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4l80e question

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scotto0609

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My 91 cc has started kicking out of overdrive I think. Dont have a tach and really cant hear it well. The speedo stopped woking today and the cruise control wouldnt hold a steady speed also. This has been going on for a few weeks now. Anybody have suggestions? thanks Scott
 
Get a scanner on it, see if the PCM is reporting proper vehicle speed. Sounds like a VSS circuit issue if cruise and speedo went weird. I think the 4L80E relies quite heavily on knowing vehicle speed, and would expect more shift issues than just OD if VSS was gone.

Greg will know for sure. :)
 
I agree the VSS would be the first place I would look, the wiring on these gets sketchy close to the connector.
 
Greg Ducato said:
I agree the VSS would be the first place I would look, the wiring on these gets sketchy close to the connector.
Click to expand...

This. The vss sensor works off of The Hall Effect. Basically a magnet senses the gaps in the tone ring. Being a 91 I imagine there may be some metal filings bridging the sensor. I'd take it out and clean/check it. Cheapest fix there is. :pimp:
 
Check the internal wiring harness connector. This is a large connector located close to the trans pan. The 4L80E has a known problem of seeping and soaking trans fluid in to this connector, and causing the connector to lose the signals to the ECM/PCM, thus causing erratic behavior.
 
Okay since last post the truck has lost the amp gauge and now the audible headlamp warning goes on when lights are selected. checked it out for a few minutes today and the alternator is functioning correctly,it is a little oily underneath by those connections so thats next. Which sensor is the one I take out the forwrd one or rear one on tranny? Will update as I find things out. thanks
 
Yep, I'd say ignore the VSS at this point, you need to figure out where all these gremlins are coming from. Sounds like a ground issue to me as well.
 
Okay since last post the truck has lost the amp gauge and now the audible headlamp warning goes on when lights are selected. checked it out for a few minutes today and the alternator is functioning correctly,it is a little oily underneath by those connections so thats next. Which sensor is the one I take out the forwrd one or rear one on tranny? Will update as I find things out. thanks
 
All right corrected most of the problems.speedo,lights,amp gauge in one fell swoop! It was the fuse! and it was one of those light up to fail ones!:doah: Anyway changed the 2 vss switches on tranny and they looked clean. Should I change the one on tranfer case? The truck is kickin out of od after a mile or so on highway. Next week i am going to drop pan and change filter. Any feedback appreciated. thanks Scott
 
If speedo and cruise are not working then that will be a function of the transfer case speed sensor or wiring, the 2wd 4L80E has a rear speed sensor for this function, the 4WD 4L80E uses the VSS on the transfer case.
 
Ok sorry i forgot to check cruise on test drive will try it out tonight. I did get the speedo back though.
 
Crazy question, but did you check the adjustment of the brake switch?

I assume the 4L80E uses the same switch as cruise to "kick out" TCC on braking?

Only reason I mention this is that my cruise will occasionally not set, and it's because the cruise brake switch eventually unadjusts itself. Pulling the pedal forward is a temporary fix/test, but laying upside down on your back looking at the brake switches is the way to adjust the switch. My pedal doesn't have hardly any play in it, but with the switch being somewhat loose, the tiny bit of play is enough.

Greg, please correct me if the 4L80E doesn't use the same switch.
 
Intermittent overdrive fallout on a gas engine 4L80E can also be caused by a worn distributor housing effecting tach signal to the computer. But check all the cheap easy stuff before you go tearing into anything.
 
I can't believe I made that connection but didn't spell it out lol. OP, are you SURE it's dropping out of OD, or is the TCC unlocking on you?

Based on Greg's agreement with my assumption, if the brake switch is not adjusted properly, or not working right (they are pretty near fail proof) not only will TCC unlock, but you'd also have cruise stop randomly, or not work at all.

I didn't mention it, but when the switch isn't adjusted right, sometimes cruise WILL set, and then just randomly stop working while I'm cruising along. All due to the switch adjustment.
 
Might as well update this. I have since changed the both switches in tranny plus did a filter change. I dont think that fixed it but as I was checking to see what type I has in the transfer case I moved some wires. Took it for a ride and tranny seemed to be fine. So i cleaned all the connections and took another short ride @10 miles an all was well. Also hed to change drivers side exhaust manifold because of warpage. Would love to take it for a long ride but have since lost the tailights and marker lights in rear! Thats another thread thou. Thanks for all the help and I for now will call this fixed!
 
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