M35A3 accelerator pedal adjustment

[RainbowDash]

Hello all! I just have a quick, yet persistent thing that I am having trouble accomplishing on my truck.

So my M35A3 doesn't like shifting into fourth gear unless I creep the pedal back after it maxes out speed in third. If I use the hand throttle, it'll shift through all gears no problem. Once in fourth gear using the hand throttle and continuing to hold the hand throttle, if I depress the foot pedal, it immediately downshifts to third, and upon releasing the foot pedal, it shifts right back into fourth. This leads me to believe it is a problem with the modulator or cabling going to it. I figure best to start with the easiest part; a cable adjustment.

In short, how do I adjust the cable going to the modulator? I can feel what seems like a nut on the very end of the the cable on the pedal side. Assuming that is how you adjust it, which direction should it go to (maybe) alleviate the downshifting?

If I am totally on the wrong track, does anyone know what might be wrong with it? I've heard of issues with the governor acting up as well, but also read that usually only causes problems with lower speeds. My mechanic also tells me it's the clutch/friction material and that it needs to come out but I'd much rather try this first.

 

[pmramsey]

What happens if you shift from 2nd to 3rd and then "D" using the shift point of 2600 rpm at each shift?

 

[RainbowDash]

Sadly, my tachometer doesn't work right now, another project for another day.

I am estimating by ear that it is around a higher rev rpm when I manually shift it, and it shifts fine, but as soon as it goes into "D" and hits that same high rev, I have to back the pedal off a bit and then it shifts into fourth. I can't use the hand throttle and manually shift at the same time though so I don't know if that makes a difference.

 

[pmramsey]

Have you sufficiently warmed up the transmission? Are you running engine oil or transmission oil in the transmission? My M35A3 came with 15-40 engine oil in the transmission. This was not uncommon. Warm ups take longer and the truck needs to be driven slowly until the trans temp comes off dead "0" when running the 15-40 in the transmission.

 

[RainbowDash]

Yeah it certainly shifts faster when sufficiently warm. Some days it won't shift at all when it first goes out. I've run both 15w-40 and dexIII with only a slight improvement with the dexIII. But like I said, use the hand throttle and it shifts through all the gears like a champ.

 

[porkysplace]

There is a troubleshooting section in this TM;
TM9-2320-386-24-1-1.pdf
TM9-2320-386-24-1-1 Unit, Direct and General Support Maintenance TM for the M35A3 1 of 2.

TM's for M35A3;
LO9-2320-386-12.pdf
LO9-2320-386-12 Lubrication order for the M35A3.

TM9-2320-386-10.pdf
TM9-2320-386-10 Operator manual for the M35A3.

TM9-2320-386-24-1-1.pdf
TM9-2320-386-24-1-1 Unit, Direct and General Support Maintenance TM for the M35A3 1 of 2.

TM9-2320-386-24-1-2.pdf
TM9-2320-386-24-1-2 Unit, Direct and General Support Maintenance TM for the M35A3 2 of 2.

TM9-2320-386-24P.pdf
TM9-2320-386-24P Unit, Direct and General Support Parts and Special Tools lists for M35A3.

 

[RainbowDash]

I've already read through all the TM's. They don't give much info other than "inspect modulator/adjust linkage"

 

[rustystud]

I've already read through all the TM's. They don't give much info other than "inspect modulator/adjust linkage"

The modulator valve is adjusted with the oil pan off. The cable actuator just pushes the modulator in and spring pressure pushes back. On the valve body there are pins that hold the valves in the bore. These pins seat against a plug that has a series of "steps" on them. By turning the plug to another set of "steps" you adjust how much pressure is on the spring. When I was still working at Allison we had a "valve body machine" that simulated being in a vehicle. We where able to adjust the valve body to any configuration we wanted to by adjusting the "steps" or adding firmer or softer springs. You will just have to guess how much tension you will need. Before you do all this, make sure the actuator is actually working. Take it off and have someone push the throttle pedal to visually see if the rod is moving in or not.

 

[rustystud]

View attachment Scan0120.pdfView attachment Scan0121.pdfView attachment Scan0122.pdfView attachment Scan0123.pdfView attachment Scan0124.pdf Hopefully this will help.

 

[RainbowDash]

So I backed the nut off the cable all the way to the point where the throttle pedal doesn't even pull the modulator cable. The truck now shifts through all ranges no problem but it still overheats. It had actually done it before too, where normally it runs around 250, now it is running 275 and I can smell it is burning the fluid. I also figure that modulator needs to be inspected as you said rustystud. I am going to see what my mechanic says about it and if he can tell me if it is good or not.

Rustystud, does the transmission have to come out to access where those steps are in the valve body? Your scans are a great help, thank you.

Anyone have any suggestions on the overheating issue? I noticed it bubbling and gurgling from the vent lines, they may be clogged so I'll blow some shop air out of them to be sure but from looking at them they appear to be unobstructed

-edit:
So it appears that for some reason the torque converter lock up is not locking up and that is causing my problem with the excess heat. I presume it's locking method is hydraulic so does that mean the torque converter is pretty much toast and needs to be removed?

 

[Floridianson]

Just a thought. What shape is the spin on filter. Might want to change out and open up the old one?

 

[RainbowDash]

That's a good idea, should probably change that now with how much burnt fluid has gone through it

I think though that my overheat issue is because my torque converter isn't locking up

It's in the shop now and my mechanic will take a look through his allison book and see what controls the lock up

 

[rustystud]

Your torque converter might be bad. The "lock-up" valve is controlled by engine RPM and vehicle speed. It's in the main valve body. Actually there could be a host of problems in your transmission. Low oil pressure due to bad seals or your oil pump or regulating valve. Valve body dirty causing the valves to stick in their bores. Torque converter clutch burnt up or one-way roller bad or stator bad or who knows what at this stage. It needs to come apart and gone through basically. This kind of problem doesn't just happen. It takes a while for the transmission to get this bad, unless there was a catastrophic component failure. More then likely someone was not doing there proper maintenance on this transmission. How long have you had this truck ? Maybe the last owner knew something was up with the transmission and that's why he sold it.

 

[RainbowDash]

The owner before me only had it a few months before another 2-1/2 ton hit the front of it and he sold it off. He only used it around his property to move stuff around. The owner before him I have no idea

I guess we'll see what the transmission mechanic says when he opens it up

 

[rustystud]

The owner before me only had it a few months before another 2-1/2 ton hit the front of it and he sold it off. He only used it around his property to move stuff around. The owner before him I have no idea

I guess we'll see what the transmission mechanic says when he opens it up

I'm going to warn you, if there is some hard parts damaged this is going to get expensive fast ! Did you get a quote from the mechanic ? A usual rebuild will go for over $2000.00 .

 

[RainbowDash]

I think I'm just going to take the truck back and pull the transmission myself. He quoted me 1200 to 1500 for a rebuild. He also said though that if we already had it pulled it would be less than that.

Oh well, it's time it was rebuilt anyway

Thanks everyone for the help