[goatmanmitch]

I have a 91 Suburban with a 350 and I plan on pulling my 70 LeMans on a trailer about 500 miles each way.

There is a trans cooler.

My question- is it okay to pull it in overdrive as long as the road is flat and the truck isn't lugging?

If pulling in overdrive is okay, I would plan on manually downshifting on hills. Sound okay?

[Chief of the 60's]

It all depends on how many miles are on the tranny. If it does not have a boat load of miles, OD is fine on flat road. Any laboring, you will have to take it out of OD. If it has miles, towing could be risky, but that goes for any tranny.

[Lemans64]

GM used to put a sticker on the door post to advise against towing in o/d due to too much stress on the o/d unit.
Even on level surface there is still a lot of load on trans, I alway towed in 3rd in my 95 2whdr light 3/4. If only going
500 each way is it worth possibly loosing trans?

[Trulyvintage]

My first tow vehicle was a 1990 Suburban 3/4 ton which did not last very long.

Regarding OD transmissions - I can tell you from personal experience if you have a manual “ lockout “ to drop it out of overdrive - DON’T use it.

Instead - downshift to 2nd or even 1st if necessary when pulling a hill.

Your tranny will last longer.


Jim

[Ken MacNicol]

I used to tow thousands of mile a year with a S10 Blazer with a 4.3 and a 700 in overdrive with no problems. Now I did put the biggest cooler I could find on it and change out the servo to firm up the shifting.

[Chief of the 60's]

Originally Posted by Ken MacNicol

I used to tow thousands of mile a year with a S10 Blazer with a 4.3 and a 700 in overdrive with no problems. Now I did put the biggest cooler I could find on it and change out the servo to firm up the shifting.

You are not pulling much with an S-10 and if you are, it is unsafe. Towing a car is a lot of weight.

[66sprint6]

I’ve noticed on my Tundra that in Tow/Haul mode, it won’t go into overdrive and it downshifts much earlier at inclines. I’ve towed two different A bodies from Kentucky and Missouri back to here in Toronto with no problem. The truck has almost 200,000 miles on it and it’s as good as ever.

[hgerhardt]

I installed an un-rebuilt junk yard '85 700-R4 into my '71 C20 Chevy truck with 4.10 gears along with a '91 TBI 350 from a wrecked Suburban and towed my '66 GTO from southern CA to Bend, OR twice, to Vancouver, Canada once, and Wichita, KS with that setup and never had any issues. On level ground with enough speed, I was in 4th gear most of the time. With a bit of grade, the ECU would start unlocking the lockup clutch and that's when I'd downshift to 3rd. Going up the infamous Grapevine on I-5, I'd be WOT pretty much the whole way, alternating between 2nd and 3rd gear. Amazing how much abuse that TBI engine tolerates, and the 700-R4 didn't complain either. There was a small external cooler in series with the rad cooler.

We've since put around 200k miles on that combo in the last 22 years since I did that conversion. I better go find a big piece of wood to knock on now!

[Sirrotica]

Mitch, if the tranny is functioning properly at this point and you keep it cool, you will probably be fine. I wouldn't push it hard though. Heat and hills are what will do in transmissions.

If you have some grades to ascend I'd pull it down manually and put the 4 ways on, and take your time. If the suburban has a tach you want to keep it around 2000 RPM at highway speeds, if it starts down below that in OD, shift it manually to drive.

The torque convertor also has a clutch internally that is probably the weak link in the whole assembly. That is what will also build heat if you over tax it, and then you get a symptom at about 40-45 MPH where the whole truck shakes, especially the dash area when the convertor clutch slips as it locks up. Burning that clutch up is what you want to avoid because things go downhill from that point.

Make sure the cooler is plumbed into the return line side of the radiator if your still using the radiator cooler too. Have fun on the ride.

[Chief of the 60's]

Originally Posted by hgerhardt

I installed an un-rebuilt junk yard '85 700-R4 into my '71 C20 Chevy truck with 4.10 gears along with a '91 TBI 350 from a wrecked Suburban and towed my '66 GTO from southern CA to Bend, OR twice, to Vancouver, Canada once, and Wichita, KS with that setup and never had any issues. On level ground with enough speed, I was in 4th gear most of the time. With a bit of grade, the ECU would start unlocking the lockup clutch and that's when I'd downshift to 3rd. Going up the infamous Grapevine on I-5, I'd be WOT pretty much the whole way, alternating between 2nd and 3rd gear. Amazing how much abuse that TBI engine tolerates, and the 700-R4 didn't complain either. There was a small external cooler in series with the rad cooler.

We've since put around 200k miles on that combo in the last 22 years since I did that conversion. I better go find a big piece of wood to knock on now!

I find this almost impossible to believe. Especially when '86 and down 700's were GM's worst.

[Ken MacNicol]

Originally Posted by Chief of the 60's

You are not pulling much with an S-10 and if you are, it is unsafe. Towing a car is a lot of weight.

I towed my race car on an open trailer everywhere

[hgerhardt]

Originally Posted by Chief of the 60's

I find this almost impossible to believe. Especially when '86 and down 700's were GM's worst.

Indeed, hard to believe. I know all about how "inferior" the pre-'87's were. Before installation all I did was pull the pan, replace filter, and replace the tailshaft & converter seals, and also rewired the TC lockup circuit (within the trans) to work with the TBI engine.

It was on the dirt floor of the office of a junk yard in Alamogordo, NM when I found it. Paid $200 for it, and this was Jan '90. Allegedly had 30k miles on it when I got it, and it obviously had never been apart. I fully expected to have to rebuild/modify it soon after installation... but it's been a great tranny! I just change fluid every 30k-ish miles and always amazed how little clutch material is in the pan. So, at this point it's got around 250k-ish miles on it and 33 years of age? Crap, I can't believe that either!