• Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!
    If you have problems registering or can't log into your account, please contact Admin.

Need Transmission Advice

SVTY SVN

Contributor
Jr. Member
Joined
Jan 6, 2022
Messages
75
Loc.
LA/TX
Good evening. Thanks in advance for any advice you can offer.

Sadly, our recently purchased EB now has a transmission that is slipping. Not something I wanted to deal with right off the bat but here we are.

Stock 302, C4, 3.50 gearing, 33” tires w 15” alloys, 80/20 highway/off-road use, sea level, flat earther. Like the current gearing span of 3.50s and the look of our 2.5” suspension lift w no body lift. But not completely opposed to 1” body lift if need be.

We actually prefer driving manuals and love their rugged simplicity but bc we’re not positive we’ll keep this EB, hesitant to go through hassle of conversion that might result in lesser appraisal.

Considering options to include rebuilding C4 myself though no experience, shop rebuild, RAT/RAN conversion, new process trans bc of granny gear and/or other options.

Open ended question I realize, but what are your thoughts? Thank you.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
43,179
The 3.50 gears with 33 inch tires are not helping your transmissions passage through time. That combination puts a lot of stress on an automatic, especially if the previous owner drove it hard.
I doubt whether by this time that it’s already showing trouble just changing the gear ratios or going back to a smaller tire is going to cure the slipping.
But it might…

When does it slip? Under what circumstances and is it all of the time?
 

gnpenning

Contributor
Bronco Enthusiast
Joined
Dec 26, 2011
Messages
1,883
Loc.
I have more questions than answers.
Paul brings up some good points, so I'm going to take a different approach.

If you want to go to a manual go to a 5spd. Yes you may have to change gears to take advantage of the overdrive, which might not be a good ROI on a vehicle you may not be keeping.

If you want to go to a 3 spd I would gladly trade you a 3spd for your C4. I have both and that would be a very unfair trade in my favor in my opinion. Some may not agree, but I think the 3spd is a step backwards.

In my 74 with the C4 I wanted a deeper gear, at the Time I didn't think I would ever be thinking about a overdrive transmission, so I went with 4:30. It made a huge difference on and off road. For me it was a great compromise between 4:11 and 4:56. It was also a great time to upgrade axles and locking diffs. Again, I don't know if that will work for your long term plans.

The C4 is about as easy a transmission for a first timer as you can get. I believe it is Bad shoe that has good videos as mentioned by other forum members in the past. Supposed to have step by step directions that are easy to follow. Viperwolf, a member on here in the past has a good list of the parts and part numbers you will need for a quality rebuild, he has posted before, you will need to search unless someone provides a link. He has been the go to guy in the past. I think we have a few others that are knowledgeable as well.
You have options and support here for whichever way you choose.
 

Broncobowsher

Contributor
Total hack
Joined
Jun 4, 2002
Messages
33,193
Going to a 3-speed manual will be a bad choice. With the current tire size and the current axle gearing it will not be happy getting moving.
The C4 you have now (ab)used the torque convertor to get fair drivability

Pick the transmission you want to drive. For modern highway driving I would look at 5-speed like an M5R2, AX-15. But not the wide ratio ZF.
But you have to gear the axles to match the transmission. That would be about 2000 RPM at 60 MPH.

The axle gearing is key. You are running a much larger tire, and have much higher drag than original. But you have the original gearing. Considering the original 3.50 gearing was the highway gearing (the 4.11 was the performance off-road option) to start with.
 

brewchief

Sr. Member
Joined
Feb 11, 2007
Messages
833
You could probably rebuild the C4 with some upgrades and regear the axles for the same cost as swapping in a manual 5 speed. I swapped from a C4 to a np435 years back and I think I ended up spending close to 1500$ to swap in a 50$ trans by the time it was all said and done, it seemed like the little stuff just kept adding up.

Sent from my SM-T510 using Tapatalk
 

blubuckaroo

Grease Monkey
Joined
Jun 11, 2007
Messages
11,735
Loc.
Ridgefield WA
The main issue with using the C4 with tall tires/high gear ratio, is its high 1st gear at 2.46. The manual three speed is only a little better at 2.99. The clutches in both the C4 and manual really take a beating.
A NV3550 has a 1st gear ratio of 4.01.
 

