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Old 03-20-2011, 07:34 PM   #1
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Diesel @2500 rpm's @ 60 MPH! MT643 Tranny. What can be wrong?

I am having an issue with my Coach. I am at 2500 RPMs at 60 MPH...2800 RPMs at 65 MPH. I am at red-line just to drive at highway speed.

1992 Safari Ivory, Cummins 6CTA8.3, Allison MT643. All Mechanical. Rear End is a 391. Tires 9R22.5, PAC Brake not connected to Tranny.

It is shifting all 4 gears but this is what I'm feeling:

I can feel it shift from 1-2, then from 2-3, and while in 3rd the rpms drop a second time, about 200-300rpms before shifting into 4th. I believe the drop in RPMs is the converter locking.

THEN;

After it shifts to 4th gear I get the initial drop in RPMs but don't get the second drop like I do in 3rd gear. Tranny not locking in 4th? I think it suppose to lock in 3rd and 4th. Though RPMs are still high in low gears.

I can remember having to downshift to get up a couple of hills by my house last time I drove it 2 years ago, but now I don't have to down shift, the RPMs are high like it's in a low gear but I know its shifting to 4th gear, MAYBE the tranny isn't locking correctly? I have no idea but RPMs seem high even in 1st gear.

PAC brake is turned off, I even removed air line to make sure it wasn't on.
Tranny Service is 3 years old, only about 1000 miles since having it serviced. Fluid looks good and no smell of burning. I don't feel any kind of slippage either.

Below are the RPMs each time it shifts. I think the RPMs are too high even in 1st gear. I thought at first Tach was off, but engine seems to be screaming and peddle is at the floor to get to 66 MPH.

0-12ish MPH 2500ish RPM 1st Gear
12-20ish MPH 2300ish RPM @12mph it shifts to 2nd gear
20-25ish MPH 2300ish RPM @20mph it shifts to 3rd gear
25-35ish MPH 2500ish RPM @35mph it shifts to 4th gear

Any ideas would be appreciated. I'm afraid if I drive it I will damage something. Engine is already leaking diesel it never leaked before, I think the High RPMs may have caused an injector and high pressure fuel line to leak.

Someone else with the same Engine and tranny as me said he is at 1900 RPMs @ 60 MPH.

Here are the links to pictures of Dashboard showing speed and RPMs.

http://i238.photobucket.com/albums/f...IdleRPMs-1.jpg
http://i238.photobucket.com/albums/f...mph1800rpm.jpg
http://i238.photobucket.com/albums/f...mph2300rpm.jpg
http://i238.photobucket.com/albums/f...mph2500rpm.jpg
http://i238.photobucket.com/albums/f...dlefloored.jpg
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Old 03-20-2011, 08:50 PM   #2
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Cummins PowerSpec Gearing Recommendations

Check the above website, plug in numbers as close to your actuals as possible and see if 2500 RPM at 60 MPH is to be expected. The 643 is a non overdrive transmission so your final transmission ratio is 1 to 1. You may have to select some wierd equipment combinations to equate your equipment but the final result will be close.
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Old 03-20-2011, 09:06 PM   #3
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My coach has a 6CTA engine with MD3060 transmission and at 60mph I am running at 1700 rpm. You are over-revving your engine as the transmission is not up-shifting. I think you need an Allison shop to run diagnostics.
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Old 03-20-2011, 09:26 PM   #4
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FleetMan: I ran the numbers as close to my specs as possible and its definately over RPMs.....

Algoma: another guy with the 6CTA8.3 and MT643 said he's at 1900 RPMs running 60 MPH. That would make me waaaay over.

The problem is the tranny & engine is mechanical is the 1992 so they can't run any electronic Diagnosis. I'm in a catch 22...I would like to do whatever I can myself before bringing it in to be checked, they always seem to find a problem whether you have one or not!

If it was a gasser I would put on a temporary Tachometer to see if my tach is wrong. From what I'm told it's not that easy with the Diesel I have. It sure would be a piece of mind if it was only the tach causing the issue.

Is there a way of adding a tach to see if mine is correct?
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Old 03-20-2011, 10:00 PM   #5
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"I can remember having to downshift to get up a couple of hills by my house last time I drove it 2 years ago."


