Tranny slip and engine rpm

Greeting XJ Doc’s

XJ:
1989, 4.0L, AW4, 3.55 diff., 215/75R15 tire (27.6” dia.), Command-Trac t-case. 279K miles. Original owner.

Need:
Formula to calculate normal engine rpm at various speeds. Trying to figure out how bad and when the tranny is slipping.

Situation:
Tranny behaving differently. Accelerating hard from stand still, I feel the tranny slip for a fraction of a second. I consider it normal given the age. But, on highway speeds, the rpm seems a bit higher than normal when holding steady speed. When accelerating at highway speeds, tranny takes a long time to downshift, and on highway inclines it won’t downshift.

Answers to anticipated questions:
Original fuel pump. Holds pressure at 35-40 psi when driving
Oil pressure at idle, 30-35 psi; when cruising, 35-45 psi; when accelerating, 50-60 psi
Tranny oil change at 40-50K intervals with filter change every other time. No particles in the pan, clean fluid, and no smell.

What should I look into?

Thanks … Maz

Maz,
heres the formula your looking for to calculate rpms at different speeds.

RPMs=(1056*Vehicle Speed*Tranny Gear*Transfer Gear*Differential Gear)/(d*PI)

the three gears values need to be entered as follows.
If your rear diff is a 3.55 just enter the 3.55, don't worry about the 1:3.55 or 3.55:1 stuff just enter the number that is not number 1, unless its a 1 to 1 ratio in your tranny or transfer case then put a number 1 in that spot in the formula.

PI=3.141592654
d=tire diameter

this should give you your RPMs at any given speed.

Just post any questions about the formula you may have.

Here are some common gear rations for th XJ.
Manual automatic
1st 3.93/3.83 2.80

2nd 2.33 1.53

3rd 1.45/1.43 1.00

4th 1.00 0.75

5th 0.85/0.79 ------

Reverse 4.74/4.22 2.39
Axle Ratio 4.10/3.07 3.55

Have a good one.

Thanks Drewman;

Not sure if I understood the ratios right.

What do I need to use, at cruising speed in 2WD, for:

Transfer case ratio (Command Trac), isn't it just 1?

AW4 final ratio (0.75 or 0.79). Is there a fifth for auto?

Thanks for the info ... Maz

Correct,

stock command trac transfer case would be 1.00 in 2hi or 4hi, or 2.72 for 4lo, the AW4 only has 4 gears.
AW4 ratios as follows.

1st 2.80
2nd 1.53
3rd 1.00
4th 0.75

command trac 1.00 or 2.72
stock diff ratio 3.55

that should get you on your way with the number crunching.
sorry about the mess of numbers in the last post.

It might not be slipping... just not getting the correct signal form the TCU.

Check the connections in your harness between the TCU (under passenger side dash) and the transmission. --ALL-- of the connections including TPS. (Check TPS ground for leakage too.)

Some older ( RENIX ) XJ's have a bad habit of doing similar tricks w/ the tranny due to oxidized connections and poor grounds in the harness. Crappy connectors don't maintain proper contact after a few years.

Mine was fine when it was very dry but humidity from rain affected it (had to rain a day or two or remain very humid outside). Cleaned the contacts & a little dielectric grease & -most- of the problem went away. For me, only did this after it rained for several days (river/creek crossings did not affect it... ambient humidity did).

:anon:

Thanks for replys.

Would wrong tranny fluid cause my AW4 symptoms?

I know Dexron III is the fluid to use. But, the last fluid change was at a new place and I am not sure what they put in there.

A couple more Q's. Is there one harness bundle between the TCU and tranny? How do you check for ground at TPS, and where are all the grounds? Also, what to use to clean oxidized connectors before applying di-electric grease?

I probably sound pretty dumb, but I don't know much about auto electronics

Many thanks for help ... Maz

Your torque converter may not be locking up, that has happened to a couple of people. One of the NAC members found that and was never able to diagnose the cause. He just installed a toggle switch to lock it manually.

Here are some speeds and RPMs for you, for 215/75R15 tires:

3rd gear (no overdrive)
--------------------------------
30 mph = 1335
40 mph = 1779
50 mph = 2224
60 mph = 2669

4th gear (overdrive)
-----------------------------
30 mph = 961
40 mph = 1281
50 mph = 1601
60 mph = 1922