hi RPMs

[4play]

My DD is a 6cyl, auto, 4wd running 235/75R/15's (about a 28.5 inch tire). I think stock gears for the 30 front/35 rear. Back and forth to work I may run 50 mph at @ 3k rpm. Tonight I ran her up to 70 mph and was hitting 5k rpm. The tach indicates redline starts at 6k rpm. Is it me? or is this running a bit high?

 

For what it's worth, I do have the power trans button in the off position and the floor shifter pattern is P, R, N, O/D, 3, 1-2. I was in the O/D position.

[DirtyComanche]

Sounds like it's not shifting into 4th.

[Hot Rod Jeep]

That's what it sounds like to me too. Try dropping it down to 3rd gear while going down the road and see if it drops a gear. You can also listen to it shift to see if it goes into 4th. 8)

[4play]

I was hoping (partially) that was the problem (at least then I would know what the problem was). However, I tried watching/listening today while on my way to work. It seemed to shift from 1st to 2nd @ 15 mph/3.2k rpm; 2nd to 3rd @ 25 mph/3.2k rpm; and 3rd to 4th @ 35 mph/3.2k rpm.

While running at 50 mph and 3.5k rpm, I dropped it from O/D to 3 and it revved up to @ 5k rpm. :hmm:

[Eagle]

The MJ never came with 235s, so let's assume that your speedo thinks you have 225/75x15 tires. Stock gearing with the automatic was 3.54:1.

 

With that gearing, in 4th gear and torque converter locked up you should be seeing about 1615 RPM at 50 MPH, and about 2250 RPM at 70 MPH. It sounds like (a) you're not getting 4th (overdrive) gear AND your torque converter isn't locking up.

 

Assuming you only have 3rd gear, the RPMs would be about 2150 RPM @ 50 MPH and 3015 RPM @ 70 MPH. If you're seeing more than that, the tranny is slipping and/or the converter isn't locking.

 

Or ... a previous owner changed the gearing.

[partsmj]

Or ... a previous owner changed the gearing.

 

Or washed the engine down. The TPS is very sensitive to water.

 

33004650 $37.00

Electrical - Powertrain control - Throttle position sensor

THROTTLE POSITION SENSOR, 4.0L, Auto Trans 1986 - 1990

[4play]

While driving home tonight I put the trans button on the "power" position. I was running an even 3k rpm and put the button in the "comfort" position. The tach did not register any change. Shouldn't it have reduced the rpms? :hmm:

 

 

Further showing my ignorance, could running with low trans fluid cause the torque converter to not lock up? Or more likely it's just worn out? (My guess is worn out. The odometer is showing @ 225k miles but I doubt its accurate - the vin check I used from a link on this site indicates I have a 4 banger, but there are a couple extra cylinders there.) :dunno:

 

My continued thanks for everyones assistance :D

[Pete M]

If I remember right, the power/comfort switch mearly changes the shift points.

[4play]

If I remember right, the power/comfort switch mearly changes the shift points.

 

OK, that would better explain no change in rpm. I assumed it worked like the "O/D off" button I am so accustomed to on the Crown Vics I drive at work. :dunce:

[Pete M]

In MJs you have to shift into "3" to turn the OD off.

[aemsee]

Does your engine actually sound like it is doing 5,000 rpm's when it shows that on the tach? A 4.0 at 5,000 is screaming. If someone changed from a 2.5 to a 4.0 maybe the tach isn't reading correctly. What does it idle at? And check your gearing. If they swapped the engine, maybe you've got some 4.88's hanging out in the back :D

[Eagle]

aemsee has a good point. Even if a previous owner swapped gearing, the most common swap is 4.10s from a 4-banger. The gearing doesn't make as huge a difference in RPM as some people think. Even with 4.10 gears, and no overdrive, 70 MPH would only be 3500 RPM. The highest (numerical) ratio available for the D30 is 4.88, and that would only run at 4150 RPM at 70 MPH.

