Stuck in limp mode

[pappyam]

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

[Blue Bowtie]

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.

[pappyam]

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

[DeltaP]

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.

[miltonkeynes]

Originally Posted by pappyam

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

[pappyam]

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

[Blue Bowtie]

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.



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