Tranny issue

[01SIERRA]

So when driving around 45-50 mph my transmission can't decide which gear it wants to be in. It constantly shifts back and forth from fourth to fifth(overdrive?) until i either speed up a bit or engage the tow/haul switch and then it goes away. Would this be a computer issue or something internal? TIA!

 

[01SIERRA]

Its a 1500 non-HD, 5.3, auto, 4x4

 

[Molson02536]

So when driving around 45-50 mph my transmission can't decide which gear it wants to be in. It constantly shifts back and forth from fourth to fifth(overdrive?) until i either speed up a bit or engage the tow/haul switch and then it goes away. Would this be a computer issue or something internal? TIA!

Put a transmission line pressure gauge ($55 shipped on Ebay, or visit a local trans shop) on the pressure test port and go for a drive to verify proper line pressure. Base pressure at idle in P,N,OD should be around 70psi and around 140psi in R. Max line pressure should be at least 190psi at WOT in 1st or 2nd, 225psi is optimum. If base pressure is low, trans or pump is worn out. If max pressure is low, check EPC solenoid. If reverse is slow to engage, and line pressure does not come up to at least 130psi in reverse, replace the boost valve with a Transgo 0.500" boost valve (easy fix in pan).

Check TPS for smooth and linear electrical response over the entire range of motion. If not, replace the TPS. Log a Tech2 scanner to see if the PCM is doing it, if not, increase line pressure for fourth at low MAP values. You might get luck also if you just need your transmission serviced, due to a dirty transmission filter and replace your boost valve also.

When you use your tow mode, it rises your line pressure by lowering your voltage to the EPC valve in your transmission. Cross the fingers and hope that a transmission service and a boost valve replacement will cure your transmission issues. Common for a worn boost valve to kill the 4L60E transmission by having it slip and gear search like what you are experiencing.

You can remove the checkball from its cage in the case in the rear of the transmission may help (must remove valve body), removing that checkbal firms up TCC engagement. It makes TCC lockup feel more like an upshift. The checkball was put there to soften the TCC feel, I dont mind a more firm feeling myself, I have done this mod myself long time ago when my Jimmy was new and the only draw back was braking traction on wed slippery roads when transmission shifts gears with the skinny peddled 3/4 down.

http://www.compnine.com/index.php?u...delid=7036&capuid=1&majorgroup=04&grouptype=B
http://www.compnine.com/index.php?u...delid=7036&capuid=1&majorgroup=04&grouptype=B

 

[01SIERRA]

Thanks for the info! Wow. Well I just did a fluid and filter change last month, and reverse does engage abnormally slow I've noticed. I'll get started on that boost valve and check my TPS and line pressure. You are Awesome Molson!

 

[Molson02536]

Thanks for the info! Wow. Well I just did a fluid and filter change last month, and reverse does engage abnormally slow I've noticed. I'll get started on that boost valve and check my TPS and line pressure. You are Awesome Molson!

Just been around guys frying stock transmission for getting more HP out of there motor and forgetting to work on the transmission and biggest transmission shop secret is to replace the boost valve every time you service the transmission. .

The reverse boost valve and sleeve in the pump should be replaced every 40,000 miles (65,000km) to avoid damage to the transmission. Damage can occur when wear in the boost sleeve causes insufficient line pressure in reverse. Many major repairs of the 4L60E at around 50,000 plus miles are because of the boost valve and sleeve. The valve and sleeve can be replaced easily during a routine service of the transmission for around $50. This should be considered a necessary procedure if you are transplanting one into your car. Oversized (0.490" versus the stock 0.470" or 0.420") reverse boost valves are available that provide a moderate increase in pressure, and are a good addition to any 4L60E.

As well, the pressure regulator valve (located under the reverse boost valve) should have two small 'flats' (~3/16" wide) ground on the second land - counting the lands from the end opposite that spring seat (or alternatively it should be replaced with an updated valve that has these flats). Remove and discard the 3/4 load release spring assemblies (5 small rectangular spring assemblies installed into the input housing, on the outside of the clutch pack). This really improves the 2-3 shift. Since your in there, and don't let a transmission shop take you to the cleaners for these updates, so call around. these things should not be more then $225.00 with the extra work on to a transmission oil filter service.