 

Methinks you need to have a bit of diagnostics run that we probably can't do for you by remote control.

[Oizarod115]

not only would a 4.0 at 5000rpm be literally screaming, but

a) rev limiter is in the low 5k's i think like 5125 or something like that.

B) you'd have to have the pedal damn close to the floor to get the revvs to stay that high

 

id check the tach... if it IS running that high then i would drive 50mph more often haha.

[4play]

The engine idles around the 1k mark. And at 5k showing on the tach, it sounds like 5k.

 

Guess I'll follow Oizarod's advice and drive slow until I have a chance to look further into this. Man, work really interferes with the important things in life.

[Eagle]

It should idle at around 650. Are you certain your tach is accurate?

 

What model is your MJ? If it's a model that didn't come from the factory with a tach, the previous idiot (err, "owner," yeah that's it) may have dropped in a gauge cluster from a 4-banger. Think about what that means.

 

In a 6-cylinder, there are three firing pulses for every revolution. For a 4-cylinder, there are only two pulses per revolution, so if a 4-cylinder tach sees three pulses, it reads 1-1/2 revolutions insteal of one. Which means it'll read 1500 instead of 1000, 4500 instead of 3000.

[Oizarod115]

to be safe though... id still run it slow.

you did say the tach redlines at 6k which leads me to beleive it isnt stock for the engine. cause my full gauge cluster (stock) definately reds from 5k up.

 

if you got brave you could punch the gas (nuetral) and hit the rev limiter while watching the tach... if the tach is right on it should rev-limit to right above 5k if its off then the tach will skyrocket 1 1/2 times or so like eagle said.

 

or you could get a cheap "tuner-tach" and wire that up to check.

 

have you tried counting shifts yet?

 

(trust me i wouldnt count on the gauge being wrong.. PO told me the temp gauge was off and read hot cause the motor was swapped... this was not the case, and i ran it at 250 for about 4 months... :eek: this guy was a naxja-er too.)

[feerocknok]

My 2.5 redlines at 5500...

[4play]

I didn't fully notice the red line in the dim light while driving at night but mine also begins the redline area at 5500, the 5 is normal colored and the 6 is red; the red line starts under the mark that indicates 5.5.

 

I did try starting in 1-2 and manually shifted up to the OD position to see how it acted. In 1-2 it never seemed to shift from 1st to 2nd, but in 3 it did seem to hit 2nd and 3rd. :hmm: Then when I hit OD it seemd to shift again.

 

There's a local retired guy that has a reputation for being really good at tranny work, if I can just get him away from the other old men that hang out telling lies and drinking coffee all day, maybe he can give some extra guidance.

[87manche]

I'm gonna take a WAG at it and guess trans solenoids.

[partsmj]

I would check the throttle position sensor.

[Eagle]

I want to know what model MJ, because I think the tach was swapped in from a 4-banger.

[GirsMJ86]

he could check or post his vin so it can be decoded. that would tell ya what it had in it stock atleast.

 

Alex

[4play]

The VIN is 1JTHR65T0HT053241

 

1 1 Country : USA

2 J Manufacturer: Jeep (USA)

3 T Type: TRUCK

6-7 65 Model: 4WD COMANCHE 7`BOX

5 R Transmission/transfercase: Automatic Transmission, Part Time Transfer Case

4 H Engine 2,5 litres, I-4, central injection

9 0 Control digit: Has passed check

10 H Year: 1987

11 T Plant: Toledo, Ohio

12-17 053241 Serial number 053241

[4play]

I would check the throttle position sensor.

 

Wouldn't the TPS also affect idle? ( :dunno: trying to learn as well as fix my truck) It seems to idle ok.

[aemsee]

If you look at the TPS it should have two pigtails comming off of it with 3 wires each. One set is for the ECU and one set is for the trans controller. So the ECU could be seeing the correct voltage and the trans controller might not, but this would only come in to play with incorrect shifting and/or no 4th gear. You need to get someone to drive your truck and verify that you have all trans gears.