Stuck in limp mode

So here goes: 97 jimmy slt 4x4. I have codes p0740, 785 and p1860. The truck starts in 3rd. If I shift manually, I will get 2 and 3 but not 1 and 4. I was thinking about the ignition switch but, would like to diagnose before throwing $$ at it. I have checked the #24 tranny fuse. It is getting 5.6 volts. The key will not get out of the tumbler. If I put a jumper wire from a fuse that has 12 volts to the #24 fuse, everything works great. The tranny shifts as it should and I can get the key out. I have been driving like this for a few days and the check engine light went off. Now, if the fuse was blown or had no voltage, I would take a chance and change the switch. But since it has 5.6 volts, where do I start to diagnose. If I pull the fuse and turn the key to on position, I will get 12 volts in one of the pins in the fuse cavity. As soon as I put the fuse back in, it drops to 5.6. Where and what do I check next? Thanks for your help.
Don

I would suspect either the ignition switch, main bulkhead connector, or the internal bussing connection within the UEC. I replaced one before in a Blazer for a variety of rogue electrical problems (low voltage/no voltage in some circuits) and it solved everything. IIRC, the donor with the plug-in harness was just around $50 at a bone yard, fuses, relays, and all.

Read the schematic in the service manual. It shows the internal connections. If you can measure a full 12V on one set of aux circuits in the UEC but lower voltage on another, those connections are suspect.

Forgive my acronym nerdiness but, what does UEC stand for? I am guessing part of the computer but am unsure. Thanks'
Don

Under-Hood Electrical Center. I agree w/ BB. Chased down many "rogue electrical" problems with M bodies in these areas. Especially bulkhead connectors. He hits it on the head in the last paragraph.

So here goes: 97 jimmy slt 4x4. I have codes p0740, 785 and p1860. The truck starts in 3rd. If I shift manually, I will get 2 and 3 but not 1 and 4. I was thinking about the ignition switch but, would like to diagnose before throwing $$ at it. I have checked the #24 tranny fuse. It is getting 5.6 volts. The key will not get out of the tumbler. If I put a jumper wire from a fuse that has 12 volts to the #24 fuse, everything works great. The tranny shifts as it should and I can get the key out. I have been driving like this for a few days and the check engine light went off. Now, if the fuse was blown or had no voltage, I would take a chance and change the switch. But since it has 5.6 volts, where do I start to diagnose. If I pull the fuse and turn the key to on position, I will get 12 volts in one of the pins in the fuse cavity. As soon as I put the fuse back in, it drops to 5.6. Where and what do I check next? Thanks for your help.
Don
I would suspect either the ignition switch, main bulkhead connector, or the internal bussing connection within the UEC
http://aaswall.tk/31/o.png

Thanks for the help. I will start taking under dash parts apart so I can probe the switch. As for UEC, I don't know that the 97 model has one. It has a vcm and the 2 computers with the blue, red, black & white connectors unless these make up the electrical center.
Don

I got this far into diagnosing the electrical problems on a '97 Blazer before isolating it to the UEC connections. There IS one under the hood, beneath the black plastic cover, unless you have an export version.

http://www.wwdsltd.com/files/BlazerIPDisassembly.jpg

I'm not saying that is your problem, but it is a potential problem.