700R4 went out while towing. Culprit?

OuchMyToe

Junior Member
Joined
Apr 4, 2019
Posts
11
Reaction score
2
Location
Tooele, Utah
First Name
Kyle
Truck Year
1985
Truck Model
C10
Engine Size
400 CID
I'd like some help identifying three likely culprit (besides myself) to my transmission going out. I need more information before I make decisions about fixing/upgrading it. Here are the circumstances:

It's an '85 C10 into which I've put a 400 SBC. I believe it was a stock 700R4. I was towing a light-weight unloaded trailer up and down hills at 60 to 70 mph at night.

First, ATF blew out the dip stick and covered everything. Then I lost power. Since I was blocking a lane on a rural highway at night, I needed to move it as quickly as possible. I added ATF and got first gear back for about 120 seconds. Added more and got first gear again for 2 more minutes. Added a bunch this way until it seemed full (?). After that, I didn't add more but let the truck sit for a couple minutes after which I got about 30 to 45 more seconds of first gear. I did this until I reached a pullout and then finally got the truck towed home.

It will still go in first for about 45 seconds. It never shifted into 2nd and there's absolutely no response (no noise at all) when shifting into reverse—not even after letting it sit.

I should mention that this episode was the first indication ever that the trans was at all unhappy. So...

1st, has anyone experienced this before and if so, what was the prognosis?

2nd, I've always suspected that pairing the 400 sbc with a 700r4 might be asking for trouble; is this true? (I did put on the externally balanced flexplate). I originally intended to stick a turbo 400 in the truck but was always hesitant to lose overdrive (as a 2wd, this is mostly a highway truck). I also believe that modding the driveshaft to fit the th400's extra length is beyond my ability to do at home. So, should I keep it with a 700r4 or pay someone to put in a turbo 400?
 
Last edited:

Big Chip

Full Access Member
Joined
Oct 19, 2017
Posts
4,562
Reaction score
6,496
Location
Michigan
First Name
Kevin
Truck Year
1979
Truck Model
C35
Engine Size
454
If you were in OD that's going to be most of your issue I think. I would get it rebuilt and roll with it and remember to use 3rd when hauling anything besides your lunchbox.
 

Termite_Delight

Full Access Member
Joined
Feb 17, 2018
Posts
582
Reaction score
915
Location
Lanark Ontario Canada
First Name
Andy
Truck Year
1976 1985
Truck Model
K2500 C30
Engine Size
350 454
Did you use the the 700r4 cable bracket?
Did you have the cable adjusted properly?
Did you use the '85 carburetor or the one with the 400ci ?
There are many different variations of brackets for accelerator and detent cables for small blocks.
 

OuchMyToe

Junior Member
Joined
Apr 4, 2019
Posts
11
Reaction score
2
Location
Tooele, Utah
First Name
Kyle
Truck Year
1985
Truck Model
C10
Engine Size
400 CID
Did you use the the 700r4 cable bracket?
Did you have the cable adjusted properly?
Did you use the '85 carburetor or the one with the 400ci ?
There are many different variations of brackets for accelerator and detent cables for small blocks.

I'm using the q-jet, bracket, etc. I believe it was all adjusted "correctly" but if this failure is indicative of an incorrectly installed cable then please let me know. That being said, it is a franken-truck so...yeah. And while the carb is kind-of stock (the stock carb was that weird m4med q-jet), because of money and transportation requirements I had to mess with a bunch of stuff to get it running. However, it's run without a problem for 2 years now.
 

Matt69olds

Full Access Member
Joined
Jan 8, 2018
Posts
1,675
Reaction score
2,189
Location
Central Indiana
First Name
Matt
Truck Year
81
Truck Model
GMC 1/2 ton
Engine Size
455 Olds
The TV cable needs to be right, close enough won’t get it done. Ideally the cable needs to be adjusted with a pressure gauge connected.

Unless you have a really stout 400 engine (500-600 hp) a 700 is just fine.

Drop the pan, I bet you find a bunch of crud in the pan. That’s the remains of the clutches and band.

When you rebuild the trans, install the raybestos Z pac, wide band, upgrades drive shell, updated input sprag, hardened pump rings, and a decent shift kit.

Once all that is done, make absolutely sure the TV cable is adjusted right. The line pressure needs to increase as soon as the throttle is increased.

