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Transmission Discussion of preventative/corrective maintenance and other technical issues regarding your coach's transmission.

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  #1  
Old 06-15-2018
keen keen is offline
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Default How to check allison transmission codes?

'87 PT40 - check trans light is lit.

Allison HTB-748R, from what I can tell - 4 speed, trans retarder, push button shifter.

I have in the various documentation a lot of information - including a list of code numbers to meanings - but I can't find anywhere, including via google, how to actually read the codes.

The BB owners manual suggests it's possible to read it via the light.... but doesnt give any tips on how to do it.


So, how do I read these codes? I know how to -clear- them, that's documented in the allison owners book.


Trans shifts reasonable, ran through all the gears up and down driving. fluid levels seem normal. that suggests it's a "minor" code, at least.
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david raistrick
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1987 PT40 8v92
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  #2  
Old 06-15-2018
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https://www.wanderlodgeownersgroup.c...ad.php?t=22063

https://www.wanderlodgeownersgroup.c...t=OM2157EN.pdf (Page 19)
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  #3  
Old 06-15-2018
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Dieselbird01 Dieselbird01 is offline
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Quote:
Originally Posted by keen View Post
'87 PT40 - check trans light is lit.

Allison HTB-748R, from what I can tell - 4 speed, trans retarder, push button shifter.

I have in the various documentation a lot of information - including a list of code numbers to meanings - but I can't find anywhere, including via google, how to actually read the codes.

The BB owners manual suggests it's possible to read it via the light.... but doesnt give any tips on how to do it.


So, how do I read these codes? I know how to -clear- them, that's documented in the allison owners book.


Trans shifts reasonable, ran through all the gears up and down driving. fluid levels seem normal. that suggests it's a "minor" code, at least.
1. Start and run the engine at idle with the shift selector in neutral.

2. Place and hold the Electronic Control test switch in the on position.

3. The Check Transmission light will flash the most important diagnostic code if one is present (Example: flash-pause-flash-flash-flash indicates Code 13).

NOTE: The Check Transmission light not will flash if no codes are present.

If you don't have a transmission test switch, the Check Transmission light is supposed to flash stored diagnostic codes when wire 216A is grounded... you may want to verify that before grounding any pins.
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John Wyatt
Titusville, Florida
1991 40' WLWB-WTB
Body Number F095567
My Location: http://www.bbirdmaps.com/user1.cfm?user=4

1991 40' WB ...From 2008 - Present
1984 ½ PT-36 .From 2000 - 2008
1973 FC-31 .....From 1991 - 2001
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  #4  
Old 06-17-2018
keen keen is offline
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thanks guys - now to try to get that to trigger something tomorrow

my trans test switch - when i've flipped it before - doesnt seem to do anything. and like many switches doesnt have a clear indicator of which direction is on or off.. . (it definitely doesnt have any momentary position)
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1987 PT40 8v92
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  #5  
Old 06-17-2018
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Dieselbird01 Dieselbird01 is offline
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Quote:
Originally Posted by keen View Post
'87 PT40 - check trans light is lit.

Allison HTB-748R, from what I can tell - 4 speed, trans retarder, push button shifter.

I have in the various documentation a lot of information - including a list of code numbers to meanings - but I can't find anywhere, including via google, how to actually read the codes.

The BB owners manual suggests it's possible to read it via the light.... but doesnt give any tips on how to do it.


So, how do I read these codes? I know how to -clear- them, that's documented in the allison owners book.


Trans shifts reasonable, ran through all the gears up and down driving. fluid levels seem normal. that suggests it's a "minor" code, at least.
I’m not sure this applies to an “87” PT but my 91 WB has a normally open 3 psi engine oil “pressure switch” (screwed into an engine oil manifold under the coach in the vicinity of the starter) which “closes” when engine oil pressure is detected. If that pressure switch were to fail to close, the “Check Trans” light would stay on.

