700r4 no overdrive

[engineeradam]

i have an 87 gmc sierra 4x4 half ton. i put a new tv cable and its properally adjusted and i by passed the tps so the tc lockup gets 12v unless the brakes are apllied. the truck runs and shifts fine, overdrive and everything until it warms up and you allow it to shift outta overdrive. then it wont go back in. downshifting to 3 does nothing hence the no overdrive. what could it be? can i fix it myself? what can i try and do? money is tight due to awesome tuition fees (sarcasm) so a tranny shops outta the question.

[Falconsprint]

Sounds like your loosing the ground to the TC solinoid. The coil could be open when heating up. It can even be the OBC causing it. (loss of ground) Check all your wiring connections/connectors before you go too far. The solinoid is in the pan, and the pan has to be dropped to get to it. it's not that expensive, but dropping the pan to get to it makes it more expensive. Check your wire connections before you go to far.

I remember this from owning several GM vehicles and having the TC solinoid go bad.

Goodluck

[Robyn]

The TCC is the only thing controled by the electronics.

If moving the shift lever to 3 and back to OD does nothing, then the issue is not with the electronics.

The symptoms point to a 4th oil problem, be it a valve issue or a band apply problem.

The band that make 4th gear happen also provides second gear, so if second is OK then the band is probably fine. Second gear is actuated by a different servo than is used for 4th.

There are several things that could cause this but as the oil warms up it thins out and if there is a leak (bad seal) this can cause the trouble.

A worn valve bore/sticky valve could do it too


If the 1-2 shift is firm and is not displaying signs of slippage then the issue is probably in the Valve body or the 4th apply piston seal could have failed`

4th (OD) is accomplished by applying the 2-4 band after the 3-4 clutch is applied.

This runs one set of planetary gears and gives the OD ratio.

I would suggest dropping the oil pan on the tranny and having a look see at whats in the pan as far as CRAP in the bottom.

If there is a large amount of junk in there and the oil is nasty brown or burnt smelling then the tranny is going to need to come out and apart.

The OD apply servo is on the RH side of the tranny under the large round cover..


TRY this test

Right directly behind the servo cover (round item on the RH side of the tranny) ARE A PAIR of pipe plugs.

The lower one just above the pan rail is the 4th oil pressure tap. (Lowest and closest to the servo)

Install a tranny pressure gauge (long hose) on this tap and do a test drive.

Check the pressure in OD when the thing is cold. (Write down the pressure when in 4th)

Once the unit warms and it will not go into OD then see what the pressure is, or if there is any.

The servo cover is held in with a steel internal snap ring that fits in a groove in the case.

If the 4th oil seal ring (teflon has failed the band will not hold)

The 4th apply piston is the first item you will come to once the cover is removed.

To remove the cover, press in on the cover (clean well first) then using a small screw driver, pry out the snap ring.

This can be an oily mess of a job so be prepared.

The setup is under some spring pressure so you may have to fashion a suitable device to push the cover in.

I suspect that the servo may be leaking or something in that circuit.

Do inspect the pan for JUNK and if no junk found, change the filter and top off the fluid.

Let us know what shows up.

If the servo seal or ?? has failed and the band is slipping in 4th, the band can be destroyed in short order if run this way.

Missy

[engineeradam]

Hey Robyn, i haven't done anything yet due to lack of time in exams now, but the first to second does seem to slip a bit. Should i just try and change the servo on the right side of the tranny with one from a junkyard? I don't have a pressure gauge to do the tests you recommended.

[DmaxMaverick]

It sounds like your TV cable is NOT adjusted correctly, or the T-valve is worn out (which means it can't be adjusted correctly, even if it's adjusted "correctly"). If you have a "sloppy" 1-2 shift, and a problem keeping or going into OD, it is likely a TV issue.

If it's the 2-4 band, then you'd lose 2nd as well as OD once the problem occurs.

What was the reason for replacing the TV cable, and how was tranny performance before?

[engineeradam]

i replaced the tv because the old one almost seized in the casing because it started to "fray" under the air intake. Before the switch it shifted fine and then i started to notice really late shifts so inspecting the tv showed the fray of the cable so i bought a new one. Adjusted it and it shifted fine.

[engineeradam]

any thoughts robyn? Should i take the servo out? and replace the seals on those actuators? Or DMax? should i do more research into what a tv valve is?

[Robyn]

Yank the dip stick on the tranny and take a sniffffffff

If it smells nasty (burnt, or is really brown/black) then the tranny is fried.

second gear does not take a lot to hold it, but OD takes a lot of holding power on the 2-4 band.

Likely that the band is shot due to ????
Once the band slips its over.

