MT643 Downshift 3rd lockup to 2nd under load

[Mountain Gnome]

perhaps remove the wiring harness from the ECM and test for continuity from that connector, pin 56, to the relay connector, pin 56.


Wiggle wires as you do this. if the continuity fluctuates at all, there is likely your problem.

 

[Simplicity-SKO]

perhaps remove the wiring harness from the ECM and test for continuity from that connector, pin 56, to the relay connector, pin 56.


Wiggle wires as you do this. if the continuity fluctuates at all, there is likely your problem.

I did test 56 on the connector using a tone tracer. It tones well. What I didn't test was pin 56 on the actual ECM.

I found in the diagnostic manual that the ECM receives engine load information from the accelerator position sensor and the idle validation switch by comparing feedback from the two. I'm assuming the APS is working properly since there is no engine warning light, no codes and the bus runs great in all other respects.

Since the APS is what also helps the ECM determine injector needs, it must be working properly.

So, if the APS sensor is good, and pin 56 on the ECM connector all the way through to and including the modulator are good, the only piece that seems to be not good is the ECM signal to pin 56.

Considering our MacGyver test today had the tranny working exactly as it should when modulated, it seems as even more evidence that it's an ECM issue.

Because the problems with the electric modulator are so well documented, I was thinking of how I could use one of the other two models of modulators. Yet, since the Williams micro-switch is available, it seems to be the easiest and more reliable solution.

 

[Mountain Gnome]

So if I'm following you, the ECM grounds the relay. You should be able to test this ground with a test-light, except how would you, since the vehicle needs to be running/driving, and that would require the ECM harness to be connected.


Good luck and Aloha!

 

[Booyah45828]

The relay receives power from the fuse box when you turn on the ignition key. The relay receives ground from either the ww williams switch, or the ecm at the appropriate throttle position/engine load. When given ground, the relay coil energizes, flipping the switch and sending power to the modulator coil, pushing the plunger out and giving the transmission full modulator pressure.

There is no designed fail safe of it being on all the time. And the only immediate telltale of failure would be softer, sloppy shifts, no kickdown, and no automatic downshift when going up a grade. 98% of drivers would be oblivious if any of those symptoms happened. Then, after having no modulation for a while, the soft shifts take their toll, and the transmission becomes worn out and needs rebuilt. Typically, it isn't rebuilt though, and the thing ends up slipping a bunch, ruining the fluid, and burning itself up in catastrophic failure.

The new/rebuilt transmission is thrown in, the modulator operation was never checked, and you burn the next transmission up in a short amount of time. Then everyone blames the at545 for being a junk transmission.

Don't get me wrong, the at545 isn't a great transmission by any imagination. It gets the job done, but it's adequate at best. We've replaced several behind the cummins isb, and those used a cable modulator. My bus has a at545 w/ cable modulator, and I had to adjust it right after I bought it, because I noticed it had soft, early shifts.

The electric modulator compounds the issues, with it's unreliability and the weak nature of the at545 it's required to work for any hope of the transmission lasting. Adjusted it and it's running perfect.

The more robust mt643 isn't as impervious to a failed modulator, but it isn't immune to it either. Without him noticing the engine lugging going up a grade, simplicity would likely have never known his modulator had failed.

 

[Simplicity-SKO]

Without him noticing the engine lugging going up a grade, simplicity would likely have never known his modulator had failed.

Booyah is right on about my ignorance of how my tranny should work. If it wasn't for my detailing my testing and him encouraging me to continue addressing the high engine load "no shift" issue, I'm sure I would have been one of the many who unintentionally ruined their tranny.

Between cadillackid, booyah and others, my tranny is strong, my knowledge more and I'm really happy to be a part of this online community.

 

[cadillackid]

So the circuit isn’t a reverse energized circuit booyah? I haven’t looked at my diagrams but the way it was made out to sound is that the NC contact on the relay is used to energize the modulator, so when the ECM commands the modulator to turn off it grounds the coil. So of the ECM pin fails or the fuse blows to the relay coil then the relay NC keeps power all the time to the modulator unless the modulator is bad or the fuse to the modulator circuit itself blows.
The fail safe being a bad relay or bad ECM pin would turn on the modulator.

Or are there multiple variants? Where on some it’s a direct engagement where as you say once the relay coil gets power then the modulator goes on?

I never studied the semantics too much as once I discovered my bad modulator then bad trans I said to hell with this stuff..

I’ll have to investigate how my red bus did it before I tear it all apart. I never removed the wiring or the relay when I did the trans swap

 

[Booyah45828]

So the circuit isn’t a reverse energized circuit booyah? I haven’t looked at my diagrams but the way it was made out to sound is that the NC contact on the relay is used to energize the modulator, so when the ECM commands the modulator to turn off it grounds the coil. So of the ECM pin fails or the fuse blows to the relay coil then the relay NC keeps power all the time to the modulator unless the modulator is bad or the fuse to the modulator circuit itself blows.
The fail safe being a bad relay or bad ECM pin would turn on the modulator.

