"Fooling" the ECM?

[DrummerDudeNJ]

Saw an interesting post on another forum where a person went to a shop for a gear swap, they flashed the ECM with the new ratio but the gears were never installed. So in short the person was taken advantage of. But it made me curious, is that even possible?? Wouldn't the shift points be all over the place and throwing codes?

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[The Last Cowboy]

Not if the tire size was changed.

 

[DrummerDudeNJ]

Not if the tire size was changed.

How do you mean??

 

[The Last Cowboy]

If bigger tires were put on, and it was flashed for the correct tire diameter, then it would shift as it should and the speedometer would read correctly, even if the gear ratios were not changed.

 

[DrummerDudeNJ]

If bigger tires were put on, and it was flashed for the correct tire diameter, then it would shift as it should and the speedometer would read correctly, even if the gear ratios were not changed.

Gotcha. Still makes me think that there would be a mechanical issue somewhere... it could eventually hurt the trans no?

 

[Pape]

I would go with the allowed diameter difference in size allowed by Jeep. I my memory is correct Jeep allow 10% difference in size per tire on axle before there is a issue. For example 33 on the left and 34 on the right this is 3% difference.

 

[The Last Cowboy]

Gotcha. Still makes me think that there would be a mechanical issue somewhere... it could eventually hurt the trans no?

Having gearing too high for the tire diameter is like trying to start pedaling on a bike with the chain on the big chain ring (gear). It strains things more than they ought to be, and can lead to broken things off road. Usually that will be axles and U joints, or burnt clutches, not the transmission.

So what tire size and what gears are you asking about?

 

[grimmjeeper]

If bigger tires were put on, and it was flashed for the correct tire diameter, then it would shift as it should and the speedometer would read correctly, even if the gear ratios were not changed.

That depends on exactly how the transmission is set up.

The computer needs to know two things. First is the input shaft RPMs. Second is the output shaft RPMs. Because the torque converter lets the engine and transmission spin at different speeds, most transmissions have a speed sensor on the input shaft. The rear depends on how the transmission is designed. Some transmissions have a speed sensor on the tail shaft. Others do math to figure it out.

The ones that use math rely on other sensors. Used to be that the transfer case had a speed sensor. So all you needed to know was whether you were in low or high range. That ended with the TJ. Starting with the JK, the computer went to using the wheel speed sensors. To get the math right you need to know high/low range and what axle ratio you have. But they also factor in tire size because that affects driveability.

I'm not sure how the JL transmissions are set up for sure. I think they continue with the same basic design as the JK. But someone who knows for sure should correct me.

If you have axle ratio off with correct tire size, you'll get weird shifting. If you have tire size off with correct gear ratio, same thing will happen. Now, you can have both wrong. And if they're both off by about the same percentage, they can cancel each other out and end up working fine.

 

[jadmt]

pretty sure on an automatic it would go into limp mode. I have read many instances of that happening. where someone regears and fails to input the proper ratio.

 

[DrummerDudeNJ]

Yeah so I don't have enough info from the post I saw on what the exact specs are for the issue, but let's use my Jeep as a pseudo test subject:

2018 Wrangler JL Sahara Unlimited 8spd auto
3.45's swapped to 4.56's
Tire size is just under 35", but resting size is 34"

If the new tire size and new gear ratio were flashed in, but gears never installed, would that cause issues??

 

[grimmjeeper]

Yeah so I don't have enough info from the post I saw on what the exact specs are for the issue, but let's use my Jeep as a pseudo test subject:

2018 Wrangler JL Sahara Unlimited 8spd auto
3.45's swapped to 4.56's
Tire size is just under 35", but resting size is 34"

If the new tire size and new gear ratio were flashed in, but gears never installed, would that cause issues??

Very likely.

 

[jludave]

If the new tire size and new gear ratio were flashed in, but gears never installed, would that cause issues??

Yes, sir.

 

[DrummerDudeNJ]

See that's what puzzles me. How could that happen and someone NOT notice anything?? I'm sure the shifts wonky, probably some codes being thrown, etc.

 

[2nd 392]

Yeah so I don't have enough info from the post I saw on what the exact specs are for the issue, but let's use my Jeep as a pseudo test subject:

2018 Wrangler JL Sahara Unlimited 8spd auto
3.45's swapped to 4.56's
Tire size is just under 35", but resting size is 34"

If the new tire size and new gear ratio were flashed in, but gears never installed, would that cause issues??

I went from 3.73’s-33’s to 4.56’s-35’s. A bit lower overall (4.10-35’s closer to =). No issues and I can even add 3.5 mph to 65 mph in my head, so no recal. It took a few weeks but I got “Service 4WD”, — “input shaft speed/wheel speed” .

 

[Tread4Lo]

So, for me to do the math. It seems they would flash a smaller tire size. Here is my math and tell me if I am looking at it wrong. Let's look at overdrive 8th gear.
Trans ratio x diff ratio x tire size
.67 x 3.73 (diesel) x 35" tire = 87.46

If the customer say wanted 5.13's in the axle, you program a different tire size (down) therefore everything would look appropriate
.67 x 5.13 (supposedly new gears) x 25.5(new tire size) = 87.65

Which says to me you would be in the same power band, speedo and shift points as before. If the gear installer is flashing the ECM for gears he has the opportunity to flash for tire size too. Speedo would be correct

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