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

[Eagle]

 

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?

 

That was my thought (torque converter lockup) as well.

 

I don't think the t-case is in low range. The '86 would have an NP207, with a low range of 2.6:1. If the correct speed should be around 3,000 RPM, multiply that times 2.6 and you get 7,800 RPM.

 

What model MJ is it? Could it be a swapped in instrument cluster out of a 4-cylinder? Lemme think again which way that goes. 4-banger has two pulses per revolution, 6-cylinder has three pulses per revolution. So say a 6-cylinder is turning 1000 rpm. That would be 3000 pules for the tach to read 1000.

 

A 4-cylinder only has two pulses per revolution, so the same 1000 RPM would be 2000 pulses. Feed 3000 pulses to a 4-cylinder tach and it'll read 1500 RPM, not 1000 -- in other words, 50 percent faster than the engine is actually turning.

[xjrev10]

It definitely acts like it shifts into a 4th gear. I was questioning myself too after a few hours of researching the trans, transfer case this thing has. This is really starting to bug me. This is the reason I got it for a steal BTW! The PO was fed up..

[xjrev10]

Interesting about the tach. I'm not sure what the model is...

 

 

[Eagle]

Interesting about the tach. I'm not sure what the model is...

 

What's the 8th character of the VIN?

 

7 = X (base)

8 = XL

9 = XLS

F = Custom w/ Metric Ton

G = X w/ Metric Ton

H = XL w/ Metric Ton

J = XLS w/ Metric Ton

[xjrev10]

I'll look tomorrow.

[mpace6a]

 

 

 

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.

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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Bingo.

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Speedo is GPS surrogate, therefore tach must be off.

Whats confusing, is that, up to high speeds, tach seems quite accurate. Then again i could also be remember wrong...that was several months ago, and no i can't go past 35 mph without getting death wobble...so i'll have to fix that, find an empty downhill highway with some tailwind and try again.

[Oyaji]

 

 

 

 

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.

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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Bingo.

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Speedo is GPS surrogate, therefore tach must be off.

Whats confusing, is that, up to high speeds, tach seems quite accurate. Then again i could also be remember wrong...that was several months ago, and no i can't go past 35 mph without getting death wobble...so i'll have to fix that, find an empty downhill highway with some tailwind and try again.

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No need. Speedo is still busted, so no point. Tach  function depends on engine speed and is independent of running speed, so just check it against a known good instrument - your driveway works just as well as a run "downhill with a tailwind". 

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Incidentally, I have seen electric tachs work fine for the first portion of their range,  but then go on the fritz for the high RPM part of their range. The condition you describe would not be unique.

[Oyaji]

 

 

 

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

.

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

.

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

That was my thought (torque converter lockup) as well.

 

I don't think the t-case is in low range. The '86 would have an NP207, with a low range of 2.6:1. If the correct speed should be around 3,000 RPM, multiply that times 2.6 and you get 7,800 RPM.

 

What model MJ is it? Could it be a swapped in instrument cluster out of a 4-cylinder? Lemme think again which way that goes. 4-banger has two pulses per revolution, 6-cylinder has three pulses per revolution. So say a 6-cylinder is turning 1000 rpm. That would be 3000 pules for the tach to read 1000.

 

A 4-cylinder only has two pulses per revolution, so the same 1000 RPM would be 2000 pulses. Feed 3000 pulses to a 4-cylinder tach and it'll read 1500 RPM, not 1000 -- in other words, 50 percent faster than the engine is actually turning.

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But xjrev10 reported that "you can feel the engine just cranking like a mother". Jim reported 1,900 RPM @ 75 MPH (which really does seem low, given that a 4.0L I-6 turns more than that at 65 MPH); how sure are you of the 3,000 RPM for a 2.8L V-6 at 75 MPH? I agree your figure seems a  more realistic one...

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"You can feel the engine just cranking like a mother" at 75 MPH is not a good quantitative observation - does that mean 3,000 RPM? 4,000 RPM? 5,000 RPM?  or even faster? If Eagle's observation is accurate that 3,000 RPM @ 75 MPH is what a 2.8L V-6 should be turning, I think he must be on target with his rationale of a 4-cylinder tach swap replacing a 6's sometime in the past. Seems too easy though... but there is no way to tell until comparing the in-dash tach with a known good one.