Timmy390

Contributor
Bronco Guru
Joined
Jan 1, 2011
Messages
5,207
Loc.
Conway, AR
4.11's or 4.30's will get you where you need to be with 33's. 4.56's might be better for the C4 depending on how hilly the terrain is in your area.

No matter what you do you need gears

Tim
 

Broncobowsher

Contributor
Total hack
Joined
Jun 4, 2002
Messages
33,193
He said flat ground. So 4.11s would be a good choice.
And for decades the go to gear for 33s and no overdrive (C4, 3-speed, NP435) has been 4.11s. The engine sounds like it is screaming, but the old carburated V8s don't mind it a bit.
What you are hearing is the cooling fan. May I interest you in an Explorer accessory drive as the next upgrade after the gears? Gets you a clutched cooling fan that can actually keep the V8 cool when it is hot outside (providing you use a fan shroud with it) but doesn't make so much noise all the time.
 
OP
OP
SVTY SVN

SVTY SVN

Contributor
Jr. Member
Joined
Jan 6, 2022
Messages
75
Loc.
LA/TX
Thanks fellas. Still pondering things but do appreciate all the info.
 
OP
OP
SVTY SVN

SVTY SVN

Contributor
Jr. Member
Joined
Jan 6, 2022
Messages
75
Loc.
LA/TX
The 3.50 gears with 33 inch tires are not helping your transmissions passage through time.
Totally agree. Tough question, but how long would you guess a rebuilt C4 w 3.5 gearing and 33s might last? I feel like if I can get ten years out of a rebuild and learn how to rebuild it myself that’s a viable long term option.
When does it slip? Under what circumstances and is it all of the time?
Looong story short, it currently starts out ok but slips more as it warms up. During trans service last weekend I found a 1/4” piece of clutch material in pan and smaller fragments. Concur that it is toast?
 
Last edited:
OP
OP
SVTY SVN

SVTY SVN

Contributor
Jr. Member
Joined
Jan 6, 2022
Messages
75
Loc.
LA/TX
If you want to go to a manual go to a 5spd. Yes you may have to change gears to take advantage of the overdrive, which might not be a good ROI on a vehicle you may not be keeping.
I really like the ROI perspective. Initially I leaned toward the NP435 but maybe the AX15 is a better ROI choice if we go manual.

The C4 is about as easy a transmission for a first timer as you can get.
Would love to learn this valuable skill but slowly realizing I do not have enough bandwidth right now. So I lean toward professional C4 rebuild or manual conversion.

You have options and support here for whichever way you choose.
Very appreciative of this fact!
 
Last edited:
OP
OP
SVTY SVN

SVTY SVN

Contributor
Jr. Member
Joined
Jan 6, 2022
Messages
75
Loc.
LA/TX
You could probably rebuild the C4 with some upgrades and regear the axles for the same cost as swapping in a manual 5 speed. I swapped from a C4 to a np435 years back and I think I ended up spending close to 1500$ to swap in a 50$ trans by the time it was all said and done, it seemed like the little stuff just kept adding up.

Great point. And I’m still not sure how to tackle the automatic shifter/gear indicator on the steering column should we do auto to manual swap.
 
Last edited:
OP
OP
SVTY SVN

SVTY SVN

Contributor
Jr. Member
Joined
Jan 6, 2022
Messages
75
Loc.
LA/TX
The main issue with using the C4 with tall tires/high gear ratio, is its high 1st gear at 2.46.
Completely agree! Believe I understand the argument to regear to 4.11. However, not to be controversial, if the chosen trans has a first gear low enough, it could effectively be the same overall ratio as stock, or better, despite staying w 3.50 gears and running 33s, right?

Not positive how to do this math but why not try. Using attached WH gear ratio chart:

2.46 C4 first gear divided by
3.83 AX15 first gear
Believe that’s a 64% improvement.

3.54 stock diff divided by
4.11 diff
Potential 14% improvement.

27” stock tires
33” aftermarket
18% degradation

I realize this is not in any way an apples to apples comp but hopefully it conveys the line of thinking…
 

Attachments

  • 79AA05CF-3A90-4B4E-B969-E1E5EFE36CDA.jpeg
    79AA05CF-3A90-4B4E-B969-E1E5EFE36CDA.jpeg
    218.3 KB · Views: 9
Last edited:

Broncobowsher

Contributor
Total hack
Joined
Jun 4, 2002
Messages
33,193
You just pointed out to yourself that 4.11s and 33s will put you pretty much even with stock tires and stock (highway) gears.