Here’s some of what I have posted in the past about the “Check Trans” light circuit on my 91 WB:

When the ignition switch is first turned on, voltage is fed to one side of the “Check Trans” light, and the normally closed contacts of the “Check Trans” relay provide the ground path for the light. Therefore, the light comes on letting you know that the “Check Trans” light is working.

Once the engine is running and has built enough oil pressure to close the “ECU oil pressure switch”, it sends power to the coil of the “Check Trans” relay which picks up, opening its normally closed contacts and the “Check Trans” light goes out.

Transmission faults can interrupt the ground path for the “Check Trans” relay coil causing it’s normally closed contacts to close, which turns on the “Check Trans” light. That's why you can use the Check Trans light to see fault codes.

The “ECU oil pressure switch” also serves as a second source of power for the “ECU Relay” insuring that the Allison ECU relay has power even if the signal from the ignition switch were to drop out while the engine is running.

For what it’s worth, Allison literature indicates that a second source of power is usually only required for emergency or off road vehicles.

Since the “ECU oil pressure switch” also serves as a second source of power for the “ECU Relay”, if it fails in the "closed" or "partly closed" position like what I found on Jim Nichols' coach a few years ago, it can power the Allison ECU without the key even being in the ignition. That one caused several lights associated with the transmission to come on at random in the middle of the night.

If your coach is set up the same way, you should check the pressure switch with a volt meter to make sure the "normally open" contacts of that switch "close" when the engine is running. If the contacts don't close, replace the pressure switch.
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John Wyatt
Titusville, Florida
1991 40' WLWB-WTB
Body Number F095567
My Location: http://www.bbirdmaps.com/user1.cfm?user=4

1991 40' WB ...From 2008 - Present
1984 ½ PT-36 .From 2000 - 2008
1973 FC-31 .....From 1991 - 2001
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  #6  
Old 06-17-2018
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Randy Dupree Randy Dupree is offline
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So John.
I know we have talked about this before,but just as a refresher that oil pressure switch is located in the frame rail back by the engine,correct?
And its opened/closed by engine oil pressure,right?
So when the engine is running and we have proper oil pressure the switch is "made" and that turns the "check trans light" off,correct?
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  #7  
Old 06-17-2018
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Quote:
Originally Posted by Randy Dupree View Post
So John.
I know we have talked about this before,but just as a refresher that oil pressure switch is located in the frame rail back by the engine, correct?

On my coach, the manifold with that pressure switch is mounted to the bottom of the frame rail just a few inches from the starter. The oil pressure sender for the dash gauge is screwed into the same manifold.

Once again, I don't know whether an 87 PT with an Allison HTB-748 uses the same “Check Trans” light circuit as my 91 WB (never tried to research it for an 87) but it appears that an HTB-748 at least had the capability to be set up the same way.


Quote:
Originally Posted by Randy Dupree View Post
And its opened/closed by engine oil pressure,right?
Yes it watches ENGINE oil pressure.

Quote:
Originally Posted by Randy Dupree View Post
So when the engine is running and we have proper oil pressure the switch is "made" and that turns the "check trans light" off,correct?
Yes, once the engine is producing oil pressure, the pressure switch closes. When the pressure switch closes, it energizes a "Check Trans" relay and the normally closed contacts of the "Check Trans" relay open, turning off the "Check Trans" light so it also serves as a test for the "Check Trans" light.

The Transmission ECU uses the same "Check Trans" relay to flash out Trans fault codes.

The pressure switch also has a second job in that it also supplies power to the "ECU relay" which in turn supplies power to the Transmission "ECU" - acting as a redundant source of 12 volt power for the ECU relay.

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John Wyatt
Titusville, Florida
1991 40' WLWB-WTB
Body Number F095567
My Location: http://www.bbirdmaps.com/user1.cfm?user=4

1991 40' WB ...From 2008 - Present
1984 ½ PT-36 .From 2000 - 2008
1973 FC-31 .....From 1991 - 2001
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  #8  
Old 06-17-2018
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Randy Dupree Randy Dupree is offline
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Thanks John,we have lots of newbies here on WOG.
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