The friction lining is make of a special PAPER and along with oil thatr soaks it the friction quality is good, but when they slip the friction material turns to a carbon and its toast (literally)

If you find some yucky oil, then I would not bother to fiddle with it, just yank it out.

Missy

[engineeradam]

ok from what i can tell the oil is good. but new intelligance has been aquired. today driving i was playing with the shift lever to no avail. as in no change when cruising when shifting from 4th to 3rd. Except, and heres the clencher. when coasting? if i let off the throttle the engine revs drop drastically. so if you down shift when cruising the revs increase. so i am led to beleive when coasting the tranny is shifting into 4th. which makes me think of this tv valve dmax is mentioning. so what is a tv valve? how does it work and can i change it?
thanks. adam

[DmaxMaverick]

TV = Throttle Valve. It's connected to the other end of the TV cable you replaced.

I hate to say this, but I think Robyn and I are both right on this one. It sounds like a misadjusted TV cable, followed by a failed 2-4 band. This is usually how it happens.

I may be wrong, if you can get it to shift into OD and stay there under power. In this case, the TV cable is not adjusted properly. If you continue to drive like this, the band will fail.

How did you adjust the cable?

[engineeradam]

pushed the release with my right hand and hit full throttle with the left pulling the casing out and then released. did i do it wrong? i dont think 4th is done quiet yet.

[DmaxMaverick]

The throttle at the pump doesn't necessarily equal the throttle at the pedal. It will sometimes adjust correctly the way you did it, but less often in my experience.

Press the button and pull the cable adjuster back. Slowly press the pedal all the way to the floor, like trying to hit passing gear. The adjuster will take it up. Make sure your pedal isn't obstructed, and is getting full travel. If the pedal is running out of travel before it's fully depressed, the throttle cable may be suspect (and could explain some shifting anomalies).

I also suggest returning your TCC operation to original until you get the shifting issue ironed out. This way, we can help you diagnose it best, with predictable results, rather than trying to account for unpredictable changes.

If you have a lot of miles on the tranny, I recommend adding a pint of Trans-Medic (Gunk product) to the fluid. If you can't find Trans-Medic, use Trans-X. This will renew the friction surfaces. If/when the tranny returns to normal operation, do a full flush/fill with fresh fluid after 500-1,000 miles. Flush the cooler circuit separately, making sure it flows well. If it doesn't already have an external cooler, adding one will help over the long haul.

[engineeradam]

still losing fourth. I adjusted the cable and added gunk trans medic. I still think its downshifting too soon as if under heavy throttle. Ive heard of corvette servo's. Good or bad idea? Any other ideas?

[Robyn]

The Vette Servos are the ticket, but not the complete answer.

You first need to figure out whats going on.

The addition of the vette servos adds some much needed holding power to the 2-4 band, but if your loosing 4th then you need to figure out Why.

[engineeradam]

could it be a worn out tv valve? and could you recommend a servo?

[arveetek]

The 1990 model 700R4 I transplanted into my old '81 I used to own had a quirk like this. Once in a while, especially after towing hard, OD would disappear. It would stay in 3rd, and pulling the lever between OD and 3rd made no change. However, I also noticed the same symptom as you when coasting to a stop, that it acted like it actually was in OD, in that if you pulled the lever down into 3rd, the rpms would jump up like you were downshifting.

Sometimes it would go for several days like this, and then all at once, OD would reappear. It was really odd. But it would only happen to me after towing hard in 3rd for long periods of time. I assumed at the time that something was getting hot and lodging into place in one of the valve body passage ways.

Sorry, I never did figure it out, and then I sold the truck. The new owner ended up rebuilding the tranny shortly after he got it from me.

But, you're not alone, as I had the exact same symptoms. I don't think it's a TV adjustment issue. Mine was perfectly adjusted, and the symptoms would come and go with no change being made to the TV cable.

After reading your first post again, I think we have something in common: you said yours acts fine until it warms up. Mine would only fail after towing hard. I think there is something inside the valve body or servo that is heating up and lodging in place. What that is or how to figure it out, I'm not sure!

Casey

[engineeradam]

I am going to try replacing the servo with a corvette one. But looking on ebay there are so many options and i dont know which one to get, im hoping somebody can recommend a setup. They seem to range drastically in price from 10$-200$. Some come with a 1-2 actuator and some come with a longer pin. Also some come with a boost valve? What is that? Is it needed? Being these kits are meant for gases and diesels i don't have a clue which one is ideal for me. The truck is my daily driver and also my work truck used for hauling relatively light loads and snowplowing.

Anyone with experience with vette servos advice is much appreciated.
Thanks again.
Adam.