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Old 06-27-2010, 09:21 PM
CF Beginner
Join Date: Jun 2010
Location: Maryland's Eastern Shore
Posts: 2
Default HELP!!! OBD II Codes Suggest Transmission/Transfercase Troubles

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2001 Silverado Z-71, 4 W/D, Automatic Transmission

Yesterday while pulling boat and trailer from water up ramp with my truck in 4 wheel drive high, the transmission seemed to not engage correctly. I placed transmission in 2 wheel drive and then back to 4 wheel drive high. The truck pulled boat and trailer up ramp but transmission again did not seem to engage correctly and again I returned transmission to 2 wheel drive. I let the truck run while I prepared the boat for travel. When I returned to the cab, the check engine light was on. I installed OBD II code reader and the following codes were activated.....PO 740 (TCC circuit malfunction),
PO 785 (Shift/Timing Solenoid Malfunction), P1860 (TCC PWM Solenoid Circuit).
Please advise me of troubleshooting procedures if applicable or if my transmission is about to take another crap??? Thanks In Advance for Your Assistance! Gorilla
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Old 08-01-2010, 07:53 PM
CF Beginner
Join Date: Aug 2010
Posts: 4

I see this problem often. It also happened to my 1999 Suburban. On my Suburban, it was the power circuit 1020 to the shift solenoids. I had trouble codes P0740, P0753, P0758, P0785 and P1860. The tach and speedo did not work either. On the Suburban, there should be +12volts to the #20 fuse on the IP fuse box. If not, the ignition switch needs to be replaced. If so, the 1020 circuit (pink wire) needs to be traced to the tranny (4L60E). My problem was a short in the circuit.... there was 12.7 volts at the fuse box, but only 1 volt with fuse in place. It was grounded, I don't know why it didn't blow the fuse. If power is good to the IP (instrument panel) fuse box, make sure there is power to the circuit at connector 122 @ the tranny, should be a pink wire. The 20-pin connector has been known to go bad. I have attached the wiring diagram for the 4L60E on the Suburban, should be similar to the Silverado. If I can find the TSB for this problem I will post it.
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File Type: pdf suburban.pdf (85.4 KB, 345 views)
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Old 04-26-2011, 07:29 PM
CF Beginner
Join Date: Apr 2011
Posts: 2
Default 97 suburban with same codes/problems

RE: jeremyt....
Hey man. I have a 97 suburban with similiar problems as your 99(tach and speedo dont work when the tranny starts to slip w/codes P1860,P0740 and P0753. I tried your method of putting volt meter to the #20 fuse and got a reading of 11.71V. Do you think the -.29v would cause the ign switch not to send proper signal to the shift solenoids? Im stuck between buying a $155 part or bringing it to a tranny specialist who may not diagnose it right
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Old 04-27-2011, 07:51 AM
CF Beginner
Join Date: Aug 2010
Posts: 4

Quick Diagnostic: If you can remove the lower plastic piece on the steering column, it will reveal the ignition switch on the right hand side. With the motor running, tap the ignition switch a few times with the butt of a screwdriver and see if your gauges come alive. If they do, which they probably will, replace the ignition switch. If not, tranny specialist. Hope this helps!!
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Old 04-27-2011, 10:50 AM
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Join Date: Apr 2011
Posts: 2

After a few taps, the Tach came back on but engine light is still on. I'll try a new ignition switch..thanx
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Old 04-27-2011, 10:50 AM

740, astro, chevy, circuit, cod, code, ii, malfunction, obd, p0753, po, po785, repair, suburban, tcc, van

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