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700R4 towing safety

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Ukjimmyfan

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Hi, I have been told not to tow any thing heavy or not to tow in Overdrive.
No one explained how to lock out overdrive though.
Do I leave the selector in 3 or manually shift?
Won't it still try to use 4th?
What is the toggle switch mod and can that be done on early gear boxes?
Thank you.
 
Leaving the gear selector in 3 will stop the trans from shifting into 4th but it will shift normally 1-2-3.
A toggle switch modification is for torque converter lockup control.
How heavy are you wanting to tow?
Is this a truck, K5, suburban? 1/2 ton 3/4ton?
 
Ukjimmyfan said:
Hi, I have been told not to tow any thing heavy or not to tow in Overdrive.
No one explained how to lock out overdrive though.
Do I leave the selector in 3 or manually shift?
Won't it still try to use 4th?
What is the toggle switch mod and can that be done on early gear boxes?
Thank you.
Click to expand...
I towed plenty with my 700r4, if I felt the transmission was shifting back and forth more than once a minute, I downshift to 3 and let it go till it's flatter.
I don't like to keep it in 3 all the time so I just feel when it's not happy and bring it down.
One time I was going on a steep hill for 20 miles, it was more comfortable in 2nd at half throttle so I kept it in 2nd, so it didn't try to uplift and then downshift within seconds like it was trying.
So in general if you can prevent the constant shifting up and down which gets it hot and burns it up, then you are fine
 
Hi, it's 1/2 ton.
Maybe tow trailer and small car 100 miles no mountains.
Cooler in front of rad.
 
How do I do the torque converter mod for lockup?
Never stripped that down before.
Is it just wires and switch?
I'm just trying to avoid the unnecessary repair/Breakdowns I don't have time to do ATM.
 
Some factory setups lock the converter in 3rd gear. You might start by making note of how it behaves now. A simple test is to slightly apply the brake pedal while cruising. If the brake switch is adjusted right, this will unlock the converter and the RPM should go up and down maybe 200 RPM as it locks and unlocks.

What you want to avoid is extended periods with the converter unlocked, as this is the main source of heat from a trans. Also, in 4th gear, less oil is flowing, so the trans may run hotter. A temp gauge on the transmission is a good idea.

Also, It really helps the transmission to have differential gearing low enough for the size tires you're running. This reduces the torque through the trans/converter and helps it run cooler.
 
Your 6.2 converter has a very low stall speed, under 1300 rpm stall, you won't need to go to the bother of making any lockup modifications. I assume the cooler lines run from the radiator, then to the auxiliary cooler then back to the trans? If not, that is the way it ought to be for best cooling. Tow in third gear, it is easier on the trans and you probably won't be going fast enough to really need the overdrive gear anyway.
 
Greg Ducato said:
Your 6.2 converter has a very low stall speed, under 1300 rpm stall, you won't need to go to the bother of making any lockup modifications. I assume the cooler lines run from the radiator, then to the auxiliary cooler then back to the trans? If not, that is the way it ought to be for best cooling. Tow in third gear, it is easier on the trans and you probably won't be going fast enough to really need the overdrive gear anyway.
Click to expand...
Hi Greg
My 6.2 with 700r4 is running now with the lockup switch broken.
I am mostly around town anyway but if I plan on going on a long trip on highway should I worry about it?
I am trying to find a switch in the meantime
 
It would be of benefit to have the lockup function for extended driving in overdrive in terms of cooling and lube flow.
 
It's 1982 about December build (I rang GM a few times for the date - never sorted it out). Probably want me to get certificate and pay...
I deliberately kept the tire size smaller to stop transmission damage. Even on manual wranglers it's a bad thing.
Besides it's not got LS power and big tires sap HP.
I as a default with most vehicles i use D and let it do its thing. The overdrive is automatically selected even in D?
It's just that somewhere else contradicted that. What would be the point of automatic gearbox that didn't auto select all ratios?
I will be bypassing the coolant rad when it's reconditioned straight to external cooler, a new bigger one hopefully.
Is it best to stick to the original GM dextron 2 (Mobil?) or other?
The fluid is decades old probably!
 
