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4L80E won't shift until it is warmed up

JohnShead

JohnShead

Member
Messages
64
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Location
Kingsport, TN
My truck intermittently started having erratic speedometer behavior, a flickering check gauges light, no gear selection light, and occasionally no tach. I thought my grounds were going bad again so I cleaned them up and checked voltage drop across them and they were good. After about 10,000 miles of this it started refusing to shift into 3rd giving DTC 81 and sometimes DTC 82. For a while I could clear the codes and it would work fine. It got to the point the code wouldn't stay cleared until the transmission had warmed up by driving for about 20 minutes, then it would work fine until I parked and it cooled down. I rigged up a box with 3 toggle switches to manually operate the shift solenoids to attempt to keep the truck operational a bit longer but it still wouldn't shift into 3rd until the transmission had warmed up after which I get all gears including torque converter lock up. With this device I discovered that when the truck is in a different gear than the ECM is calling for it causes the speedo, tach, gear indicator, gauges light issues mentioned earlier. I checked all of the wires from the ECM to transmission for resistance and they were good. I checked voltage at the transmission connector and there is 12V. The transmission has a good ground. At this point I'm still blaming electrical because of the codes so I replace the shift solenoids. While I was inside the transmission I checked the internal wiring harness and I get 12V at all the connectors when I call for that solenoid with my manual shifter. The new solenoids did not change the behavior so now I think it has to be mechanical. I have the valve body out now and the 1-2 and 2-3 valves are not sticking. Has anyone heard of anything like this or maybe have an idea of how to fix it?
 
You have a power or ground issue somewhere in the vehicle. The ecm has no input to the cluster other than the ses light and sts light. The speedo receives it's signal from the vssb which also sends a signal to the ecm, radio, and abs. The tach comes off the alternator, and everything else runs off it's own senders. The only way for all of these things to interconnect is either power or ground.
 
How do all the dash symptoms happen as soon as I switch on the 2-3 solenoid? I see the ECM calling for 2nd gear on my scanner and everything is working when I'm in 2nd, as soon as I hit the 2-3 switch speedo & tach go to zero, check gauges on, gear indicator off. That's when it's not shifting, after it warms up and starts shifting when I hit the 2-3 switch I get 3rd gear and the dash is normal.

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Sounds to me like you have a power issue. Turning the 2-3 solenoid on is pulling the voltage down. The cluster and trans I believe both receive power from the same portion of the ignition switch as well as the vssb. Sounds to me like you need to get a voltmeter on that power circuit when it's acting up. If the solenoid was shorted out, I would expect it to pop the 10 amp fuse in the fuse panel inside the truck, unless somebody has put a larger fuse in it.
 
Fuse is 10 amp and doesn't blow. I'll look into power. Scanner says system voltage is 13.1, idk where is picking it up though.

Sent from my Pixel XL using Tapatalk
 
You're going to have to check the voltage going to the trans when it does it. The systems you arehaving problems with are all powered via thefuse panel inside while the ecm is all in the underhood panel.
 
Problem solved. It was the ignition switch again. It just had me confused because it so predictably would work after it warmed up. The switch wasn't that old either, it didn't last nearly as long as the original one did.

Sent from my Pixel XL using Tapatalk
 
Glad to see you got it figured out. The electrical gremlins are a pain sometimes to figure out
 
I knew it had to be power supply based off of what you were saying was happening. The trans is still powered by the ignition switch via the fuse panel inside the truck, so when you said your manual box did the same thing, that meant it had to be power supply.
 
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