Automatic transmission downshifts at innapropriate times

ask a mech to test drive,
we cant feel that.

There's no such thing as a downshift relay.Trans downshift is cable actuated.Go to the end of cable at the carb/throttle body,open the throttle fully by hand and depress the black button in the cable connection block,the cable housing will pop and when your driving will automatically reset to where it needs to be.Your done.

I have one that does exactly this. I have confirmed no codes in trans or engine; I have watched the data on the scan tool and it appears that the PCM is choosing to shift without a change in throttle or speed to cause it. I am looking into a new PCM - ouch!

Most modern automatic transmissions have a chip in them that will prevent the car from downshifting, say, into first gear when you're moving at 60 mph. Still, I wouldn't try it. When climbing a hill, only downshift if the AT can't decide which gear to be in and switches back and forth--choose the lower one. When descending use brakes to decelerate. However, once you've braked, do use a low gear to coast in, to prevent you from freely accelerating down the hill.
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GM Tech2 GM Diagnostic Scanner With C card

The hold bottun is basically a downshift button , just a silly gadget. It will not Damage transmission transmission shifting from drive to neutral , but why would you? Drive the car as it was designed. Owners manuals are copy righted , and are only availiable new from dealer . May try amazon.com or something like that. Have seen them in glove boxes at salvage yards if you got some time and a good pair of boots.Good Luck!

Try checking your timing to solve the ping problem, your timing might be too far advanced. For the downshifting problem, I have this same issue on my odyssey, On the top of your efi manifold you will see 2 wires that control your throttle, One of them extends to the transmission and basically lets it know when you are pressing firmly on your gas pedal, telling it to downshift. I would adjust it so the cable has some more slack in it, therefore requiring a harder press on the pedal before downshift, and hopefully a sooner upshift. You can also slightly adjust idling problems if the van is running high while in park or drive while sitting. As for the gas vapor purge code, I would first try replacing the egr valve, its only a couple bucks and takes no more than 5 minutes.

have your transmission checked by a professional, It sounds like you have a shift solenoid going. Had something similar on a van. I don't know where it is on your vehicle so I don't know how deep into the transmission you may have to go.

your shifts correctly, as your driving it downshifts and stay stuck at that gear when u stop and try to take off but intell u turn off your car and restart it it shifts correctly intell a few mins later. i am thinking that is a speed sensor (out signal problem) mounted on top of your transmission next to the battery tray, you will see couple of cable linkages next to it ...

When does it do that and what gear does the transmission downshift? What kind of speed does this happen?

Hyundai has a TSB for this: 07-40-010 - AUTOMATIC TRANSAXLE HARSH AND/OR DELAYED
UPSHIFT OR DOWNSHIFT". This one will also refer to TSB# 06-40-005 - AUTOMATIC TRANSAXLE CONTROL MODULE -RESET AND RELEARN ADAPTIVE VALUES. either need to be performed by the dealer.