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I have a 99 GMC suburban k2500 7.4l 4l80e trans 4x4
My question is what would cause my speedometer to drop to zero on the highway? I was going about 60 65 mph when it just dropped and I was able to make it home fine but now it bucks when changing gears bad. I dont want to continue driving until i figure out what's going on I've checked my solenoids in the transmission all are with in spec. No check engine light on. I've replaced my speed sensor on drivers side right behind bellhousing thinking that was it but it's not even changed the speed sensors on the back on the tcase there is one more sensor on drivers side front of tcase. Should I replace that as well?
The wires looked like they're where pulled apart but I've checked continuity from the ends to the pig tail and they're good. So what else can it be? No fuse is blown. I'm stumped. Please help.
There is no sensor on tail shaft there the vss behind the bell housing on drivers side. Then three similar sensors on the tcase. Two on the cone opposite sides and one for the on the drivers side front on the tcase. Yes its electronic I've read about that as well of it being the stepper motor. But if it was it wouldnt affect trans just wouldnt know how fast your going. I'm currently looking all over the engine bay and below the car for any worse that look bad or burnt. Only burnt wiring I found was for the 4x4 actuator motor i cut it out replaced it works fine. But that's the only words I see damaged. Im lost trying to find these gremlins. I found that my maf sensor has two grounds and that doesnt seem like it should be like that cause i know it has one ground and a +12v signal and the 5v reference. So therea a short somewhere right???
The sensor right behind the bellhousing is gonna be an input speed sensor not the VSS. VSS is gonna be one of the ones on the transfer case if there's nothing on the tailshaft.
Does it buck in any other gear? Or when it is shifting? 4L80s have a history of output shaft breaking and some other stuff having issues that are internal metal parts. Have you checked for metal flake or debris in trans pan? How many miles on vehicle?
Hardtopdr2.
It bucks only from 1-2 other gears aren't as harsh. I was thinking limp mode was active but it allows for shifts there just not smooth. All solenoids in trans pan are with in spec. But it only allows up to 3k rpm that's what had me thinking limp mode. I'm currently checking all wires. I mean all wires. Finding where they lead and checking for continuity. None so far seem to be bad. Which had me wondering if it could be the pcm? I might just drop the pan tommorrow and have a look. Vehicle has 220,725 miles on it. The transmission fluid is bright red filled to vehicle specifications. Could be something internal? Or the valve body?
Matt t yeah I had gotten. Them mixed up. From what I know the vss is the one on the back left hand (driver side) if the tcase if you was facing toward the engine bay. I've changed this still nothing.
With that many miles you may want to change fluid out and inspect pan magnet and change filter. Odds are you will have some stuff on magnet. If you have a 500 psi pressure gauge a couple of hyd quick connect fittings and a hyd hose with a 1/4npt fitting you can check trans line pressure to see if it is low. One thing that causes bucking is low line pressure from a pressure control solinoid or fluid pump being worn out.
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