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A number of auto parts stores offer a free (just a refundable deposit required) loaner tool deal - best, cheapest, and easiest to just run down there and compare your tach with one of theirs, I think.

[xjrev10]

Just got home from work. Have not checked the vin yet.

 

Remember the trans was rebuilt. Could something have been out in wrong or backwards? PO said he didn't know if it was "normal" before he got it but after the trans was rebuilt.

[Oyaji]

Just got home from work. Have not checked the vin yet.

 

Remember the trans was rebuilt. Could something have been out in wrong or backwards? PO said he didn't know if it was "normal" before he got it but after the trans was rebuilt.

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Maybe?

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Need to start checking the cheap/free stuff, then move on from there.

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Check the tach across the whole RPM range against a known good one - free check at the auto parts store.

[Eagle]

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But xjrev10 reported that "you can feel the engine just cranking like a mother". Jim reported 1,900 RPM @ 75 MPH (which really does seem low, given that a 4.0L I-6 turns more than that at 65 MPH); how sure are you of the 3,000 RPM for a 2.8L V-6 at 75 MPH? I agree your figure seems a  more realistic one...

.

"You can feel the engine just cranking like a mother" at 75 MPH is not a good quantitative observation - does that mean 3,000 RPM? 4,000 RPM? 5,000 RPM?  or even faster? If Eagle's observation is accurate that 3,000 RPM @ 75 MPH is what a 2.8L V-6 should be turning, I think he must be on target with his rationale of a 4-cylinder tach swap replacing a 6's sometime in the past. Seems too easy though... but there is no way to tell until comparing the in-dash tach with a known good one.

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A number of auto parts stores offer a free (just a refundable deposit required) loaner tool deal - best, cheapest, and easiest to just run down there and compare your tach with one of theirs, I think.

 

I'm 100 percent certain of my figures (to within about 1 to 2 percent deviation). Years ago I went to tire manufacturers' spec sheets and looked up their actual revolutions-per-mile data. That's the most accurate way to correlate engine speed to road speed, and those data were what I used to generate the spreadsheet I built up to show engine RPMs are various road speeds for all the tire sizes typically used on XJs and MJs, and for all axle ratios from 3.07 through 4.88.

 

But the OP didn't say 75 MPH, he said 3700 RPM at 65 MPH. He also didn't say what tire size he has, and I don't remember what sizes were stock in 1986. I took a guess at 215/75-15 and used those numbers.

 

I just realized, after re-reading the description of how his transfer case works, that he has the NP228 Selec-Trac case, not the NP207 Command-Trac case. So I don't think his truck is a base model. Therefore, unless/until he says otherwise, I think we can generalize on 225/75-15 for the tire size. I don't know if the 2.8L was ever sold with 3.55 gears. I have the remains of an '86 XJ with the V-6 and the 228 transfer case out back -- it has 3.73 gears, which I verified. That's why I assumed that the OP probably has 3.73 gears. But ... what if he has 3.55s?

 

For 3.55 gears with 225/75-15 tires, 65 MPH  is 2800 RPM. Now ... if he has a 4-cylinder tach in there, 1.5 x 2800 = 4200. Too high -- so that may not be the answer. And if he does have 3.73 gears it would get even worse.

 

I would suggest that the OP beg, borrow or buy (but not steal) a hand-held tachometer, or get some time on a shop scanner, and run the engine against a calibrated tach to compare what the diagnostic tachometer reports compared to the one in his instrument cluster.

 

Second gear ratio in that tranny is 1.55. If he's supposed to be turning 2800 RPM at 65 MPH, but the tranny is stuck in second, that would work out to

 

2800 x 1.55 = 4340. Nope -- that's not the problem, either.

 

Could be that the torque converter ISN'T locking, and that it has a lot of slip.

[Rockfrog]

Given the 904 trans (having no O/D)

NP228 or NP207 (same ratios anyway)

and 215/75/15 tires

 

geared 3.73 = 3108rpms at 65 mph

 

geared 4.10 = 3416rpms at 65mph

 

now ... if he happens to have 4.56 gearing for some weird reason ... that number becomes 3799 rpms at 65mph ...

 

methinks one should pull the covers, or find a tag if still there and verify gear ratios.

[xjrev10]

Vin number indicates that it's a base model X per Eagles chart above.

