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Towing with a 700r4

gone huntin

gone huntin

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I know it's been discussed before and to tow in 3rd gear, but what's too heavy? I need to tow my crew cab about 600 miles with my '90 Suburban. It has a 350/700r4/241/14bff with 4.10's on 33" tires. The trailer and crew cab weigh in at 8,500lbs. Will the 700r4 handle this kind of weight?
 
gone huntin said:
I know it's been discussed before and to tow in 3rd gear, but what's too heavy? I need to tow my crew cab about 600 miles with my '90 Suburban. It has a 350/700r4/241/14bff with 4.10's on 33" tires. The trailer and crew cab weigh in at 8,500lbs. Will the 700r4 handle this kind of weight?
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I can't find any 'real' info online. I see ratings for rebuilt beefed up units, but nothing stock or used. Towing in 3rd for 600miles would suck. I'd say reach cruising speeds, then switch to OD, coupled with an external cooler past the factory cooler inline. And switch to 3rd for hills and what not... but I'm not sure about 8500lbs. What I have read also is keep your RPMS around 2000 while crusing at 70mph in OD. So would need to adjust axle/tire accordingly. The main problem is heat, 'toughness' when towing is your final drive. But like I said, that info was gathered from the interwebs.

Best thing you could do is go on the freeway with your trailer and crew cab, and see if while at crusing speeds, you keep going in and out of lock. That's the main problem. If it stayes locked you should be ok. If it keeps going in and out, you might have to regear one way or another. My 86 Blazer had 3.73 with 35s, and I was always going in/out of lock on the highway, keep forgetting where that chart is with the RPM range and gear ratio. I needed 4.10 minimum and that was just to stay in the 'economy' range. 4.56 would have been ideal. Hard to say cause you're towing, and have 33s. I'll see if I can't find that chart.
 
Well, it has been rebuilt and does have an external cooler. The last rebuild came after it failed while I was towing. I was towing my car trailer with the cab, front cap, and two sets (8) of doors for my crew cab home after purchasing them. I was doing as described, going to 3rd on hills and going back to OD on the flats. It let go and blew out the front seal. The fluid pouring out was so hot that it melted the pavement. I sat there 10 hours waiting on someone to come get my wife, kids, and me along with my truck and trailer. I don't want to go through that again.

When it was built this last time I told them to build it for towing, but I didn't have a big budget for upgrades. It does have good firm shifts, but I'm gun-shy. I was basically just wondering if anyone out there is towing these kinds of loads with a 700r4 on a regular basis. I know I need to get a tranny temp guage.
 
gone huntin said:
I know I need to get a tranny temp guage.
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Here's a good quote from bowtie overdrives on that.

"Remember, the temperature you observe on the gauge is telling you the fluid temperature after it's been cycled through the radiator heat exchanger! This reading has nothing to do with how hot the fluid got inside the torque converter. If it reached 300 degrees F inside the torque converter and you only see 150 degrees F at the sensor in the pan, it doesn't mean the fluid wasn't overheated and degraded! We do not know of a reasonable way to monitor the fluid temperature inside the converter, so we always assume it is very high during hard work situations. Locking the converter during high heat production situations is the only way to prevent the overheating from occuring in the first place!"

Another thing to keep in mind I guess, is OD is .70, so 4.10x.70=2.87 with a Suburban, hauling 8500lbs. 3rd you'll be at 4.10 of course. I'm believing in 3rd you'd be at 48mph at 2000rpm, 4th 68mph at 2000rpm. 3rd gear 60mph is 2500rpm. Like I said, I've always known heat to be the downfall, never anything mechanically/broken gears etc. Keep it in lock you should be ok. I'm not 100% sure of the lock speed though. Another thing, I know when I had my 200r4 built, if I didn't clean the radiator with that pressurized cleaner stuff for the cooling lines/radiator. My warranty was void. So hopefully when the external cooler was put in, that cleaner stuff was ran though, and a flush.

All that said, I have personally never towed 8500lbs in a 3/4ton suburban with 700r4.. so take it with a grain ya know. But IF it were me. I would have no problems doing it granted my gear ratios were corrected due to tire size, weight, rpms..etc. to keep that tranny temp down. Then again, GM put what tranny in that Suburban, with what axle/gear ratio, factory specs say tow what weight? They know what they were doing :)
 
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Yes, I cleaned the cooler and the radiator with the pressurized cleaner. I had it bench built and installed it myself.
 
You should get a bigger OD servo (I like the TCI one that Summit carries). That will greatly improve the issues with towing in OD that these have by increasing the 2/4 band clamping force. I would also replace the 2-shift servo while you're at it, since that's one of the really important upgrades anyways.