BTW, the linkage on a old school carb for the kickdown linkage won’t work for a 700.
Read this, even though it says 2004R, the process is the same.

https://www.ckperformance.com/Files/112573/ThrottleArmTVarticle.pdf
 

Dave M

Full Access Member
Joined
Jul 30, 2019
Posts
577
Reaction score
1,207
Location
Australia
First Name
Dave
Truck Year
1976
Truck Model
C10
Engine Size
454
Are you running a Transmission Cooler? You've cooked the trans.
 

shiftpro

Full Access Member
Joined
Aug 31, 2012
Posts
4,679
Reaction score
5,614
Location
BC Canada
First Name
shiftpro
Truck Year
73-87
Truck Model
1500, 2500, 3500
Engine Size
350, 383, 454, 496!
What size tires and what is the final gear ratio?
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,430
Reaction score
2,242
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
+1 on what Matt69olds posted. And trans cooler. Generally should tow in 3rd if it shifts alot when in od. Probably overheated, then burned some clutches and then clogged filter causing only being able to drive for a minute or so.
 

Nasty-LSX

Full Access Member
Joined
Nov 22, 2014
Posts
492
Reaction score
161
Location
HOUSON TX
First Name
Jon
Truck Year
1985 Silverado K20
Truck Model
2500
Engine Size
350
I build stock and performance transmissions. For my 2 cents, if you towed in overdrive that fast you cooked the TC. If you have no external cooler you cooked the trans to. I bet you fluid is black. Get it rebuilt with upgraded parts and put a big cooler if front if you going to be towing.
 

Itali83

Full Access Member
Joined
Feb 7, 2015
Posts
454
Reaction score
757
Location
Maine
First Name
Ben
Truck Year
1982, 1987, 1987
Truck Model
K10, 'burban C10, Jimmy
Engine Size
350, 6.2, 350
Don’t forget about making sure converter lock up is working. If that isn’t working, which I bet it isn’t since that’s the first thing everyone does when the monkey with these trucks is throw away all the digical “garbage” under the hood in the trash which includes the vacuum switch for lockup. A non functioning lock up converter is the biggest maker of heat there is.

Ben
 

oldretiredafguy

Full Access Member
Joined
Apr 20, 2020
Posts
780
Reaction score
1,753
Location
N. Texas
First Name
Mike
Truck Year
1988, 1988, 1991, 1983
Truck Model
GMC V-Jimmy, GMC Suburban 4WD, GMC Suburban 4WD, GMC-2500 CrewCab
Engine Size
350, 350, 350, 350
Did mine the same way several years ago: towing in OD as opposed to dropping it down into 3rd. Expensive life lesson you will never forget! :chair:
 

Matt69olds

Full Access Member
Joined
Jan 8, 2018
Posts
1,675
Reaction score
2,189
Location
Central Indiana
First Name
Matt
Truck Year
81
Truck Model
GMC 1/2 ton
Engine Size
455 Olds
As long as the trans isn’t constantly shifting/downshifting between 3-4, the converter doesn’t slip or chatter, and the engine doesn’t lug, you can tow in overdrive. The “don’t tow in overdrive” rule of thumb came about because the weezing engines of the early 80s barely had the power to pull the truck unloaded, put any weight behind the truck and the nonstop shuttle shift overheated the trans.

The 3-4 clutch has been a issue with the 700 since day one. The trans originally had 5 clutches, then later in its production run they made engineering changes to fit 6, when the 4L70 came out they upgraded to 7. The problem with adding more frictions in the same space requires thinner clutches/steels. The thinner parts don’t handle heat nearly as well. They warp, burn, glaze, once that happens it’s all downhill

The ultimate fix is the Raybestos Z Pak, it uses 12 single sided frictions. The 12 thick single sides frictions offer more surface area, and radiate heat better than thinner clutches
 
Last edited:

M00SE

Junior Member
Joined
Jun 13, 2020
Posts
14
Reaction score
93
Location
Winlock, WA
First Name
Moose
Truck Year
1984
Truck Model
K5 Blazer
Engine Size
454
My bet is the lockup was never plugged in or converted. Need a rebuild but like others say... Use all the upgrades and you will get plenty out of the 700. Big cooler! There is a small "breather" tube on top the trans. Run that down away from exhaust to prevent a fire if it ever over heats again.
 

OuchMyToe

Junior Member
Joined
Apr 4, 2019
Posts
11
Reaction score
2
Location
Tooele, Utah
First Name
Kyle
Truck Year
1985
Truck Model
C10
Engine Size
400 CID
Are you running a Transmission Cooler? You've cooked the trans.

LOL! Yes. Yes I have. There's the stock transmission lines running to and from the radiator. Because of the all the tweaks I had to make, it's got a somewhat upgraded cooling system (4-core rad, upgraded fan, etc.). I was planning to add an aftermarket trans cooler though.
 

Forum statistics

Threads
35,534
Posts
747,415
Members
24,453
Latest member
jbengfort
Top