700R4 Random, split second OD drop out

[X4X4X]

Hello everyone! I have a 1990 K1500 with a freshly rebuilt TBI 350, 700R4 with a rather large transmission cooler and around 50k miles on a rebuild, 6 inch lift, 33x10.50r15 tires and 3.73 gear ratio. The 700R4 in this truck shifts very well, and when I did a fluid and filter change at around 30k miles, the fluid was still cherry red and there were no signs of clutch material in the pan.
This issue is very random and rare. It drops out and back into tcc lockup once for a split second (so fast that you have to really pay attention to even notice it) during windy conditions and going up a grade at speeds of 65-70 MPH. I seem to recall this only happening after I installed the 6 inch lift kit. Does this sound normal to anyone more knowledgeable than me? Or is this a possible early sign of transmission failure? Thank you to anyone who provides their input!

 

[NickTransmissions]

Hello everyone! I have a 1990 K1500 with a freshly rebuilt TBI 350, 700R4 with a rather large transmission cooler and around 50k miles on a rebuild, 6 inch lift, 33x10.50r15 tires and 3.73 gear ratio. The 700R4 in this truck shifts very well, and when I did a fluid and filter change at around 30k miles, the fluid was still cherry red and there were no signs of clutch material in the pan.
This issue is very random and rare. It drops out and back into tcc lockup once for a split second (so fast that you have to really pay attention to even notice it) during windy conditions and going up a grade at speeds of 65-70 MPH. I seem to recall this only happening after I installed the 6 inch lift kit. Does this sound normal to anyone more knowledgeable than me? Or is this a possible early sign of transmission failure? Thank you to anyone who provides their input!

If this is only happening when you're in fourth gear, climbing a hill or grade, I'd venture to suspect your larger tires may be playing a role...When you in install larger tires but don't change the final drive ratio, you essentially decrease the amount of mechanical leverage available...So in effect, instead of simply cruising up a mild hill in fourth, your TCC is unlocking to give you a little more torque multiplication to make up for the loss of mechanical leverage.

This is just a guess as 3.73s should be sufficient for 33s, though 4.10s would be ideal.

Other causes include a weak TCC solenoid, electrical connectivity issue between your PCM and transmission (Check your external wiring harness), vacuum issue, trouble with the 3-4 clutch pack and so on...

You will need to do some additional diag work to rule out some of those things...

 

[1998_K1500_Sub]

I seem to recall this only happening after I installed the 6 inch lift kit.

I *think* in 1990 the ECU will show TCC lockup commands in the scan info.

If you can put a scan tool on it and monitor, you'll be able to see if the ECU is commanding the TCC to unlock and then re-lock. If this is what's happening, then there's no issue with the transmission.

The 6-in lift kit you mentioned might (might) have increased your wind drag, leading to the condition mentioned by @NickTransmissions

 

[X4X4X]

If this is only happening when you're in fourth gear, climbing a hill or grade, I'd venture to suspect your larger tires may be playing a role...When you in install larger tires but don't change the final drive ratio, you essentially decrease the amount of mechanical leverage available...So in effect, instead of simply cruising up a mild hill in fourth, your TCC is unlocking to give you a little more torque multiplication to make up for the loss of mechanical leverage.

This is just a guess as 3.73s should be sufficient for 33s, though 4.10s would be ideal.

Other causes include a weak TCC solenoid, electrical connectivity issue between your PCM and transmission (Check your external wiring harness), vacuum issue, trouble with the 3-4 clutch pack and so on...

You will need to do some additional diag work to rule out some of those things...

Nick, thank you for your suggestions. I have verfied that the PCM to transmission wiring is in good health, so no issues there. I do know that the TCC solenoid is original to this truck even after the rebuild, so it has close to 300k miles on it. I think I will go ahead and change it just to rule that out and then go from there. I will also plan on regearing to 4.10s asap as that has been in the back of my mind since putting on the larger tires.

I *think* in 1990 the ECU will show TCC lockup commands in the scan info.

If you can put a scan tool on it and monitor, you'll be able to see if the ECU is commanding the TCC to unlock and then re-lock. If this is what's happening, then there's no issue with the transmission.

The 6-in lift kit you mentioned might (might) have increased your wind drag, leading to the condition mentioned by @NickTransmissions

Good idea, I have a MT2500 scanner and I recall seeing a TCC lockup command in the info section. I'll hook it up and go for a long drive tonight and then report back with what I find. And yes, after installing the lift it definitely has increased wind drag. I live in a very flat and windy area and when this truck catches a gust it pulls me around fairly hard. I feel like this is probably what is causing the "issue", but I just wanted to double check on here to ease my mind. Thanks again to both of you.

 

[1998_K1500_Sub]

Good idea, I have a MT2500 scanner ... I'll hook it up and go for a long drive tonight and then report back with what I find.

Let us know how it works out

As Nick pointed out, re-gearing to 4.10 or even 4.56, would help in many ways.

