In this page you will learn why GM codes P0740, P0753, P0758, P0785 and P1860 when they occur at the same time, the problem is not in the transmission, but they are the result of a faulty ignition switch.
This is what these codes mean ( for your reference) :
P0740 : TCC clutch circuit malfunction
P0753 : Shift solenoid A electrical
P0758 : Shift solenoid B electrical
P0785 : Shift/Timing solenoid malfunction
P1860 : TCC solenoid electrical fault
If only one of these codes appears when you scan your vehicle, it is possible that the problem will be a malfunctioning solenoid in the transmission, but, like we mentioned earlier, if all of these codes appear at the same time, the problem is caused by a malfunctioning Ignition switch, the same circuit that provides the electrical current to these solenoids provides the electrical current for some of the gauges in your dash, ( circuit 1020 ) if the gear selector indicator light is not working, the tachometer, speedometer are not working, those are also signs of a faulty ignition switch.
Usually the transmission will default to third gear when this problem occurs, this particular problem can be found in several GM vehicles including the following:
2004-2005 Buick Rainier 1999-2000 Cadillac Escalade 2002-2005 Cadillac Escalade, Escalade EXT
2003-2005 Cadillac ESV 1997-1999 Chevrolet Full size pick-up,Suburban,Tahoe( old style)
1997-2005 Chevrolet Blazer, Corvette, Express, S-10 1999-2005 Chevrolet Silverado ( new style)
1999-2005 Chevrolet Blazer RH Drive ( Export) 2000-2005 Chevrolet Suburban, Tahoe
2002-2005 Chevrolet Avalanche, Trailblazer, Trailblazer EXT 2004-2005 Chevrolet Colorado
1997-1999 GMC Sierra, Suburban, Yucon ( old style) 1997-2001 GMC Jimmy
1997-2005 GMC Savana, Sonoma 1999-2005 GMC Sierra ( new style) 2000-2005 GMC Yukon, Yukon XL
2002-2005 GMC Envoy, Envoy XL 2004-2005 GMC Canyon, Envoy XUV 1997-2004 Oldsmobile Bravada
2004-2005 Pontiac GTO 2003-2005 Hummer H2
Equipped with 4L60-E, 4L65-E, 4L80-E Automatic Transmission ( RPOs M30,M32,M33,MT1,MN8)
Important: Take note that sometimes this problem occurs when installing an aftermarket remote starter or alarm system.
The steps shown below explain how to test the ignition switch in a 1997 Chevy Truck, you must realize that these steps will be similar for all the other vehicles listed above, this truck is not equipped with aftermarket devices, so the troubleshooting was quite simple.
The truck we are working on was experiencing the symptoms described earlier: transmission stuck in third gear, malfunctioning instrument cluster ( gauges and gear indicator ).
The first thing we did was to scan the truck, the codes we found were: P0740, P0753, P0758, P0785, P1860.
We tested for power circuit 1020 ( the fuse showin in this picture), to test for power, turn the ignition switch on and using a test light, test for power, if the fuse has power, the problem will be in the transmission most likely, but if the fuse doesn't have power, the problem is very likely to be a bad ignition switch.
AS you can see in this picture, the truck we are working on didn't have power at the fuse.
Next, remove all the screws that hold the lower dash cover in place.
Pull gently on the lower cover, if you want to remove it completely you will need to unhook the emergency brake cable, otherwise, leave the cover hanging down.
Next, remove the steering weel tilt lever and set it aside.
Next, remove the screws that keep the lower steering column in place.
Next, pull gently on the lower steering column and set it side.
Pull the plastic cover off the ignition switch, turn the ignition on, and test for power on all circuits, the wire you are lloking for is the white wire located at the top, that is the wire that feeds the transmission and the instrument cluster.
What we are doing here, is using fused jumper wire and the test light, we are sending power to the white wire from the red wire ( as you see in the picture), we started the truck to see if all the gauges worked, and just like we suspected, they all worked fine.
To be 100% sure that our transmission problem was in the ignition switch, we connected the fuse jumper wire from the red wire to the white wire as shown in the picture and took the truck for a test drive, the transmission worked like a brand new, in the next page we will illustrate how to replace the ignition switch.