolenoidRouted1.JPG
http://www.omargarciaportfolio.com/Solenoid/SolenoidRouted2.JPG
http://www.omargarciaportfolio.com/Solenoid/SolenoidRouted3.JPG
http://www.omargarciaportfolio.com/Solenoid/SolenoidRouted4.JPG
http://www.omargarciaportfolio.com/Solenoid/SolenoidRouted5.JPG

I cleaned everything and put the big (outer) o-ring back in (it was in good shape and stayed in place nicely) and I decided to not use the small o-ring at all again and stuck it my toolbox.

I carefully(!!!) cleaned the area on the transmission one more time before remounting the solenoid and bolted it down. I reconnected the cable (although it does nothing now) just so that everything was where it should be.

I took her out for a highway drive and, lo and behold, in and out of 4th gear like a champ!! I noticed that it switched a little later (at 50 MPH vs. 45 MPH before the fix), but that's no problem for me.

I must admit, I was reluctant to do this fix until I found this:
http://www.ipdusa.com/products/4793/110397-automatic-over-drive-solenoid-bypass-kit

IPD is selling THE EXACT SAME FIX for $39.95! It is just a cap with a channel routed in to permanently connect the two holes in the solenoid.

Summary: if you aren't worried about towing heavy trailers or driving up and down mountains, you can disable the solenoid with Roger's fix above. You can do it in two ways: buy the IPD Overdrive Solenoid Bypass Kit for $40 or route it out yourself. If you are having problems with 4th gear and have checked everything else in the overdrive circuit, your solenoid is probably bad anyway, so there's no point in saving it - you'll have to buy a replacement at some point if you want the "overdrive" feature.

In any case, my problem seems to be solved - I'll post back in a few weeks to confirm the repair. But if the problem keeps happening, I'm may have to cough up the big bucks.