 

[01SIERRA]

Thanks for the tip. I may have to go to a transmission shop since I've never even pulled a pan before. Its the only part of a vehicle I've never messed with.

 

[Molson02536]

Thanks for the tip. I may have to go to a transmission shop since I've never even pulled a pan before. Its the only part of a vehicle I've never messed with.

Since you mentioned that you just had your transmission serviced, you didn't have it flushed and just the oil and filter changed??? Flushing the transmission will in most case cause the checkball to stick after and cause what you have described. You might get luck by adding a transmission fluid additives to get the checkball not to stick, additives not like Lucas which is thicker. But additives that clean your valve body, just read the transmission additives.

http://www.gunk.com/CAT_M3616.asp

 

[01SIERRA]

They supposedly only did a fluid and filter change, I turned down the flush.
So I replace the TPS today and took it to the most reputable tranny shop in town and they found no trouble codes and took it for a drive and couldn't get it to reproduce the problem, its barely noticeable anyway but it still drives me nuts.
He also stated that upgrading the boost control valve or AFL valve wouldn't have any affect on line pressure. Then I started to distrust him so I went to an automotive repair shop that has treated me well and had them do a diagnostic test on the engine and they also could find no codes logged in the computer or symptoms of any misfires.
I see now that I may have to upgrade the boost control valve on my own. Are there any write-ups on this job anywhere? Should I do the AFL valve too or would that not help. Can these thing be done by just dropping the pan or does it get ridiculous in there?

 

[Molson02536]

They supposedly only did a fluid and filter change, I turned down the flush.
So I replace the TPS today and took it to the most reputable tranny shop in town and they found no trouble codes and took it for a drive and couldn't get it to reproduce the problem, its barely noticeable anyway but it still drives me nuts.
He also stated that upgrading the boost control valve or AFL valve wouldn't have any affect on line pressure. Then I started to distrust him so I went to an automotive repair shop that has treated me well and had them do a diagnostic test on the engine and they also could find no codes logged in the computer or symptoms of any misfires.
I see now that I may have to upgrade the boost control valve on my own. Are there any write-ups on this job anywhere? Should I do the AFL valve too or would that not help. Can these thing be done by just dropping the pan or does it get ridiculous in there?

NO worried for now as you mentioned that the issues goes away when you use your tow mode. Just use it if it's doing searching gears as you mentioned between 3-4 and with it on the searching goes away. With the tow mode on, the PCM will send less current to the ECP in the transmission to raise the transmission line so your transmission stay in the gears longer and not search between gears.

As mentioned the Boost valve and sleeves after so many miles will wear out and your transmission line will drop and the ECP can no longer compensate for it and that's when your start to have the gear searching between 3-4. Next time you have your transmission serviced, have them do the upgrade i mentioned earlier to keep your transmission running 110%. I have the original transmission in my 96 Jimmy with 365,000 miles and still going strong with replacing the boost valve every transmission service at 60,000 miles.

Yes i have done the up grades mentioned earlier and shifts like a Corvette, but hey, i can live with it and the Smile it gives me as the stop sign warrior Oh and there will not be a DTC codes unless there is transmission slippage or shift issues, the scan should have been to see what perimeters your transmission was in when it was gear searching as to the Engine RPM and Load and Vehicle speed. A Tech2 can reprogram you transmission to shift at a lower RPM or higher RPM so you will not have that gear searching issues at that speed you have indicated, this is not in a transmission shops best interest cause your transmission will last a life time.

 

[01SIERRA]

That last paragraph probably sums it up. Now I guess I'd better get dirty. Is that boost valve something I can replace in the driveway?

 

[Molson02536]

That last paragraph probably sums it up. Now I guess I'd better get dirty. Is that boost valve something I can replace in the driveway?

It's going to be messy as you have to drop the transmission pan that's why i was trying for you to wait till you get the transmission serviced again and have transmission shop you can trust. But here is a link for you to understand how to replace the boost valve and the other mods i have mentioned.

Feel free to asked more questions and it's not a hard job to do with a pit of a lift to get under the Sierra. I may self hate showering in transmission fluid so i wait till i get my transmission serviced to replace the most valve every time.

Keep me posted and be safe friend, this should get your transmission to wake up and take as much HP as your engine can give. http://www.sonnax.com/instructions/HP-4L60E-01-IN.pdf