Transmission has trouble shifting into 6th with A/C on - Dodge Cummins Diesel Forum
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post #1 of 7 (permalink) Old 02-19-2016, 10:58 AM Thread Starter
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Transmission has trouble shifting into 6th with A/C on

2011 Dodge Cummins 6 Speed Auto. Truck has 158k miles on it and is totally stock.

When I am driving with the A/C on, it doesn't want to shift from 5th to 6th. I have to let off the gas for a moment and then hit the gas again suddenly to get it to shift into 6th. It shifts fine when the A/C is off. I am not towing anything when this happens.

It has never thrown a code. I bought the truck used last year and noticed it not long after purchase.


Some of the things I have ruled out:

It doesn't matter if the truck is at operating temperature or cold

Trans fluid level is good and has normal color

Outside temp doesn't matter. It happens when it is 70 or 100.


Is this in need of a filter/fluid change or is it a bigger issue?

It is more of an annoyance than a problem.

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post #2 of 7 (permalink) Old 02-19-2016, 11:04 AM
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if you don't know the service history of the trans, it would definitely be a good idea to do filters and fluid. maybe an electrical or ground issue with the circuit for the a/c compressor?

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post #3 of 7 (permalink) Old 02-19-2016, 12:49 PM
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Have you tried with your tow/haul mode on/off?

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post #4 of 7 (permalink) Old 02-20-2016, 09:56 AM
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Hmmmm... not a problem I've seen before. There is no direct connection (that I know of) between A/C usage and trans shift pattern. My first guess would be that when A/C is on, you have to tip deeper into the throttle (to counteract the compressor load), and maybe you're running right on the edge of the 5-6 shift point. So at heavier throttle (A/C on) you are still running in the 5th gear zone, but when you turn A/C off and let up on the throttle slightly, that lowers the 5-6 shift point and puts you in the 6th gear zone.

Or maybe something is screwing up your throttle position sensor reading when A/C is on (making it show a higher throttle than actual). A dealer or shop with a scan tool could monitor the throttle signal while driving (at constant throttle) and see if it jumps around when A/C is turned on or off.

One other thing to consider is that the 68RFE will not shift into 6th gear until the Torque Converter Clutch (TCC) is engaged. Again, there should be no direct relationship between A/C and TCC engagement, but your scan-tool-equipped tech can also monitor TCC state and confirm whether the TCC is engaging or not (when A/C is on).

If everything looks "normal" when monitored with a scan tool, check this: Hold the throttle at 50% open as you accelerate through the gears, and see when the 5-6 upshift occurs. At 50% (45 degrees) throttle, the 5-6 shift should occur at roughly 2540 output RPM (assuming TCC is engaged). See if you get that same shift point with A/C on or off.
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post #5 of 7 (permalink) Old 02-20-2016, 10:11 AM
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Quote:
Originally Posted by TransEngineer View Post
Hmmmm... not a problem I've seen before. There is no direct connection (that I know of) between A/C usage and trans shift pattern. My first guess would be that when A/C is on, you have to tip deeper into the throttle (to counteract the compressor load), and maybe you're running right on the edge of the 5-6 shift point. So at heavier throttle (A/C on) you are still running in the 5th gear zone, but when you turn A/C off and let up on the throttle slightly, that lowers the 5-6 shift point and puts you in the 6th gear zone.

Or maybe something is screwing up your throttle position sensor reading when A/C is on (making it show a higher throttle than actual). A dealer or shop with a scan tool could monitor the throttle signal while driving (at constant throttle) and see if it jumps around when A/C is turned on or off.

One other thing to consider is that the 68RFE will not shift into 6th gear until the Torque Converter Clutch (TCC) is engaged. Again, there should be no direct relationship between A/C and TCC engagement, but your scan-tool-equipped tech can also monitor TCC state and confirm whether the TCC is engaging or not (when A/C is on).

If everything looks "normal" when monitored with a scan tool, check this: Hold the throttle at 50% open as you accelerate through the gears, and see when the 5-6 upshift occurs. At 50% (45 degrees) throttle, the 5-6 shift should occur at roughly 2540 output RPM (assuming TCC is engaged). See if you get that same shift point with A/C on or off.

Thanks for the info, he should have a load test done on his batteries and or AC circuit. Whatever is easiest to test first, I guess.

Perhaps there is enough draw somewhere to cause a significant voltage drop.

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post #6 of 7 (permalink) Old 02-24-2016, 03:46 PM Thread Starter
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Quote:
Originally Posted by 1400ho View Post
Have you tried with your tow/haul mode on/off?

I've never used the tow/haul mode. It is always off.
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post #7 of 7 (permalink) Old 02-24-2016, 03:50 PM Thread Starter
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Quote:
Originally Posted by TransEngineer View Post
Hmmmm... not a problem I've seen before. There is no direct connection (that I know of) between A/C usage and trans shift pattern. My first guess would be that when A/C is on, you have to tip deeper into the throttle (to counteract the compressor load), and maybe you're running right on the edge of the 5-6 shift point. So at heavier throttle (A/C on) you are still running in the 5th gear zone, but when you turn A/C off and let up on the throttle slightly, that lowers the 5-6 shift point and puts you in the 6th gear zone.

Or maybe something is screwing up your throttle position sensor reading when A/C is on (making it show a higher throttle than actual). A dealer or shop with a scan tool could monitor the throttle signal while driving (at constant throttle) and see if it jumps around when A/C is turned on or off.

One other thing to consider is that the 68RFE will not shift into 6th gear until the Torque Converter Clutch (TCC) is engaged. Again, there should be no direct relationship between A/C and TCC engagement, but your scan-tool-equipped tech can also monitor TCC state and confirm whether the TCC is engaging or not (when A/C is on).

If everything looks "normal" when monitored with a scan tool, check this: Hold the throttle at 50% open as you accelerate through the gears, and see when the 5-6 upshift occurs. At 50% (45 degrees) throttle, the 5-6 shift should occur at roughly 2540 output RPM (assuming TCC is engaged). See if you get that same shift point with A/C on or off.


With the A/C off, it shifts at about 2100. With the A/C on, I can hold it steady at 3000 and it won't shift until I "goose" it, and then it shifts into 6th like I described.

I keep thinking it's a sensor issue of some kind, primarily the throttle position sensor like you mentioned. Would there be any other sensors that could would come into play?

I am going to change the tranny filter and fluid, and if I don't see a change, I will take it to a nearby transmission shop and have them put a diagnostic computer on it and see if they find anything out of the norm. I will let you know what I find.
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