MT643 Downshift 3rd lockup to 2nd under load

[s2mikon]

Listen to Booyah. Remember you just re geared this up 25% that means you are putting more load on the transmission. You need to make sure it is properly set up. These 600 series transmissions are very reliable but do not suffer low line pressures at all. The modulator modulates the line pressure to compensate for load. If it is running low it WILL burn up the clutches. Also make darn sure that the manual cable is adjusted properly and does not have slack in it. If it does it will not hold the detents properly and WILL burn the clutches out in short order. I had a good friend that was an Allison tech and he used to tell me that 95% of the time a 600 series needed rebuilt was because of mal adjusted cables and modulators.

 

[Simplicity-SKO]

That's fine and dandy that you can shift it manually and get it to function how you want, but it still isn't right.

It should downshift to 3rd automatically once you get below a certain rpm. If it doesn't, your modulator likely isn't functioning. I would say 1600 rpm is well below that speed.

It should also drop down into 3rd gear when you floor it going 35 mph. Once again, if it doesn't your modulator likely isn't functioning.

Also, seeing how your shifts happen below governed speed, and your lockup occurs at 1450 is kind of a telltale of a failed modulator as well.

You can drive it as is, ruin your trans, I really don't care. But the most common reason for failure on the old mechanical allisons is a broken, misadjusted, or non-operative modulator. Yes, the MT series transmissions are more robust then the AT series, but that doesn't make them bulletproof and impervious to this issue.

I'll run some more tests and see what it does.

Do you think the switching the diff from 5.11 to 4.10 has anything to do with it?

If it's still not down shifting at 35mph or automatically during hill climbs, I'll take out the modulator and test to see if it works with a KOEO accelerator floored.

Thanks for looking out for my tranny!

 

[Booyah45828]

The trans doesn't care what gear is out back. The only thing that would change is the mph figure you test at. But that # isn't all to important. The important thing for the kickdown test is that the trans is under low load in 4th gear in the middle of the rpm range. I've not calculated your gearing or tire size, so I can't tell you the exact speed or rpm you should be traveling.

But, in that scenario of mid rpm and low load in 4th, going full throttle should cause the trans to downshift into 3rd. If it doesn't and the engine just sits there and lugs, then you're modulator is very likely to be a culprit.

Having the torque converter lockup drop the rpm so low also leads me to believe the modulator is not functioning either. The rpm that occurs at is changed by modulator position as well.

 

[Simplicity-SKO]

UPDATE:

Thankfully, Booyah questioned my assumption that my tranny was working fine. So, I tested more and it did not downshift when I was going 35mph and applied full throttle.

I disconnected the harness connection from the modulator and tested the wiring. Nothing. So, we found the relay and disconnected it.

We got continuity from the coil connections, but when we tried to test the switch connections nothing happened (yes, we know the switch is normally closed), yet we applied voltage across the coil connections and still nothing.

Before we did anything to the relay, I took a picture. As you can see, there is a whitish discoloring around a couple of the connection blades, and one blade had a bit of a light bronze coloring.

I'm assuming this relay is bad, which isn't surprising considering all the forums and posts I've gone through.

Keep your fingers crossed for me that it's simply the relay.

 

[Booyah45828]

I'm glad you're looking into it.

More often then not it's the relay that's failed, at least in my experience.

Relays are typically pretty durable and robust devices, but some scenarios they just don't seem to hold up well in.

 

[s2mikon]

Is that a universal 5 pin relay?

 

[Booyah45828]

Not sure what's considered a universal relay, but that is a 5 terminal pin type weatherpack relay. They're pretty common and are used by most oems. You should be able to take that into your local parts store, and if the counterperson is any good, they'll find it on the shelf.

Napa #ar153 is what I have on the shelf.

 

[Booyah45828]

Make sure it's the metripack pin type, and not a blade type. The blade type is more common and is more of what I would consider a universal relay.

 

[Simplicity-SKO]

Well, I'm certainly learning a lot! Metri = round to me. :whistling:

The pins in the old relay are apparently flat, and I've already ordered them online, so I'll have to go with it for now. But, I'll try to find the Metri pin relays if there's a next time (probably, just hope it's a while).

I'm hoping this resolves this issue so I can see how the bus operates when the modulator is working.

Great info and, as always, thank you!

 

[Booyah45828]

Metripack is a terminal/connector line from delphi. They're used on many many vehicles as a seal style connector.

That particular relay uses the 280 series metripack terminal/connector. Yes, those terminals are flat.

I goofed when I called it a weatherpack relay. Weatherpack uses rounded pins, instead of the flat ones in metripack.

 

[Simplicity-SKO]

Metripack is a terminal/connector line from delphi. They're used on many many vehicles as a seal style connector.

That particular relay uses the 280 series metripack terminal/connector. Yes, those terminals are flat.

I goofed when I called it a weatherpack relay. Weatherpack uses rounded pins, instead of the flat ones in metripack.

No problem on proper naming, even Cadillac and Booyah can make a little error every now and then. :whistling:

I try to do my research before commenting so I don't look like a total idiot. The only picture I got that matched metri was round male tongs, so I "assumed" that's what it was.

Man, between inverter/chargers, relays, modulators types, DCAs, coolant types, etc., this has been a hell of a learning curve!

As always, thankful for those who have the knowledge and take the time to help the less knowledgeable folks.

Awaiting the new relays. Soooo hopeful that is the solution and I can feel her run the way she's supposed to.

