Reprogramming For Gear Ratio Change

[Benesesso]

I am about to change the final drive of my 1995 Transport 3.8 from 3.06 to
3.33. Anyone know how I am supposed to compensate the computer for the lower gear? Do I change a speedo-gear or is there a way to "reprogram" the computer?

[LMP]

This is irrelevant. If I remember your first thread about this, you are to perform this change at the chain level. Speedo pickup is taken at the output shaft,
www.avigex.ca/xport/4T60cutaway.jpg
so it reads the actual wheel speed and does not care about what has happened ahead..in fact, if it were the case , it would have to change the logic on each and every gear..so good luck...and PLEASE take photos of the operation and post them here.
p.s. With rear wheel drive of former times, the differential was AFTER the transmission speed pickup so an adjustment was required...I had FOUR differentials for my 1968 DODGE GTS ( 2.76, 3.23, 3.54 , 4.10) and consequently, I had four gears ready to adapt to the speedo cable)

[Benesesso]

Thanks for the reply. No, I won't be changing the chain sprockets--I'd have to pull the trans. out. But the final drive ratio (planetary sun gear and the pinions, I assume) can be changed easily w/o pulling the trans. I just hope my planetary ring gear is OK-it's supposed to be another weak area in these stupid trans. The ring gear cannot be changed without major disasembly.

Instead of reprogramming, I need to find out about changing the speedo gear. The gear ratio change is only ~10% but it should be a big help.

I will take photos and post them here.

[LMP]

My reply is still valid: the speed pickup is taken on the wheel output shaft, hence AFTER the differential. There is no gear, just a magnetic pickup....SO the computer will behave normally: engine pickup OK, and speed pickup too. ANd the pressure distribution in the transmission will build up and make normal shift balance points in the transmission..this is totally transparent to the system. Interesting.
I wish the larger planet/pinion gears will have exactly the same tooth angle and size which they need to engage the large planetary ring gear you plan to keep.

[Viala]

hello I try too understand , what result do you hope after passind from 3.06 to 3.33


Thanks for your answer

[Benesesso]

The reason for the gear change is to allow the engine to spin a little faster for the speed of the van. I think it's asking way too much for a little 3.8 engine to pull a big van at ~60 MPH at only 1,600 RPM. Sorta like climbing a hill on your 10 speed bike in a high gear. Add the A/C load in the summer and a few thousand feet of elevation, and 4th gear (OD) becomes useless unless you're going downhill.

By letting the engine spin a little faster it can generate more HP easier, so OD will work better. The engine RPM will only go up ~10%, to ~1760 vs. 1600. But even this small a difference will make a much better running van.

It would be nice if these transmissions had 5 total speeds, with 3, 4 and 5 spaced fairly close. But they don't, so you're either lugging along at a low RPM in OD or spinning kinda fast in 3rd gear, because the ratios are pretty far apart.

Most people who drive vans like these neither know nor care about gear ratios--they sure aren't sports cars or motorcycles. But it's pretty cheap and easy to get that 10%, and for me it makes it worthwhile.

In the event I ever have to pull the trans. for a rebuild, I'll change the 35/35 chain drive to 32/37, to get another few % too. It will probably cost me about 1-2 MPG, no big deal.

[Benesesso]

LMP,
Update time. I finished the swap of the 3.06 for a 3.33 final drive. Van runs much better now, except that the computer does NOT like it at all. It senses
there is now a mismatch between the input RPM and the output and frequently, but not always, will not command the shift to 4th (OD).
The orig. 3.06 gearset had a 31 tooth output/speedo reluctor ring, while the 3.33 has a 30 tooth. I'm surprised the computer allows the shift to OD at all, but it does. I believe that I need a 34 tooth reluctor ring, which would generate ~10% more pulses/sec than the orig. 31, which should compensate for the fact that the output is now spinning ~10% slower for the same input RPM. I'm not sure if GM makes a 34 tooth reluctor, so I may have to make one myself.

[LMP]

Indeed based on the engine speed/output shaft speed ratio, the computer should think that the lock up clutch slips (either in 3rd or 4th gear) and then generate a code accordingly....(no problem with OD itself though), so matching the two could indeed require the change you suggest, but of course the speed reading of your speedometer and mileage of odometer would then start showing 10% more than the actual....where now it counts 3% less because of the switch from 31 to 30.
I would rather like to know if access to the computer code could allow for changing that in the computer instead...those who have built or procured the computer interface --I think of nicke66 - could give us a clue on that....

[nicke66]

Originally Posted by LMP

I would rather like to know if access to the computer code could allow for changing that in the computer instead...those who have built or procured the computer interface --I think of nicke66 - could give us a clue on that....

It is not possible to change any parameters in the ECM or PCM with the software I use. It is only possible to read data and turn some devices on or off, for instans the AC clutch. I am quit sure this is not possible with any software using the ALDL serial data link. There is nothing of this in the Factory service manual. I think you have to re-program the eprom in the ECM/PCM for this, if possible.

[Benesesso]

I now have quite a few miles on the van since I lowered the gear ratio. It drives much better, but could still stand to be lower yet. If I ever have to pull the trans. I will change the chain drive from 35/35 to the lower ratio.

Some days everything works fine. Other days it won't shift above 3rd gear, unless I momentarily turn the key off. Seems that resets the computer and all is fine for a while. It does not correlate with temp. I am lucky--the trans. itself is good and solid--so far.