I have a 700r4 in my 77 k20 suburban, behind a mild built 355. This trans was purpose built for my big heavy truck.
I have towed many miles(moved 250 miles away when retired) loads were not over 3500lb. My boat is @ 5k it gets towed in 3rd never over 45mph.
Currently running only an external cooler. 67000 btu with its own fan. Truck only trans stays under 200° F. Towing light loads(dune buggy and single axle aluminium trailer) up 10% grade in 3rd and 2nd temp has climbed to 220.
all of that history was done with 4.88 gear ratio and 33" tires. Recently upped the tire size to 37s. Have one buggy tow since. Trip went well used 3rd gear for the few hills.
All told trans was done in late 2018, by a GM trans specialist shop in Pomona Ca. The Toy Shop. @ 30k miles on this trans.


Answers to your questions.
Yes D will automatically shift in to overdrive. Normally over 40mphs and part throttle.

If you can find Dexron III that is the correct oil for 700r4, if not Dexron VI is acceptable.

Throttle valve cable adustment is very very Important to the life of this trans. When you sevice the fluid, this is the best time to/ adjust the cable as you can see the cable lever ans tv plunger.

Second is cooling besure to get a large quality cooler, esp ifvyou bypass the rad.
Greg Ducato makes his living and has tremendous experience with automatic transmissions, Phoenix Transmission in Texas, weight what he says more than others. Including me


On your brake light switch there will 2 plugs. 1 is normally open, brake lamp circuit. The 2nd is normally closed,this Torque Converter Control(TCC), or lock up. Installing a toggle switch in 1 wire on the normally closed side will allow you to disable TCC.

I recently added a TCI vacuum switch to my TCC circuit. This switch is adusted to unlock my converter at @6" Hg engine vacuum. Unlocking allows the converter torque multiplication to assist,and for me @ 250-300 more rpm into power band.

You read about my trials, tribulations, and some solutions here https://ck5.com/forums/threads/wess-burb-build-update-maintance.336847/
 
Ukjimmyfan said:
I will be bypassing the coolant rad when it's reconditioned straight to external cooler, a new bigger one hopefully.
Click to expand...
I would go through the radiator first, then to the auxiliary cooler, then back to the trans. Water will pull that heat out of the system very well, why discard the primary source of cooling?
 
I think people *want* to bypass the radiator because 200F seems like heating the transmission fluid, but in many cases, this is a good thing. When you drive in cold weather and never warm the transmission, you can get condensation in it. The external cooler is great because now you're feeding the transmission a cooler temp than the engine, but what the radiator gives you is more stable temperatures, whether you're at lighter loads or pulling a house up a hill. I don't know the UK much, but temperatures seem to vary a lot throughout the year.
 
Your engine temp is usually measured in a cylinder head, the bottom part of the radiator, where the trans fluid exits, is the water that is getting pumped back into the engine after being cooled down, it really does a great job pulling heat away from the trans fluid. Think in terms of quenching a hot iron in water versus cooling in ambient air.

You are correct about needing to get trans temp up enough to dissipate condensation, the torque converter does a pretty decent job of that for the trans no matter how we try and cool the fluid.
 
Very interesting experiences.
I couldn't understand why the engine was really smoking.Eye watering bad. Plus it would come and go.
Eventually!!! worked out had trouble with the coolant and oil cross mixing at the rad.
When engine oil pressure was high, oil pushed into water.
Coolant water pressure high, water pumps into engine oil.
Hence suspicion of oil to water ooolers.
Yes good working. Cost of engine or trans high if rad fails. Probably more common on the Chinese aluminium rads today.
Suppose it means get a good rad and change it regularly. Ron davis≠$1000+!
 
"The overdrive is automatically selected even in D?
It's just that somewhere else contradicted that. What would be the point of automatic gearbox that didn't auto select all ratios?"

Some confusion may stem from the fact that GM apparently used different shift indicators throughout the run.

If the vendors are correct, 82-83 used a D/3/2/1 indicator, later was OD/D/2/1.

4th gear is overdrive regardless what the indicator shows, 3rd gear is 1:1. To my knowledge the 700's won't upshift to 4th from 3rd.
 
dyeager535 said:
"The overdrive is automatically selected even in D?
It's just that somewhere else contradicted that. What would be the point of automatic gearbox that didn't auto select all ratios?"

Some confusion may stem from the fact that GM apparently used different shift indicators throughout the run.

If the vendors are correct, 82-83 used a D/3/2/1 indicator, later was OD/D/2/1.

4th gear is overdrive regardless what the indicator shows, 3rd gear is 1:1. To my knowledge the 700's won't upshift to 4th from 3rd.
Click to expand...
You beat me to it
 
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