....Just a shot in the dark, I'm not that familiar with a DP, but if it was a gasser, I would be looking for some restriction in the exhaust, like a muffler that the baffles have let go and plugged the exhaust making the engine work to hard, kinda like driving with the brakes partially on. Setting for 2 yrs, anything can happen. You check with some older mechanics that learned the hard way as I did before the computer age, and you will find it's highly likely with any engine, they gotta breathe. Good luck, and travel safe. PS, let us know what the fix is, OK.
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Old 03-21-2011, 10:41 AM   #6
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I used a small tachometer brand called Tiny Tach on a small diesel engine that did not have a tach. The sensor attaches to a injector line and reads the injection pulses. I don't know if it would work on your engine so you might Goggle tiny tach for information.
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Old 03-21-2011, 11:30 AM   #7
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Todays tachs work off the alternator sine wave that rides on top of the 12 vdc voltage. They can just plug into the cigarette lighter or 12 volt aux plug and work.
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Old 03-21-2011, 05:51 PM   #8
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If you think it is the tachometer that is wrong I have a couple of suggestions. Your idle speed should be 700rpm. The max rpm should be 2500 as the engine is governed. If your tach does not agree with these you need to recalibrate. As Mike said, a tachometer often works from the pre rectified output of an alternator so there is no direct correlation to engine rpm. Your tachometer will have a potentiometer which you can adjust to calibrate.
A long time ago, showing my age here, I did this on a marine engine and my dealer loaned me a mechanical tachometer to do the calibration. It has a cone which you insert into the crankshaft pulley and a dial then gives you the engine rpm. Easy then to adjust the potentiometer to the same reading. If you ask around maybe you can find someone who can loan you one.
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Old 03-26-2011, 07:35 PM   #9
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UPDATE ON RPM's

I bought a handheld Digital Tach and verified my Dashboard Tach is working correctly. I had my wife hold the rpms at 1000, 1200, & 1500. The dash Tach matched my handheld tach reading off a pulley on the engine.

Now I know I have a much bigger problem than just a bad tach!
I know it's shifting all 4 gears and I don't feel it slipping at all, is there anything else on a mechanical Tranny ( my MH doesn't have a computer ) that could cause the high rpms?
The rpms seem high right from 1st gear to 4th gear.

I guess I'll change the tranny fluid and both the filter inside the Pan and the external Filter. Do I just drop the pan and change the filter like I do on all automatics? I'm not sure if the MT643 needs to be done a different way. I'll also change the external screw on filter.

I drove the coach for 2 hours at 2500-2800 RPMS not realizing I had an issue!! Now I have 2 diesel fuel leaks and I am guessing it was the high Rpms that caused them. 1 High Pressure fuel line is leaking and 1 injector.
Here is a picture of the high pressure line leak. http://i238.photobucket.com/albums/f...tor_leak_3.gif

The injector is leaking quite a bit. Here is a couple of pictures, I used a paper towel and plugged the opening below the injector. The paper towel absorbed all the diesel fuel that was leaking down the engine to the ground. Heres a picture of the injector leak top and bottom.
http://i238.photobucket.com/albums/f...ector_leak.gif
http://i238.photobucket.com/albums/f...tor_leak_2.gif

I will post the diesel leaks in the appropriate forum for help, also might help someone else.

Now that I know for sure the RPMs are really high please post any ideas you might have on the reason for it. Any idea would be a help! i would like to do this without bringing to the shop.

Thanks, AL
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Old 03-26-2011, 08:30 PM   #10
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I did some calculations using a 38.2" diameter tire, a 3.91 rear gear, and 2500 rpm.

If you had 5% slip in your conveter and with a 1:1 4th gear ratio then at 2500 rpm you should be at 69-70 mph. Which it is not.

Did the same calculation assuming it was stuck in third gear with a 1.39 3rd gear ratio and that would put you at 52 mph at 2500 rpm. Which it is not.

All the above calculations assume a 5% torque converter slip. Guessed at that so if it locks up 100% the the speeds would be a little higher.

Have you verified your speedometer against either a GPS speed or a chase car speed?

You can check the math out here
http://www.tciauto.com/Pages/418/calculators.aspx
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Old 03-26-2011, 09:30 PM   #11
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Algoma: I bought a Handheld digital Tach and verified Rpms from a pulley on the engine. (FYI..the Handheld Laser Tach works great!)
Today I had my wife hold peddle to 1000, 1200, & 1500 RPMs. Handheld Tach & Dashboard Tach are matching within 50 rpms or less. I was glad to know that my tach is working, but now my problem is probably much worse than the cost of a new tach!

Mike Canter: I havent matched my true speed. I will do that with my gps. I am pretty sure its very close. If I'm at 2000 RPMS where I should be, everyone is passing me like I'm not moving. BUT, I'm going to check anyway, it would be good to know that its accuate or not.


Thanks, AL
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Old 03-27-2011, 06:16 AM   #12
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My previous MH was way off on the speedometer. Didn't realize it until I starting using GPS. It was about 6-7 mph off at 65 mph. Found a loose connection on the back of the speedometer.
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Old 03-27-2011, 08:08 AM   #13
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Here is my BIG QUESTION.

Have you always had this problem of high rpm when driving or did it just suddenly start one day?
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Old 03-27-2011, 09:31 AM   #14
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hey cummins 8.3 you should probably check your kick down cable to make sure its not stuck wide open ive attached pic of where its located on trans the other end should be near the injection pump follow cable from trans to pump the end near the pump should have slot in my moneys on its sticking or stuck open from sitting do not take it off the trans unless you have the instructions for adjusting it.lube your linkages or cables don't use wd40!!!!!
hope this helps
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