As for the not a ZF, it is a fine transmission. If you are into driving big truck transmissions. Think of it as the stock 3-speed manual, with a granny gear and an overdrive. Not like any modern vehicle. With my daily driver that has a 6-speed in it, I can be in any number of gears depending on what level of performance I want. And that is how most car type transmissions are. For most any speed, you have 2 different gears, maybe 3 or 4 that it is happy to be in. The ZF, and those wide gear spreads, you just row the gears. For any given speed, there is one good gear to be in, maybe two. For your 80% highway driving, one of the more car like shifting and ratio spreads would be better. If you were pushing more of a 50/50 rig, ZF starts looking better. Give up some road manors for off-road performance.
 

Timmy390

Contributor
Bronco Guru
Joined
Jan 1, 2011
Messages
5,207
Loc.
Conway, AR
First gear isn't the whole issue picture. You need to be in a better rpm all around to keep the C4 happy. 3.50 and 33's slips the clutches and generates heat and it all starts compounding from there.

A better shifting ZF is the M5ODR2. I just installed one. Only a few miles on it but first impressions.....I like it.

If you do the conversion to stick keep in mind the ZF or the M5R2 require a hydraulic clutch. It's a bit different than the stock fork clutch. Going to take some getting used to. It's a can of worms as some transmissions bolt right up and some need an adapter. All need an adapter to bolt up to the D20. Most will require a twin stick conversion or at least relocating the shifter/hole.

Let the rabbit hole begin....engine torque plays a role too. All about being in the power band.

Tim
 
OP
OP
SVTY SVN

SVTY SVN

Contributor
Jr. Member
Joined
Jan 6, 2022
Messages
75
Loc.
LA/TX
Broncobowsher and Timmy390,
Believe we are saying the same things. To that end, I lean toward either refurbing C4 (and regearing unless likely to get multiple years out of C4 w 3.50 gearing and 33s?) or swapping to AX15 and staying w 3.50 gearing. Very much appreciate any input.
 
Last edited:

Timmy390

Contributor
Bronco Guru
Joined
Jan 1, 2011
Messages
5,207
Loc.
Conway, AR
How long the C4 lasts with 33's and 3.50 gears will all depend on the amount and kind of driving you do. Most will say "borrowed time" but then again it might last years....

Not an auto expert but if I did go with the C4 and no regear I would add an additional cooler if possible. Heat kills

Tim
 

Broncobowsher

Contributor
Total hack
Joined
Jun 4, 2002
Messages
33,193
An AX15 with 33s and 3.50 gears isn't going to be happy either. That 5th gear you now have, might get to use it in low range, or coasting down the back side of a mountain.
3.50 is about the target gearing for an effective gear ratio when in overdrive on 33s. That is axle ratio X overdrive = about 3.50

Running that tall of a tire with that little of a gear on a C4 is done because the torque convertor stays loaded, multiplying torque. About 6 months ago or so I did a huge long post explaining torque, driving force, axle ratios. For the moment with the C4 you need more driving torque than the gearing can deliver. You are low enough in the RPM band that the convertor is multiplying torque. This is fine for short blasts, pulling away from a stop. But sustained it is harmful. It produces a lot of heat. Which is both hard on the transmission and wasteful of fuel. But it is also hard on the internals of the transmission as well. The torque convertor multiplies the torque before the transmission. You might be making 150 ft-lb at part throttle cruise, but what is going into the transmission could be 200 or 250, sustained. It is getting enough torque to the tire to get you down the road, but not happy about doing it.

As much as you are trying to fix this with transmission alone, the real fix is axle gearing. Not going to get around that.
 

Eoth

Contributor
Bronco Guru
Joined
Mar 10, 2004
Messages
1,642
I have a C4 with 33s and 4.11 gears and couldnt be happier... Since this is new to you, just checking off some boxes.... What trans cooling do you have going on? I know its overkill but I have both the rad cooling and external cooler going on. Since automatics seem not to be your preferred choice, are you sure that it is at the proper level (check isnt the same as checking the oil). Lastly, what fluid is in it? Beyond the concern of old burnt fluid, an original unrebuild C4 should be using "Type F". If you have something else in there it will slip. If it has been rebuild, most people will tell you to use modern Mercron or Dextron.
 
Top