97 Yukon...loss of hp to transmission after dash swap

Rider

Original poster
Member
Feb 28, 2017
5
St
Just swapped out dash boards(cracked/broken) on my 97 Yukon & everything seems to be working but tranny. Vehicle engages in each gear but drops in harder w/more force while sitting stationary w/foot on the brake. Very little fwd movement occurs while depressing gas pedal down in drive even when manually putting in 3rd, 2rd or 1st (seems to have a bit more power, not sure if due to lower gear). Reverse seems to be fine. I kept orig ECM, electronics(except PS module) and wiring harness. Check eng light on, cleared memory & after approx 30 seconds it comes on again. Lent out my scanner & will be getting it back tonight. Swapped fuses to rule out, no change. Anyone have any idea's or suggestions as next steps?
 

littleblazer

Member
Jul 6, 2014
9,265
Maybe it's the linkage? It seems coincidental and frankly unlikely that the swap affected the transmission but maybe the linkage somehow was messed up? It seems to me like it isn't fully engaging.
 

Rider

Original poster
Member
Feb 28, 2017
5
St
Hooked up the scanner and received the following codes - P0740/P0753/P0785/P1860. Seems like the ignition module suspect or solenoids. Wondering about potential wiring issues under dashboard possibly having effect (open circuit, wire damaged etc). Recently purchased Yukon and wiring under dash had quite a bit of "personal touches" to it. I did re splice & fix a couple damaged wires but stopped as to not cause additional issues until dash change was complete. Ideas/thoughts?
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
OK, stupid questions that have to be asked.

Did you check ALL the fuses? Not just visually but for voltage/continuity?

Did you check the connector for the transmission?
 

Rider

Original poster
Member
Feb 28, 2017
5
St
Not stupid questions at all Mosseman, can't take anything for granted when troubleshooting. Not yet, I'm off work tomorrow and will be spending time testing/isolating as much as I'm able. Will post back what i find or ask for assistance if I hit a wall. Thanks for the posts/help, I appreciate it.
 

Rider

Original poster
Member
Feb 28, 2017
5
St
Validated 12v passing through fuses & also have 11.9v at tranny connector on pink wire(E). Does anyone have additional testing points or steps I should try?
 

Blackwater

Member
Aug 14, 2015
477
Lawton, OK
since you replaced he dash, you could have jarred or cut a wire from the TCM. It could be there. Then again it the codes call out solenoids and a torque converter. If it where me I would start that after I check the TCM. Then the most likely culprits, the solenoids. Torque converter is a little more a pain in the *** than the solenoids.
 

coolasice

Member
Oct 27, 2013
1,019
Northern Maine
The reason its so underpowered is the transmission has no control voltage. It is defaulting to third gear.

1. With the ignition switch in the RUN position, test for battery voltage at the circuit fuse in the transmission solenoid power supply circuit.
2. If battery voltage is present at the fuse, inspect the ignition voltage circuit between the fuse and the transmission for possible opens.
3. If battery voltage is not present at the fuse, test for continuity between the fuse and the ignition switch. If NO continuity is between the fuse and the ignition switch, repair the open in that circuit.
4. If you have continuity between the fuse and the ignition switch, the most likely cause is the ignition switch.
 
Last edited by a moderator:

Rider

Original poster
Member
Feb 28, 2017
5
St
Update....
I pulled dash out, cleared ECM and it ran normally. I inspected wiring & connectors but found no visible problems. I re taped the harness, especially where it splits or Y's when it comes out of firewall. I did notice a tight fit at the Y where harness fits into a metal raceway. I suspect wire(s) were being pulled there or at the ECM plug as it's a tight fit. The bad part is dash pad cracked by windshield...arrrg.
Thanks for the all the help gents, it's very much appreciated!
 
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Blackwater

Member
Aug 14, 2015
477
Lawton, OK
That's a bummer on the dash.
 

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