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'86 Rpm Issues... Way Too High For 65?

xjrev10
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xjrev10 Comanche Fanatic

xjrev10

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86, v6/auto/np207

Leaving its home



Arriving at its new home



Took it to town. Something is wrong here... 3700 rpms at 65 with stock tires..



At the DMV



Truck is wet in the following pictures but it really is that shiny! Been repainted.









POs pics











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xjrev10 Comanche Fanatic

xjrev10

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What the heck could be wrong that it turns super high rpms? The trans has been rebuilt recently and the t case has been apart 3 times I guess. PO thinks it has something to do with the t case. He also says 4lo doesn't work but I haven't tried it yet.

 

Lookin at SBChysler swaps.. But it does run nice right now.

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xjrev10 Comanche Fanatic

xjrev10

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Look at my Black Betty build for a ChryCo Magnum V8 swap. It's been fun!

Already have. Seems you like wiring! I don't have the patience. But I know a guy..

 

Jim, you can feel the engine just cranking like a mother. I thought of that too, but I don't thnk it would be possible to hit 65-70mph. I'm going to play with it tomorrow to see if I can learn more.

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Oyaji Comanche Addict

Oyaji

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86, v6/auto/np207

 

 

Took it to town. Something is wrong here... 3700 rpms at 65 with stock tires...

.

1986MJ,V6, not auto but 5 speed...

turns 1850-1900 at 75. Never paid attention what it does at 55 and 65

.

What the heck could be wrong that it turns super high rpms? The trans has been rebuilt recently and the t case has been apart 3 times I guess. PO thinks it has something to do with the t case. He also says 4lo doesn't work but I haven't tried it yet.

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Maybe stuck in Low Range? Surely you can hear an engine over revving. Check the tach against a known instrument

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Jim, you can feel the engine just cranking like a mother. I thought of that too, but I don't thnk it would be possible to hit 65-70mph. I'm going to play with it tomorrow to see if I can learn more.

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Auto transmission clutch(s) slipping? Torque converter slippage? Waaaaay too high RPM.

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Evidently the transfer case low range ratio is 2.60:1, and high range is "straight through" or 1:1:

 

The NP 207 is one of the weaker Jeep transfer cases, but can be upgraded. The NP 207 has an aluminum case and chain driven planetary reduction. Thankfully the NP 207 was installed behind some of the weakest Jeep power trains every used. It featured a 2.60:1 low range and a straight through design high range, but featured a driver side front output. It was used in one year of the Jeep YJ Wrangler 1987, and in the early XJ Cherokee’s from 1984-1987.

http://www.4x4review.com/jeep-transfer-case-identification-guide/

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there's no overdrive in that auto trans.

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So assuming top gear at 1:1 and transfer case also at 1:1, if you were stuck in low range, then RPM in high range would be your RPM in low range divided by 2.6. At 3,700 RPM @ 65 MPH, this would yield :

.

 

3,700 / 2.6 = 1423 RPM.

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That's just not realistic... hmmm. Besides, since low range is 4X4, I guess that would have torn something up in the 4X4 drivetrain, but you reported no problems.

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Let's look at final drive next. I dunno how accurate/applicable this info is (there is prolly better info on this site, but I don't know where to look), but here is the Wiki info for the XJ - it will do to set up the math (you can substitute better values if anyone has them, then recalculate):

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Jeep XJs came in several standard gearing ratios:

  • 3.07:1, manual transmission, I6 engine.
  • 3.54:1, automatic transmission, I6 engine with Dana 44 rear differential.
  • 3.54:1, manual transmission, I4 engine with Dana 35 rear differential.
  • 3.55:1, automatic transmission, I6, V6 engines; manual transmission, I4 engine.
  • 3.73:1, automatic transmission, I6, Tow Package, UpCountry Package.
  • 4.10:1, 2.5L I4 engine
  • 4.56:1, automatic transmission, I4, offroad or tow package.

http://en.wikipedia.org/wiki/Jeep_Cherokee_%28XJ%29

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According to this site, top gear for the AX-5 is 0.85:1:

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AX5 Gear Ratios: First - 3.93

Second - 2.33

Third - 1.45

Fourth - 1.00

Fifth - 0.85

Reverse - 4.74

http://www.smartpartsauto.com/manual_transmission_ax5_jeep.html

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So, assuming that the above info is good and that OE was 3.55 final drive ratio, that Jim's 5th gear ratio is 0.85:1,  that top gear ratio in your auto transmission is 1:1, that axle changes were made sometime in its history, and that the tachometer is working properly (as Jim mentioned, you should definitely check yours against a known good instrument), let's see what swap might have been made that matches your reported RPM:

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per Jim's reported RPM:

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1900 / 75 = x / 65

cross-multiplying yields 75(x) = 123,500

solving for x: x = 1,646 RPM @ 65 MPH.

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Correcting for a 1:1 ratio top gear from the AX-5 0.85 top gear:

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1,646 / 0.85 = x' / 1

cross-multiplying: 1,646 = 0.85(x')

x' = 1,937 RPM in 1:1 top AT gear.

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Getting there. We know from the above assumptions that final drive should be 3.55:1 ratio, so let's set up one last proportion:

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1937 / 3.55 = 3700 / r 

1937® =13,135

x = 6.78, or 6.78:1 ratio final drive.

