GMC Sierra Transmission Shudders

2004 GMC sierra 2wd. Repairs = Haven't made any attempts at repair yet. Asked a question here before. 4L60 Transmission shudders going from reverse to drive when cold. After it warms up some it doesn't do it as bad if at all. It has started to slip in reverse and forward gears, usually at slower speeds, around corners or starting uphill.

It was starting to act up prior to a filter and fluid change about 3,000 miles ago. The trans pan looked fairly normal as far as the amount of clutch sediment for a 100k service interval. The fluid level is good and no apparent or visible leaks. The fluid also smells/looks normal. With the theft of my scanner, I have no means of pulling any diagnostic codes out of it. Any ideas? Pretty frustrating. Never had any trouble out of it before this.

Answer:
Sounds like a possible problem in the forward accumulator in the valve body. Possibly binding, or a seal has rolled. Or a problem in the forward clutch or forward clutch piston.

There is also a service bulletin for a failed reaction sun shell, gear and or bearing. But this usually will be a no reverse condition. Although the problems you are having may be a pre-cursor to it failing. Would need to be disassembled and inspected for these areas.

GMC Trucks

Question:
Please advise me on how to replace transmission pan seal on a 1997 GMC Sierra pick up automatic transmission. (1500 2 W/D) Thanks.

Answer:
If you are going to do it yourself, be ready to have a mess on the garage floor. The best way I have found is to remove the rear 4-6 bolts first. Then work your way toward the front by removing 1 at a time from each side. When you get to the front 4-6 bolts, back them out less than half-way so the fluid will start to run out the rear of the pan. Let fluid drain out rear for a while, then hold your hand on the pan to position it back flush with the trans., then remove remaining bolts and SLOWLY lower the pan. No real way to not spill.

Clean off magnet in the pan, clean off gasket mating surfaces on pan and trans., install new gasket on pan and raise into position. Install a few bolts by hand so pan will stay up by itself, then install remaining bolts.

Question:
1999 GMC Sierra 4.8L Vortec. Truck falls on its face at 100 mph only for a few seconds then kicks back in again till 100mph then wont take fuel. Could this be throttle actuator or accelerator pedal sensor.

Answer:
Your truck is too old to have an electronic throttle control system. This problem would not be caused by that anyway. It could be the throttle position sensor (TPS) but that will usually set a check engine light and code. You may be running into the Rev limiter, since you are talking about 100 MPH.

The computer would shut off the fuel injectors or ignition to prevent excessive speed and engine RPM. If that is your condition, there is no way around that other than getting an aftermarket PCM re-flash of the computer. Some companies offer a way to get around that limiter, but not all. You would have to check with the different companies to see what they off. It is usually a speed or custom shop that does that sort of thing.

Question:
2005 GMC Sierra. So i have a couple dash lights out and one came back on when i hit the dash but is now off again...also my prnd123 indicator light is usually not fully illuminated but once and a while will be when i start my truck. Any ideas as to what the problem is? Thanks!

Answer:
For the few lights that only come on when you hit the dash cluster, that means the cluster has an internal problem and needs to be removed, sent for repairs, or have a rebuilt unit installed. After either repair, the dash will need to be programmed to your truck VIN # or it will not work properly. The gauges will not work and the odometer will show ERROR and not function.

For the PRNDL shift indicator, that could also be because of the cluster being bad, but can also be a bad gear indicator switch on the side of the transmission. This is pretty common. The switch and connector tend to melt together and cause this problem. But, with the dash acting this way, it would need further diagnosis to confirm that. A scanner with transmission range switch data, a schematic, test light and multimeter would be needed to test the switch.

Another possibility is a bad ignition switch for the shifter lights. That switch feeds power to the cluster and many other things. If it is putting out low voltage, it can cause this problem too. A tap on the head of the key and watching it will show if that is the problem.


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