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Steve, Auto Service Technician
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Experience:  25+ yrs experience as a professional working technician; ASE L1 master technician
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91 Dodge ram 50: auto trans..check engine light comes..tranny

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Hi,
What would cause the auto trans on my '91 Dodge ram 50 truck
to downshift when the check engine light comes on then upshift
when it goes off? The light comes on ocassionally for a few
seconds at a time and whenever it comes on the tranny will
downshift a gear. when the light goes off it will shift back up. I
notice it more on the highway than in city driving. Is the tranny
shot?
Thanks, Customer

The way your going to find out is to retreive the codes that the computer stored when the check engine light came one. Here's the procedure to do that.



To call up codes, cycle ignition switch on, off, on, off and on within 5 seconds. Stored codes will be indicated by flashes of the power loss/check engine lamp. The codes will be indicated as two digit numbers, with a four second pause between codes. An example of a code is as follows:



  • Power Loss/Check Engine Lamp illuminated for approximately 2 to 3 seconds as a bulb check, then turns off.

  • Lamp flashes 2 times, pauses, then flashes 6 times.

  • Lamp pauses for approximately 4 seconds.

  • Lamp flashes 3 times, pauses, then flashes 1 time.

This would indicate that code 26 and 31 are stored. The lamp will continue to flash until all stored codes have been displayed.


Once the lamp starts flashing the stored fault codes, it cannot be stopped. If a code is missed, the entire procedure must be repeated. The lamp will not indicate if oxygen feedback system is switching lean-rich or if idle motor system is operational. The lamp also cannot be used to perform actuation test mode, sensor test mode or engine running test mode.


After all fault codes have been displayed, then power loss/check engine lamp can be used to verify switch function. The lamp will turn on and off as the switch is turned on and off.

Customer: replied 11 years ago.
Reply to Tech501's Post: Hi Tech501,
Thanks for your swift response and for the info on the codes.
Once I find out what codes are flashing (by the way can I find
code info in a Haynes or Chilton service manual?) then I should
be able totrack down the problem right? Also, how do i reset or
turn off the flashing check engine light afterwards?
Thanks again, Customer
We'll be glad to supply the code definition for you later. Even if you accept the question now, just leave it open and follow up with the codes. After the problem is fixed, you can disconnect the battery for a minute and that will clear the codes. It will also reset all the fuel adaptives so it may have to learn the idle and mixture strategy after that. That's no big deal. It will do it on it's own right away.
Tech501, Auto Service Technician
Category: Car
Satisfied Customers: 747
Experience: 38 years exp, ASE Mastertech with L1 Advanced Engine performance
Tech501 and 5 other Car Specialists are ready to help you
Customer: replied 11 years ago.
Reply to Tech501's Post: Thanks again Tech501.
I will try finding out the codes and will let you know what comes
up. Customer
This information is incorrect; this vehicle has a MMC control system. You cannot retrieve service codes from this vehicle through the key cycling procedure used on other chrysler vehicles...
Customer: replied 11 years ago.
Reply to Steve7654's Post: HI Steve,

Thanks for the correct info. I was just getting ready to try the
key cycling procedure. I guess it's different because it's a
Mitsubishi. I have no idea what a MMC control system is. Should
I just take it to a tech and have them fix the problem or can I do
anything myself? I just want to get it fixed.
Thanks, Customer

MMC= Mitsubishi Motors Corporation


The procedure tech501 describes does work on most chrysler corp. vehicles, but not on the mitsubishi built systems, such as you have.


Since a scan tool is needed to access the service codes and data stream on this vehicle, I would recommend taking this particular problem to a technician for diagnosis. Although there are many repairs that can be done easily at home, there are some, such as this one, that are so much easier with the right equipment and diagnositc information available...

Steve, Auto Service Technician
Category: Car
Satisfied Customers: 5494
Experience: 25+ yrs experience as a professional working technician; ASE L1 master technician
Steve and 5 other Car Specialists are ready to help you
Customer: replied 11 years ago.
Reply to Steve7654's Post: Thanks Steve,
I'll take it to a pro and let him find the problem and fix it. Customer
Customer: replied 11 years ago.
Hey guys,
I took my truck to a shop and they found the problem. It was the
throttle position sensor that had to be replaced. Now the truck
runs great, no check engine light and no downshifts on the
highway. thanks for all your input and help. Customer

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