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Holy smokes - that isn't realistic either! 6.78:1 ring and pinion sets??

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Something must be wrong in the assumptions, or else it must be a transmission slippage problem as mentioned earlier. Jim, your 1900 RPM @ 75 MPH seems low - any chance it could be in error? But even if it was substantially off, that would still mean an unrealistic final drive ratio, I reckon.

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No, I think transmission slippage from torque converter and/or internal clutches (and maybe even coupled with a bad tach reading) is likely here.

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Oyaji Comanche Addict

Oyaji

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Sounds to me like it is stuck in second gear.

Started previous  post very late, finished it this morning without checking for more recent posts... and right now I am too tired to look up 2nd gear ratio and do the math. But this makes perfect sense.

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  Simplest solution might be it. xjrev10, have you counted how many times it shifts?

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Oyaji Comanche Addict

Oyaji

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My 2.8 5spd 4x4 spins ~2300 rpms consistently at any speed above 55 and lower than 93mph, since that's where it tops out. I doubt I have the power to spin any more in 5th gear.

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That just cannot be.

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Think about it - engine is coupled with direct gearing to the ground via drivetrain and tires. RPM is always directly proportional to ground speed (minus maybe a tiny slippage error between tires and  pavement along with an  even tinier clutch slippage error). Tachometer must be in error, possibly speedometer as well. Like Jim said earlier - check against a known good instrument.

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xjrev10 Comanche Fanatic

xjrev10

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Played with this a little more this am. It most definitely shifts into fourth gear. Taking off really slow it shifts into 4th at about 35-40 mph. MPH is correct, I checked with GPS.

 

I played with the 4 wheel drive a little. WTF? Looks like a vacuum motor that I presume runs off the selec-trac switch on the console. I could not the the linkage to move at all to move it presumably into 4lo. The lever on the t case itself is all the way back as in spun clock wise to the rear of the truck.

 

The case itself is leaking so ill have to pull it apart to seal it up away. I've been inside a 231 a few times so I'm not too scared of this one.

 

 

 

 

 

 

 

 

 

 

Look at my Black Betty build for a ChryCo Magnum V8 swap. It's been fun!

 

Already have. Seems you like wiring! I don't have the patience. But I know a guy..

:shake:

Guy in my jeep club is a mopar guy. He's got a few buggies with magnum power.
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xjrev10 Comanche Fanatic

xjrev10

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This did get a little more techy then I originally thought. Thanks for moving.

 

 

I can't figure out how to change the title of this thread. I did it once on the old CC program but never on this. Maybe 86 RPM issue or something like that would be more fitting.

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mpace6a Comanche Addict

mpace6a

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My 2.8 5spd 4x4 spins ~2300 rpms consistently at any speed above 55 and lower than 93mph, since that's where it tops out. I doubt I have the power to spin any more in 5th gear.

.

That just cannot be.

.

Think about it - engine is coupled with direct gearing to the ground via drivetrain and tires. RPM is always directly proportional to ground speed (minus maybe a tiny slippage error between tires and  pavement along with an  even tinier clutch slippage error). Tachometer must be in error, possibly speedometer as well. Like Jim said earlier - check against a known good instrument.

No speedo. Hasnt worked for the past 4500 miles or so. GPS is what my speed is based off of, checked between two at that time. Tach being off i can believe, because the engine screams at 3000 rpm (according to tach) like its redlining.

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Eagle Obsessive Comanche Disorder

Eagle

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Played with this a little more this am. It most definitely shifts into fourth gear. Taking off really slow it shifts into 4th at about 35-40 mph.

 

4th gear? I thought that was a 3-speed tranny. And my 1986 Comanche FSM confirms this. First gear is 2.74:1, second gear is 1.55:1, and third gear is 1:1. With the V-6, the most likely axle ratio was 3.73. If the stock tires were 215/75-15, with 3.73 gears the engine should be turning 3047 RPM at 65 MPH. If it has 4.10 gears, it should be turning 3349 RPM at 65 MPH.

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Oyaji Comanche Addict

Oyaji

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My 2.8 5spd 4x4 spins ~2300 rpms consistently at any speed above 55 and lower than 93mph, since that's where it tops out. I doubt I have the power to spin any more in 5th gear.

.

That just cannot be.

.

Think about it - engine is coupled with direct gearing to the ground via drivetrain and tires. RPM is always directly proportional to ground speed (minus maybe a tiny slippage error between tires and  pavement along with an  even tinier clutch slippage error). Tachometer must be in error, possibly speedometer as well. Like Jim said earlier - check against a known good instrument.

No speedo. Hasnt worked for the past 4500 miles or so. GPS is what my speed is based off of, checked between two at that time. Tach being off i can believe, because the engine screams at 3000 rpm (according to tach) like its redlining.

.

Bingo.

.

Speedo is GPS surrogate, therefore tach must be off.

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Oyaji Comanche Addict

Oyaji

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Both of my V6 powered X/MJ autos were three speeds. Dunno?

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Per Pete M's info, top gear if no overdrive should still be 1:1 ratio, therefore earlier assumptions hold.

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xjrev10 reported four shifts. Could the 4th shift be the torque converter locking in a 3-speed?

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