M1088 transmission trouble

[thetruckguy]

So, I have a 98 m1088 with the wtec iii transmission. When I first got it, it would shift into drive and reverse, but going down the road wouldnt shift out of 3rd gear. The manual shifter sometimes wouldn't respond either. Then it started to shift normally, but the problem was still intermitten. I did the fluid swap to tranny fluid, and put some miles on it. Now, the problem is more constant, at only a certain temp (I believe). When it is cold, it runs great. Then when it warms up a bit, the shifting problem comes back. I take off from a stop, and it won't upshot unless I put it in neutral and coast for a second, then put it back in drive. If I am stopped, it will not move when shifted into drive for a few seconds, then the 7 starts to flash for a few seconds, and it slams into gear. When I shift it in reverse, it flashes and refuses to go into gear. After. A few minutes of screwing around it acts fine. Yesterday I drove 5 miles stop and go, after it acted up 5 minutes into the drive and it was great. Even went into reverse fine when I parked. It i at the Allison shop right now, but they can't connect to it to diagnose because apparently the military use a different diagnostic plug than civilian. They seem to think, switching out the internal harness and temp sensor should be the first step.
So, I have 3 questions. Has anyone else had this issue?
Is there an adapter available for the diagnostic port?
What do you think is likely the problem?

 

[snowtrac nome]

you need to pull codes.

 

[wheelspinner]

If they are suggesting swapping parts before understanding what could even be wrong by codes, well how do you say this nice?....they idiots.

You can pull codes through the keypad, if they didn't even know that .......

 

[snowtrac nome]

Right pull codes through key pad. In Iraq we had issues with the internal harnesses but it also exceeded 115 degrees out side I maintain a fleet of Allison 3000's here and have had no problems with them here in the us. it should use a dutch conector if uou have a wtecIII transmission and controller if you have a hybred with a wetecII controller than it uses an obd 1 gm conector and they will need a prolink to acess it.

 

[NDT]

I suspect the keypad has water in it. Can you swap it with a known good one?

 

[Suprman]

Careful with that. I had a wet wtec3 keypad that liked to put the truck into drive by itself. Water drips down around the lights above the windshield and over time ruins the keypad. Arrow up and arrow down in neutral will start flashing codes.

 

[thetruckguy]

Update, I talked to the shop today, and they did find the connector they needed to connect. Apparently they were looking in the wrong place. They drove it with it plugged in and saw the temp sensor spiking and dropping off. Also the outside connector on the internal harness is supposedly broken. I ged get a code off the keypad of low temp. They want 1500 to replace the harness with labor. I am going to try and get the part and replace it myself. If this doesn't fix it, I will look into the keypad.

 

[Suprman]

Is it the pigtail coming off the trans? Doing that stuff can be tricky. Sometimes having the dealer do it will save a lot of headache.

 

[thetruckguy]

I'm not sure yet. I am going up later to have them show me. I might just save up some money and have them do it.

 

[NDT]

In the realm of fixing Allison transmissions, that price is not bad. Assuming they warrant that the problem is solved for that quote.

 

[thetruckguy]

They said no Guarentee it would fix the problem, but they were "pretty sure" that was the problem. I know I should suck it up and pay them, but I just don't have the cash. Probably just going to have to wait until I do.

 

[Nomadic]

Update, I talked to the shop today, and they did find the connector they needed to connect. Apparently they were looking in the wrong place. They drove it with it plugged in and saw the temp sensor spiking and dropping off. Also the outside connector on the internal harness is supposedly broken. I ged get a code off the keypad of low temp. They want 1500 to replace the harness with labor. I am going to try and get the part and replace it myself. If this doesn't fix it, I will look into the keypad.

It seems suspect to me to think a broken harness would fix the problem and its risky to give them your money when they will not stand by their diagnosis. They may be guessing at what the problem is. A very slippery slope to be on.

When its cold, the trans shifts fine but after it starts to warm then the problem occurs. Let it cool and its fine. What is the fluid temperature reading is from the sensor when this happens. If it is suspect, then check the fluid temp with a thermometer of some type to what the actual temperature is. If the temperature is too high, verify your engine temperature to make sure it is actually running at a good operating temperature.

If it is starved of fluid the trans won't shift right. The fluid expands once it starts to warm. Do you have a fresh filter on it? If not, change it out. There is a good amount of Allison documentation on the web. Read through it to see if you can troubleshoot the problem on your own. Hit the low hanging fruit if you haven't already.

 

[coachgeo]

Tagging onto what Nomadic said; review Allison 3000 tranny discussions on RV boards for some ideas. Pretty much the same electronics... just different programming of the computer so there will be some cross over between the two.

 

[thetruckguy]

I drove it home today and it drove perfect all the way home. I'm thinking it was that broken connector for the internal wire harness that was causing the problem with the temp sensor. Maybe them messing with it gave it a better connection. I found a place online that sells the replacement connectors, so I am going to try that first.

 

[frank8003]

Don't leave your posts undone. Update them/it.
I having a beer and watching and dislike threads that end in middle.
You can help all by continuing to resolution and writing.

 

[thetruckguy]

Its not done, not sure what you mean. I have not bought the connector to fix the problem yet. You want to see the conclusion, you are going to have to wait until I have figured things out. Can't predict the future brother.