 

[s2mikon]

To do a skoolie there are lot of things a person has to know or be willing to learn. Everybody thinks it is easy but it's not.

 

[Simplicity-SKO]

UPDATE:

Well, It appears not to be the relay. I'll double check that the relay and modulator connections are tight, but it's still not downshifting.

I tested at 25mph, 35mph and 45mph. No downshift.

I ran it up the 2 mile 6% grade and keeping the pedal floored, no downshift. I manually downshifted at 1700 and maintained 2400 rpm at 50mph up the grade which required me to back off the throttle a bit.

A concern though was my tranny temp got up to 200-205 after the climb. I babied her home and checked the tranny fluid while running. It did not smell burnt but it wasn't a bright red either. The levels were good.

I'm going to test the connection at the modulator. If it's good, then I'll pull the modulator and see if it's actuating. If not, I guess I'm buying a new one.

Any thoughts or insights are appreciated.

 

[s2mikon]

200-205 is fine. My Allison manual states 300 max converter out sump temp 250 and 300 max in retarder mode. Intermittent. Not continous. They also state 100 as the minimum.

 

[Simplicity-SKO]

200-205 is fine. My Allison manual states 300 max converter out sump temp 250 and 300 max in retarder mode. Intermittent. Not continous. They also state 100 as the minimum.

Whew! That's a relief! Other posts about the 643 were saying they were down to 130 - 180 degrees. It's high 40s here and I thought it would run cooler.

Thanks!

 

[s2mikon]

Mine runs 180-190 converter out in lock up 4th on a 100 degree day on level road. Running through town stop and go it runs up to 245 converter out on 100 degree day.

 

[mokibrabrant]

booyah is right it will have an electric modulator.


S2mikon there is no adjustment on the electric modulator.. and the earlier the ECM the bus has the worse that modulator will work.. in fact so bad on some that WW williams in columbus Ohio created a fix where they put a Micro-Switch on the foot pedal to initiate the modulator..



the electric modulator is nothing more than a kickdown.. it is either full on or full off.. the idea was the ECM would activate it when the engine is under heavy load, whether the trans actually downshifted or not was up to the transmission valve body and governer it either was modulated or not..


Gear ratios play into part with downshifts.. to prevernt Hunting, transmissions typically wont downshift themselves right into an upshift situation.. for instance the MT643 is designed to upshift around 2500-2600 RPM on IH diesels.. (except for 4th gear which is usually 2400).. this is fully modulated..


if you have your foot to the floor and your 643 is up-shifting at much lower RPMS then it means the modulator or its electrical circuit isnt working


now back to downshifts.. on my 643 it upshift to 4th at 2400 RPM.. so lets say thats too much for it so im slowing down in 4th gear.. it will drop RPMs down so that when it downshifts back to 3 that it is going to have at least 200-300 RPM of rise before it upshfits again.. so yes because the ratios are pretty wide on most 643s (that ive encountered).. it means that to get that downshift you will be below your desired horespower band of your engine..



if you want the shift sooner, you can drop the T-handle down a gear and as lomng as the trans isnt exceeding its max governed input shaft RPM in the gear you select, it will downshift..


Lockup and downshifts on the 643.. ive found on mine that if I drop down low enough in RPM during 3rd gear, that it will unlock the converter before downshifting to 2 (this is with my foot to the floor (modulated shift).. it will try to gain extra power from the engine by running unlocks through the first part of 3rd.. if you speed up enough it will pull the lockup back engaged..



NOW if the modulator is not on (no foot on pedal) then it will stay locked all the way through third till it gets slow enough to downshift into second then it unlocks..


so for you to test.


1. from a dead stop drive at full throttle till you reach 4th gear.. you should notice that the transmission is shift from 1-2 and 2-3 somewhere around 2500 RPM.. Lockup will pull-in midway through 3rd and will feel almost like a solid shift. the 3-4 shift should occur at somewhere around 2400 RPM..


if thats not happening.. if your shifts are much lower RPMs then check the modulator and circuit..



I burned up an AT545 because of a bad modulator I was unaware of at the time.. when i fixed the modulator it was too late, damage had been done to the trans..

Thought the OP said this was mechanical and the existance if the MT644 would suggest that is so. Therefore all allusions to an ECM would seem not relevant.

Don't recall the OP stating what engine he has but guessing I would think an 8.3 as the MT643 was rarely behind a 5.9 Cummins. But then it could be a turbo and a CAT....but unless it's a V8 the load or governed T
RPMs would be 2400. Having done a few axle ratio changes in my day in fully mechanical machines I have to say I have never experienced the problems the OP claims. Other than if course the Speedometer reading out it's disparity with actual road speed

My advice would be to bite the bullet and find a competent Allison shop to sort through this. And move along.

 

[cadillackid]

if you apply +12 volts to the 2 pins of the modulator it should make a loud click and the pin should extend.. if that doesnt happen the modulator is bad

 

[s2mikon]

if you apply +12 volts to the 2 pins of the modulator it should make a loud click and the pin should extend.. if that doesnt happen the modulator is bad

Should we assume that polarity does not matter because it is just an electromagnet?

 

[Simplicity-SKO]

UPDATE:

I tested the modulator while installed, no clunk. I removed it (did you know transmission fluid pours out when you remove it? I didn't, but I do now) and tested it, no movement.

So, I need a new modulator. Since these seem to be hit and miss, including out of the box, I want to buy one from a good manufacturer. Is there such a manufacturer for these?

I washed the modulator hoping to find some markings, but there were none.

Thanks for any feedback.