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Allison Transmission Control confusion questions.

Haulin' 8.1

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Haulin' 8.1 Enthusiast

Haulin' 8.1

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Warning unless you're a real tech this post will probably confuse the heck out of you and it's a long read.

 

Anyone else out there work on these things for a living? GM Tech can you help? I've been trying to help my 8.1 / Allison get better mileage. I've noted some transmission gear descrepencies with the scan tool. What I can't figure out is the park/nuetral position (PNP) switch is broke or if it's something called a pressure switch manifold (PSM). It seems as if the Transmission Range Switch (TR) is working (on the gear shifter). I asked my transmission man but he's not totally up on the Allison yet and not being able to see it in person yet he coundn't help.

 

There are three scanner parameters for transmission gear, one for each switch I would assume but I'm not sure which parameter corisponds to which switch or how to diagnose this. For all I know it's normal but it doesn't make sense, it should be more specific as to what gear it's using if it's like other vehicles. Also it doesn't dispay many examples from the HELMS manual. The following is what my HELMS manual says about the parameter and then what my scanner indicates.

 

TR Switch: The scan tool displays the transmission gear postion. Typical data value is Park.

My TR switch readout indicates <Park> in Park or Nuetral, in any gear it reads <Drive 4> at all times.

 

Current Gear: The scan tool displays 0-4. The scan tool displays which gear the transmission is in. An illegal transmission position displays 9. A typical data value is 1.

My Current Gear readout indicates <2> at all times the ignition is on.

 

TFP Switch: The scan tool displays PARK/NUETRAL, REVERSE, DRIVE 4, DRIVE 3, DRIVE 2, or LOW. Trans range represents the decoded status of the four inputs from the Transmission Internal Mode Switch Assembly. The combination of the transmission inputs indicates the position of the transmission manual valve. Typical data values varies.

My TFP readout indicates <Drive 2> at all times the ignition is on.

 

The tranny seems to be working fine, but I've only had it for about 8 months and I don't have experience with another to compare it to.

 

Any help with this would be great.

Thanks,

Vern

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted

Ummmm, I think you need a new scanner. :banghead: If the readings were actually as you say, your trans would not even be close to operating properly. The readings you are getting are totally wrong and I would venture to say that the truck wouldn't even move if those readings were accurate.

 

Off hand I don't recall each parameter, but I can tell you that in key on, engine off, current gear is 0 and running in reverse it is 7. The TR (trans range) definitely should read out whatever position the indicator is in.... Park, Reverse, Neutral, etc., etc.

 

If your trans is operating properly and the PRNDL indicator in the cluster changes accurately to reflect the position you've selected, your scanner is wrong.

Haulin' 8.1 Enthusiast

Haulin' 8.1

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Posted
Ummmm, I think you need a new scanner.  :banghead:  If the readings were actually as you say, your trans would not even be close to operating properly.  The readings you are getting are totally wrong and I would venture to say that the truck wouldn't even move if those readings were accurate.

 

Off hand I don't recall each parameter, but I can tell you that in key on, engine off, current gear is 0 and running in reverse it is 7.  The TR (trans range) definitely should read out whatever position the indicator is in.... Park, Reverse, Neutral, etc., etc.

 

If your trans is operating properly and the PRNDL indicator in the cluster changes accurately to reflect the position you've selected, your scanner is wrong.

 

 

 

 

Thanks I'll try that. I'll goto my transmission man's shop and we'll use his, he has transmission specific software. My other scanner doesn't go past 2000 on this model.

 

 

Vernon

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