Hey all,
I have a 1999 Blazer that is popping a P0785 after it warms up. During warm up it shifts fine, 1-2,3-4 then back down no problem. After reaching operating temp (~195) it will pop this code, even if I'm in park.

I've skimmed over https://blazerforum.com/forum/diagno...plained-28394/ but need help understanding a few tests.

Before I go popping the pan off and testing the actual solenoid and the wire harness in the pan, I'd like to test the wiring going from the transmission to the PCM. I'm not sure which and what wires connect to which of the four connectors on the PCM. There was a squirrel nest right next to the PCM so I feel like this is my best place to start.

Thanks everyone!

 

Went ahead and pulled the pan off. Checked the ohms on the 3-2 shift solenoid and get 22.8 ohms which I believe in in spec, so I connected it back for the time being.

I pulled the PCM out and used my multimeter to check for a short. I'm not sure if I did this right so maybe someone can check my work?

I turned the key to "On", switched to voltage and put one probe on a ground and probed pin 13 (White wire) on the blue connector. 0 voltage. My understanding is I should have got something here. I'm not the best when it comes to electrical diagnostic.

 

Probably should have gone ahead and put the 3-2 coast down solenoid and 2 new shift solenoids in while you are there. Good possibility you had correct resistance while cold. But not once warmed up as stated in your first post that it would set the code when at operating temp. Also behind the 3-2 coast down solenoid is a valve and spring. This spring is commonly broken. Be very careful probing wires with power on them especially at the PCM. Not every thing uses a 12 volt signal. You won’t have any power until the PCM commands the 3-2 down shift to occur and then it is a PWM signal that starts at 90% and drops. Very hard to see with a multi meter. Also bad ignition switches will set this code but as a rule of thumb other codes will follow. Are you checking the correct solenoid?

 

Hey Transman304, thanks for the reply. I triple checked I was testing the correct solenoid and I will put the new one in once I figure out the problem. The A&B will also be replaced along with the harness. I really don't want to have to take the pan off again.

I put the old solenoid back on strictly of diagnostic purposes, I need to get out of the habit of just firing the parts gun at a problem and hoping it fixes it.

I'm following this post over at chevy trucks. It has some helpful diagnostic information. The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - '97 4l60e being strange... shift solenoids

I'm going to continue with the diagnosing when I get home from work this afternoon to see where I end up, but like you said the resistance might change when it's hot so I might go ahead and change out the solenoids/harness and close it back up and see if that resolves the issue.

 

Well, I replaced all the solenoids and the harness. Bolted it back up and went for a drive. All was looking good after a few miles at operating temp 1-2-3-4 and back down no problem. Parked it for a few hours and not wanting to call it fixed just yet I went for another drive. 3 miles into the drive popped a P0785 again.

I'm running low on ideas on what could be causing the issue.

Maybe it's a small break or something loose. Gonna just start shaking wires tomorrow to see if I can get the code to fire.

 

Sorry for the multiposting. Just wanted to update with what I've tried lately.

I gave all the wires a good tug coming from the PCM to maybe knock loose a short or broken wire causing the intermediate problem, no dice, code fired again. I let everything cool off and cleared the code. I warmed the PCM up with a hair dryer thinking maybe a transistor or something in there expands when it gets warm or something. After warming the PCM up a little bit, I got on the road and kept speeding up and slowing down watching the gears change 3→2 without issue. As I was slowing down for a stop sign I went all the way down to first gear and P0785 fired. It's only a "Pending" code, but it still fires the MIL and goes into limp mode.

I'm gonna find someone next week with one of those fancy scan tools that can command gears.

I know this code is an electrical malfunction, but can this be a sign the transmission is going bad?

EDIT: Just went through this vehicle's maintenance history I found in the glove box. The ignition switch has been replaced before. I wonder if they cheaped out on parts or half-*** installed that might be causing this problem...

 

Thanks for the update. When the ignition switch fails you will get several solenoid codes not just one USUALLY. As it is dropping power to the transmission for a split second and communications are lost. When you took out the 3-2 down shift solenoid did you happen to pull out the valve and spring behind it? Did you replace the wiring harness with a new one or used? A proper scanner with the right software and live data stream will prove to be the quickest way to diagnose for sure.
Good luck and keep us posted.
Brad

 

I pulled the valve and spring out, they both looked good. The wire harness is brand new.

I acquired a used PCM, had it key'd to my VIN and slapped it in real quick. After the 10-10-10 relearn I went for a drive. I was getting up to speed, it wouldn't shift into 3rd then threw the code P0758 and went into limp mode . PCM was untested so I'm taking this occurrence with an extreme grain of salt, but it's strange it's another transmission solenoid code. Wish it would have got to third at least once to check the down shift.

I put the old PCM back, went for a drive and got the P0785 again.

Oh well, I wasted the whole weekend on hacky diagnoses. I'm gonna get someone who knows what they're doing and mark this up as an L and Learn from the experience. I'll post when I find out more.

 

Hey everyone,
Finally got around to messing with this Blazer again. I couldn't convince anyone with a scanner to come out here to the boonies and scan it and I didn't wanna drive it too far. So it has been sitting for a while while I decided the next move. Using methods found here I did some thorough testing and nothing made sense. I decided to gamble the $75 on a new PCM and slap it in. I drove it around today for about 30-45 minutes over 3 drives and no codes popped. I am cautiously optimistic this solved the issue. I still find it very hard to believe the two previous PCMs were both bad.

I'm gonna go drive it some more when the sun goes down because the AC is also broken and it's summer in the south.