 

I crawled under and looked and ill be damned but it is actually a NP228 case. I can read the tag still plain as day.

 

Tire size is 215/75/15.

[Eagle]

Vin number indicates that it's a base model X per Eagles chart above.

 

I crawled under and looked and ill be damned but it is actually a NP228 case. I can read the tag still plain as day.

 

Tire size is 215/75/15.

 

Okay, then engine RPMs at 65 MPH should be:

 

3.55 ==> 2892

3.73 ==> 3047

4.10 ==> 3349

4.56 ==> 3725

4.88 ==> 3986

 

Don't discount the possibility of 4.56 gears. A number of years ago I picked up an '84 or '85 XJ Wagoneer ... with a 4-cylinder automatic. It had the same transmission and transfer case you do. I bought it only to keep it from being junked, and a year or two after I bought it I gave it to a guy in NAXJA NAC from northern Massachusetts. It was supposed to be for his daughter, but then his XJ crapped out and he used it as a daily driver for a year or more.

 

But I digress. The point was, I picked him up somewhere in MA and brought him down to my home to pick it up, and he drove it home. We talked either late that night or the next day, and he reported that it ran fine except that the tach was showing alarmingly high RPMs on the highway. He opened up one of the diffs and discovered that it did, in fact, have 4.56 gears ... and it apparently had come from the factory that way. Things like axle ratio were a lot less standardized back in the days of the early AMC XJs and MJs. The factory wasn't shy about offering optional axle ratios.

[xjrev10]

FWIW, I am the third owner of this Jeep. The original owner said they used to run on the interstate with it all the time and it never ran like that. Now that could be a miscommunication somewhere obviously. I guess that is what started the whole question. Somebody said it was fine at one time, but its not now....

 

Ill check gears. I have a MJ D44 that I'm going to swap into this eventually with 3.55s.

[xjrev10]

Picked up some gear lube and rtv. I'm going to check the gears in the d35 tomorrow.

[xjrev10]

Just found out the original owners used to pull a "big" car trailer with this truck. And also used it to plow snow. (I knew that about the plowing.)

 

Story and mystery just keeps getting better.

[Eagle]

Just found out the original owners used to pull a "big" car trailer with this truck. And also used it to plow snow. (I knew that about the plowing.)

 

That explains why the transmission went walkabout ...

[Oyaji]

 

Just found out the original owners used to pull a "big" car trailer with this truck. And also used it to plow snow. (I knew that about the plowing.)

 

That explains why the transmission went walkabout ...

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If the tach is correct (and that is still a big IF, right? free check, should do that...), and considering the operating load they placed on that weak-sauce engine, it would have made good sense to put short-leg final drives under it. I wonder just how short they geared it...

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I concur with Eagle about what that sort of heavy duty must have done to the transmission... 3 times, right?

[xjrev10]

Waitin for the garage to warm up a bit on this 23 degree morning...

 

The trans has been rebuilt once but the t case has been apart 3 times for unknown reason. I gotta run to the parts store this am to pick up some parts for the plow truck and I'm going to pick up a tach if they have a reasonable one.

[Oyaji]

Waitin for the garage to warm up a bit on this 23 degree morning...

 

The trans has been rebuilt once but the t case has been apart 3 times for unknown reason. I gotta run to the parts store this am to pick up some parts for the plow truck and I'm going to pick up a tach if they have a reasonable one.

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Unless you frequently need a tach for general use working on various vehicles, you don't need to buy one. As I mentioned earlier, many/most/all of the auto parts chain stores offer a free loaner tool deal. You put down a deposit on the tool, use it, then return it in good condition and you get your full deposit refunded.

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It's a good  program they offer - I take advantage of it for tools I seldom need.

[xjrev10]

I'd probably install it in my 88.

[xjrev10]

4.11 gears

Factory tach is off abut 300 rpm according to aftermarket tach

 

I remember the 60mph figure due to the fact that is what I last drove before I got home.

 

3200rpm on aftermarket tach @60mph.

 

Seems to me there may be nothing wrong. This isn't a highway truck obviously!!

 

It's idling at 1500rpm now all of a sudden. Must be a vacuum leak.....yippy skippy

 

 

I did end up purchasing a tach. Parts store did not have one to rent. I temporarily installed it on this truck but it eventually will make its way to the Wyomanche.