700R4 no forward gears that fixed itself ???

[midnightblue]

Hello, 700r4 in a 1984 K5. I replaced the transmission, converter and cooler 3000 miles ago. I purchased them from reputable companys. I have not had any issues till last week. I went to do a quick errand. Started it up but it had no forward gear. I put it in reverse and it backed up. Put it back in drive and it did not engage. While in drive I started to increase the RPM, between 1500 and 2000 RPM it engaged. It felt like a neutral drop. So I have been driving it since with no issues. What could cause this? The fluid is full. I did replace the filter and fluid after breakin at 500 miles, 2500 miles ago. No leaks, OD and lock up work, shift perfectly. The more i think about it the more concerned i get. Was this a one time issue or is this going to leave me stranded far from home? Did something just get stuck?, dirt? Or is this a bigger issue?
Thanks

 

[NickTransmissions]

Hello, 700r4 in a 1984 K5. I replaced the transmission, converter and cooler 3000 miles ago. I purchased them from reputable companys. I have not had any issues till last week. I went to do a quick errand. Started it up but it had no forward gear. I put it in reverse and it backed up. Put it back in drive and it did not engage. While in drive I started to increase the RPM, between 1500 and 2000 RPM it engaged. It felt like a neutral drop. So I have been driving it since with no issues. What could cause this? The fluid is full. I did replace the filter and fluid after breakin at 500 miles, 2500 miles ago. No leaks, OD and lock up work, shift perfectly. The more i think about it the more concerned i get. Was this a one time issue or is this going to leave me stranded far from home? Did something just get stuck?, dirt? Or is this a bigger issue?
Thanks

Sounds like the beginnings of gear train failure or possibly input sprag failure. Id drop the pan to confirm or deny the presense of gear teeth if it happens again or if you temporarily "neutralize" at surface street or highway speeds.

 

[midnightblue]

Sounds like the beginnings of gear train failure or possibly input sprag failure. Id drop the pan to confirm or deny the presense of gear teeth if it happens again or if you temporarily "neutralize" at surface street or highway speeds.

Thank you for the reply Nick. I certainly did not want to hear "failure" from someone with your experience, lol. So if its going to break, it should do it soon? Wife and I have a few day trips planned. We just got back from a 1500M round trip to Tennessee, sure glad it did not break there

 

[NickTransmissions]

Thank you for the reply Nick. I certainly did not want to hear "failure" from someone with your experience, lol. So if its going to break, it should do it soon? Wife and I have a few day trips planned. We just got back from a 1500M round trip to Tennessee, sure glad it did not break there

Yes, agreed - that would have absolutely sucked if you broke down while on your trip - glad it didn't shiit the bed then, haha.

The amount of time you have left before it becomes inoperable impossible to predict.

I'd drop the pan to see what, if anything at the bottom and if you see any pieces of gear teeth you obviously need to pull and overhaul the unit, replacing the gear train along with the usual other stuff. If the problem is the forward sprag, you will still have forward movement in manual low (1) and manual 2nd as the coast clutches come on and are splined to the secondary hub on that sprag gear assembly, which doesn't require the sprag clutch itself to be functional. If it's your low roller clutch (less likely), you'll loose all forward movement in all range settings (except reverse - that will still work). If it's gear train, you'll lose everything.

If it's front pump, that would tell me the rotor is in the process of breaking up and may have fractured into two separate halves (yes, believe it or not the trans will still work for a short duration when this occurs - at least until the rotor completely shatters).

The only other thing I can think of is problem w/converter drain back in that all the fluid drains out of the converter and into the sump (not common w/these transmissions or GM transmissions in general). If it happens again, immediately check fluid level to see if you're way over full - if so, its either converter drain back or pump rotor failure. Also stick a pressure gauge on the trans (main line pressure port is on the drivers side of the trans just behind the bell - look for a 7/16" plug and thread your 0-300 PSI gauge in there (don't use an oil pressure gauge for this as you may destroy it as trans pressures are much higher when driving than engine oil pressures). Pressures should be 55-70 PSI in P,N,D. Reverse should be 100-120 and Manual 2,1 should be 150-175psi.

In case you haven't already seen it, I have a 700R4 Information Thread the covers all aspects of that transmission including diagnosis/troubleshooting, parts selection and rebuilding information.

 

[midnightblue]

That is great info. Thank you. Looks like I have a good starting point and a lot of reading to do

 

[NickTransmissions]

That is great info. Thank you. Looks like I have a good starting point and a lot of reading to do

You're welcome...these transmissions are fairly easy to rebuild as long as you follow proper procedure..I'm going to update the OP in that thread w/all the special tools you'll need later today or tomorrow so you'll know what sort of tools you will need to rebuild it yourself, should you decide to take that step. You'll end up spending less money in special tools then you would in labor to have someone else rebuild the transmission and you'll have those tools for the rest of your life and use them perhaps in other builds (including 4L60Es as they will work on both).