Or are there multiple variants? Where on some it’s a direct engagement where as you say once the relay coil gets power then the modulator goes on?

I never studied the semantics too much as once I discovered my bad modulator then bad trans I said to hell with this stuff..

I’ll have to investigate how my red bus did it before I tear it all apart. I never removed the wiring or the relay when I did the trans swap

I don't know what you mean by reverse circuit.

But the plunger never pushes out until it receives power. If it was receiving power all the time, like if it was connected to the NC side of the relay, I think I would have had at least one driver complain about firm shifts, hard downshifts coming to a stop, etc. and I never have.

If it was wired up on the NC side of the circuit, simplicity wouldn't have had to do his jumper harness test, because the modulator would have been applied the entire time the key was on. Which would have gave him full throttle shifts/firm shifts all the time, downshifting coming to a stop, automatic downshifting for grades, etc.

I'll try and look at one next time it's in, but I'm fairly certain they weren't wired to the NC side of the relay.

 

[Simplicity-SKO]

Booyah, read this and see if it helps. It's my version of what happened, why, etc.

https://www.skoolie.net/forums/f33/...modulator-issues-theory-35465.html#post432755 (NOTE: Correction made in later post in this thread. ECM sends 0v to energize the coil, not 12v).


My friend and I wondered why they didn't use a relay where 85 was normally not grounded (coil therefore is not energized) and 30 / 87 were normally closed, so the coil did not have to do essentially continuous duty keeping 30 / 87 closed.

Then, when 85 was energized, it would open 30 / 87, close 30 / 87a and pass 12v to the modulator.

This way, if pin 56 wasn't sending a signal to the relay, or the relay went bad, the modulator would never receive 12v and burn out.

Of course, you'd have a shifting issue, but hopefully, the driver would notice the issue and report it before burning up a tranny.

 

[Booyah45828]

I'll read it here in a bit.

Most relay coils are operated this way. They're fed constant power and then switched on/off through the ground side. I believe it's done this way for several reasons. The big one is so that the ecm isn't supplying power, and in danger of burning itself out because of a short to ground. Shorting a grounding wire to ground wouldn't damage anything in the ecm.

Now, my thoughts on all of this.

If the modulator was on the NC side of the relay, it would have power to it anytime the key is on. And then under low load the ecm would ground the relay coil, switching the relay, and pulling power from the modulator. If your ecm isn't providing ground, or anything on the control side of the relay failed, the modulator would be on constantly, and you would have harsh shifts, and downshifting coming to a stop.

Both of those things, I feel drivers would notice and complain about.

If the modulator was on the NO side of the relay, requiring that the ecm to ground the coil under high engine load, any failure of the control system wouldn't activate the relay, which wouldn't send power to the modulator, and you'd have soft shifts, and normal downshifting coming to a stop.

The soft shifts wouldn't be as noticeable to a driver, and normal downshifting coming to a stop wouldn't be out of the ordinary.

If it was wired up on the NC side of the coil, and with the KOEO, and the ecm not/never grounding the relay coil, you should have power going through the relay to the modulator, and the new/good modulator should click immediately when you plug it in.

Does it do that?

If it's wired on the NO side, then the only time the circuit would be active, is when they activate it by grounding the relay coil, whether through the ecm, or a switch on the pedal.

 

[cadillackid]

Grounding of ECM pins to turn on circuits allows me to fuse the positive leads together for multiple circuits .

It also allows me to see if anything is at the other end of a circuit or if it’s blown open.

Plus 12 going through a relay coil and terminating at the ECM acts as a pull up and I can read the ECM pin before calling it active . If I see zero volts I would set a code for an open in that circuit or possible power fail.

Many ECMs will employ a stepped in pull down for the pin. If the circuit is shorted to plus 12 the turning on of a pull down resistor will result in much higher voltage return than expected and a code will be set.

Again I haven’t read my book on the modulator circuit my explanation earlier of why one might use the NC contact was such that no damage occurs even though the shifting would be horrible. Horrible enough the driver might take it in vs ignore the early soft shifts. I wrote that before I saw the update in which the relay uses the NO or normal style methods.

 

[Simplicity-SKO]

Update & Solution: This is the thread I posted as to the solution to my tranny modulator problem. Hope this helps someone.

https://www.skoolie.net/forums/f33/...ic-modulator-activation-35870.html#post436690

 

[Soaringcook]

Did you fix the issue and if so were did you get the modulator

 

[cadillackid]

Did you fix the issue and if so were did you get the modulator

he fixed his issue by doing a custom solution monitoring the engine load much more reliably than the factory did.. read his thread about it.. its brilliant.



i sent him my spare modulator since id no longer use it.. theres a pic of it in this thread you can get the part number.. they are available.. but id test yours on a bench first to see if it works.