I would get a temp gauge and probably a cooler while you're at it. Try shifting it in to OD once you get moving and see if it's getting too hot. The big issue is having all that load on the tranny while it's shifting. By increasing the band apply force you will be helping that a lot. A big thing is keeping the throttle from making you hunt between 3 and OD too, because the frictions wear and create heat every time it shifts. Driving in OD does create more heat too due to a lot of extra movement in there and more force on the bushings though, but you'll be able to see that pretty well on a temp gauge.

The servos are really easy to replace, you don't even have to drain the pan. It only takes about 1/2 hour to 45 minutes once you've done a couple. Only takes like a 1/2" socket and a pick or something to pull the retaining ring off. I posted a picture-by-picture writeup on a couple forums years ago that I can dig up if you'd like.
 
Random thought but what's the tow rating on a 3/4 ton Burb of that vintage? I know it's pretty low on K5's. A lot lower than folks seem to think it should be.
 
I have the BIG 3.5x13 shoes from the 1 tons on my 14bff and my trailer has brakes on both axles so I'm good on brakes. I'm going from Memphis to S.M.O.R.R. just S.E. or Springfield, MO. There are lots of big hills in S. MO., that's what I'm worried about.
 
All that I can input doesn't apply to the 700R4 as I have the 4L80E. Hauling a 12,000 lb load from Summerville, S.C. to Alliance, Oh. I was down to 2nd gear @ 35 MPH going up 8 mile in Va. on US 77. It's a 7% grade for 8 miles. It was just happy there, any faster or 3rd gear would cause the engine temp to instantly rise. the rest of the trip also included the hills of W.Va. I spent a lot of time out of OD.
 
Well, I don't plan on going into OD at all after my last experience, I just want to be sure it'll be all right in 3rd with that kind of load.
 
gone huntin said:
Well, I don't plan on going into OD at all after my last experience, I just want to be sure it'll be all right in 3rd with that kind of load.
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Well the misconception with some is that pulling hard makes it overheat, it's the gear hunting that overheats it quick, so if your tranny is holding in third at a reasonable RPM it should be fine.
If the speed you are going at is inbetween 2 gears and you don't think you can go faster, slow it down so you are in a good RPM range for the gear you want to hold, that wy your engine is not maxed out and overheating, and your tranny is not slipping because it doeasn't have enough RPM's.
Does that make sense to you?
 
imiceman44 said:
Well the misconception with some is that pulling hard makes it overheat, it's the gear hunting that overheats it quick, so if your tranny is holding in third at a reasonable RPM it should be fine.
If the speed you are going at is inbetween 2 gears and you don't think you can go faster, slow it down so you are in a good RPM range for the gear you want to hold, that wy your engine is not maxed out and overheating, and your tranny is not slipping because it doeasn't have enough RPM's.
Does that make sense to you?
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That's what I was trying to get at. On flat ground he might even be ok in OD, IF he's locked. Same thing will happen if he's not locked in 3rd. The converter slipping is what causes alot if not most of the 700r4 to overheat, like you said, hunting for a gear. If it's locked, there's no slipping, less pressure/friction/heat. Guess a decent analogy is partially engaging your clutch for your road trip.Not sure if all 700r4s lock in 3rd, but the later ones should I believe, definitly could if your last rebuild they made it that way.

You have all the ecm stuff hooked up, it locks properly? Or an aftermarket lock up kit, if the ecm was ripped out.
 
When are you going? If its tomorrow, never mind.......

But, if you have time, I would definitely put in a temp gauge. I would also try to work out a way to measure the temp coming out of the tranny.

The best way probably, would be to cut the transmission line between the tranny and the cooler, and put in a "T".
Plumb the temp sending unit into the "T", and then wrap it with insulation.
That way, even if its not in the direct fluid stream, the fitting and sending unit will quickly all reach the same temp, and will track the fluid temp pretty well.

Or, clamp the sending unit to the line, with some heat sink grease and then insulate the area.
It works with refrigeration expansion valve bulbs.

That will hopefully let you spot trouble in time to pull over and run it at fast idle in park until it cools down.

The other thing I would consider, is changing tires.

If you can find some rims and good smaller diameter tires, you can swap them and put your 33s in the towed vehicle to put on after the tow is over.

This will have the effect of changing the ratio and should let the transmission see less load, and easily stay in lock in OD.

Cheaper than gears.........

I used to have a set for my 3/4 ton Ford.
I had the 16.5/33s for hunting season, and a set of regular 16 inch wheels with Michelin radials street tread for the summer when I was using it for some towing and as a DD.

After I got the SUV, I just left the 33s on, and traded the others to a guy for a nice rifle.
 
Engine RPM at 60mph will be right at 2500 pm which should be a good pulling rpm for the small block and not totally kill you on fuel or excessive heat. If the trans was well built you shouldn't have much trouble at that speed and weight if you keep the trans temps reasonable.
 
I tow a 4000 lb boat and I have 5.13s and 35inch tires. On the flats overdive seems to work ok. Any kind of incline I shift to 3rd.

My tranny was also built stronger then stock.
 
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