4L60E Won't Downshift

[CHEVY_1999_K1500]

That seems to be what it's doing. I can manually shift but no longer shifts by itself. Will let group know what I find so hopefully it helps somebody else.

 

[tayto]

there is no manual shifting through forward gears when in limphome mode.

 

[CHEVY_1999_K1500]

Schematic from Factory Service Manual for 4L60E Solenoids. Power is from Fuse 20 off Instrument Fuse Panel Box (which is labeled PRNDL - NOT Transmission). Fed off ignition switch. Tests good to the Instrument Panel Fuse with Ignition On. Ran out of time tonight but need to get under truck tomorow and take plug off 4L60E and test there. Is throwing a bunch of codes right now (both transmission upshift solenoids and Torque Converter Lockup). More to come when I have more time to diagnose.

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[CHEVY_1999_K1500]

Mystery Solved.

You bet it was in limp mode (good call by several people). The last shop which told me it was a bad ECM were correct.

Unfortunately, they forgot to unplug their 4L60E Test Plug and plug the vehicle wiring harness back in. Once I did that, it runs like a champ.

Only fault code left is the crankshaft relearn code which the ECM Supplier warns must be reset. It says you need a Tech II (or advanced Scan Tool) to reset, so I'll have to pay somebody to do that. If anybody has any recommendations on reasonable price advanced scan tools would appreciate that.

Thanks for all the help and advice. Great to have a forum like this.

 

[CHEVY_1999_K1500]

Also meant to mention that I have the complete (four giant paper books) Factory Service Manual for the 1999 C/K Trucks/Suburban in case anybody needs a troubleshooting chart or schematic. It's almost too much information (over a 1000 pages).

 

[tayto]

I stand corrected. There is manual 2nd in limphome, my bad.

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glad you got it sorted out.

 

[CHEVY_1999_K1500]

No worries, thanks for helping. I could manually shift to 2nd with the cable and then manually to 3td. I drove it that way for awhile and was just careful. No obvious damage (yet). Didn't seem to overheat the engine - not sure what it did to the tranny. Seems toi run great now (for the time being).

 

[tayto]

when in limphome mode the trans should run at full pressure. might be a bit hard on the gear train, but clutches shouldn't slip or anything like that