 

[X4X4X]

I drove around for about an hour with my scanner plugged in on road test mode and was able to catch the issue while looking at the tcc command section. When it happened I saw that the ecm did not command the tcc to unlock. So, I stopped and unplugged the external tcc connector to rule out the 3-4 clutch pack without the tcc engaging and had no shifting issues while driving with it unplugged (I believe this was the proper way to test it. Please let me know if I am wrong). I'm assuming my problem is the solenoid, unless I am missing something else. I have a new one ordered and will update again after installing and testing. Should be here in 3-4 days.

 

[1998_K1500_Sub]

I drove around for about an hour with my scanner plugged in on road test mode and was able to catch the issue while looking at the tcc command section. When it happened I saw that the ecm did not command the tcc to unlock. So, I stopped and unplugged the external tcc connector to rule out the 3-4 clutch pack without the tcc engaging and had no shifting issues while driving with it unplugged (I believe this was the proper way to test it. Please let me know if I am wrong). I'm assuming my problem is the solenoid, unless I am missing something else. I have a new one ordered and will update again after installing and testing. Should be here in 3-4 days.

Perhaps the solenoid, or perhaps simply the electrical connection(s) to it. Just keep that thought in mind as you debug. I'm not saying it's probable, but it's possible.

Your results jibe with my expectation. I've not seen a GM ECU cause the TCC to unlock and relock rapidly, as the ECU's coded to disallow such nonsense, but it's important that you checked.

This doesn't rule out a problem in the ECU, namely the "driver circuit" that controls the TCC solenoid, but I don't think it's likely the problem. Just keep it in mind.

 

[NickTransmissions]

I drove around for about an hour with my scanner plugged in on road test mode and was able to catch the issue while looking at the tcc command section. When it happened I saw that the ecm did not command the tcc to unlock. So, I stopped and unplugged the external tcc connector to rule out the 3-4 clutch pack without the tcc engaging and had no shifting issues while driving with it unplugged (I believe this was the proper way to test it. Please let me know if I am wrong). I'm assuming my problem is the solenoid, unless I am missing something else. I have a new one ordered and will update again after installing and testing. Should be here in 3-4 days.

Do not drive around with the external case commector unplugged - all that does is generate excess heat via the TC clutch slipping comtinuously when it should be locked.

There's no way to "test" or ascertain the health the 3-4 clutch in a 700R4 beyond driving it (a 4L60E is a different story).

Hopefully the new solenoid resolves the problem.

 

[alignman88]

I believe it’s the tire size. My 2013 had 35’s and I was able to adjust shift rpm and line pressure plus cheat the tire diameter to get it to stop doing that. My 98 k1500 has 20’s not sure tire diameter and a 4l60e and it drops in and out of OD easily pretty hard like a shift. Unless I’m at 60-65+ I run it in the 3 spot on PRINDL and it’s fine.

 

[X4X4X]

Perhaps the solenoid, or perhaps simply the electrical connection(s) to it. Just keep that thought in mind as you debug. I'm not saying it's probable, but it's possible.

Your results jibe with my expectation. I've not seen a GM ECU cause the TCC to unlock and relock rapidly, as the ECU's coded to disallow such nonsense, but it's important that you checked.

This doesn't rule out a problem in the ECU, namely the "driver circuit" that controls the TCC solenoid, but I don't think it's likely the problem. Just keep it in mind.

The solenoid I bought comes with the entire internal harness instead of the splice on type. I guess it's ok, but I don't feel comfortable having splices inside of the transmission, lol. I feel it could be possible that the transmission shop may have damaged the wiring when rebuilding, so I figured I'd go ahead and rule it out just in case.
I have another known good ECU I can swap in to see if the driver circuit is possibly the issue, I'll test that tonight and let you know if that fixes it or not.
Thanks for the suggestion!

Do not drive around with the external case commector unplugged - all that does is generate excess heat via the TC clutch slipping comtinuously when it should be locked.

There's no way to "test" or ascertain the health the 3-4 clutch in a 700R4 beyond driving it (a 4L60E is a different story).

Hopefully the new solenoid resolves the problem.

Crap! I feel like an idiot now. I drove like that for a short time, so hopefully it didn't do any harm. Now I know to never do that again!

I believe it’s the tire size. My 2013 had 35’s and I was able to adjust shift rpm and line pressure plus cheat the tire diameter to get it to stop doing that. My 98 k1500 has 20’s not sure tire diameter and a 4l60e and it drops in and out of OD easily pretty hard like a shift. Unless I’m at 60-65+ I run it in the 3 spot on PRINDL and it’s fine.

After installing the lift kit, I still had 31's when this issue started occuring. I should have specified that to being with, but I'm sure the 33's are doing it no favors if that is what is causing this. I do plan to re-gear to 4.10's fairly soon as Nick and K1500_Sub have suggested. Possibly even 4.56's if that would be better. Do any of you have opinions on that? If you had 33's and a 6 inch lift, would you feel more satisfied with 4.10's or 4.56's?

Thanks again guys, I really appreciate you all taking the time to help me figure this problem out!