Can the 4+3 overdrive operation be modified with a custom chip?
The OD currently engages on start-up and I'm looking for it to engage manually when the button on the shifter is pressed. It's a 1986.
Thanks

 

Yes

 

[Quote] The OD currently engages on start-up

Really? That's a new one on me. My OD engages only when I tell it to w/the button. Same year & drivetrain.

 

Based on previous research, I believe this operation is correct for my vehicle. The 4+3 OD operation changes a lot between model years. I'm not sure why yours would be different...

 

what would be the point of having a on off switch or a relay for that matter if the overdrive was always on? there is a problem in your overdrive system................one of the electrical componets is telling the overdrive to engage on start up........................try the relay first, the insulation on the wires breaks off and there might be a short at that point.................I beleive its the one closest to the firewall next to the brake master cylinder.

 

Sorry if I used the wrong terminology... When I start the car and begin driving, the OD functions as designed for my year vehicle. What I want is for the OD never to engage until I tell it to - basically freeway driving only. As it is now, the first thing I do when I start the car is to push the button so the OD does not engage ever during around town driving.

 

I still think you have a problem...............the only time the overdrive should engage without using the overdrive engage switch is over 130 mph............. it engages by itself during normal city driving?.............anyway I have mine wired manually because I'm using an accel gen 6 computer, I just wired the overdrive switch to ground out the relay...........I can't remamber the color of the wire at the relay to ground...post this on the tech section and scorp will jump in he has all of the wiring schmatics.

 

More simply put, on 86-88 cars, the OD program in ON upon start-up. I want mine to be OFF upon start up. I believe this can be addressed with the custom chip. Correct?

 

The 4+3 function can be addressed with chip mods, but there is an easier solution if you just want the od to default to off.

The trick is to convince the car that its in 1st gear all the time. There is a place in florida that sells a switch for this purpose for $20.

Sorry can't remember the name. Hopefully somebody will chime in.

Here it is: Corvette Clinic Inc in Sanford Florida See C4 solutions
http://www.corvetteclinicinc.com/

 

I use Tunerpro, and I have a 4+3, but I have never noticed a table that changes the polarity of the OD operation on startup. As a matter of fact, there are no "manual only" or 4+3 specific tables at all.

 

My info below pertains to my experience with my 88 4+3. Earlier years may be slightly different, as all years were calibrated differently, and may have different hardware.

The 4+3 is controlled by the same parameters that control the tcc or torque converter clutch, lock, unlock, lock delay, forced shift etc.

Also there are pre-set n/v limits or rpm/mph values to distinguish 1st gear, 1st od, the other gears and max and min values for 4th gear.

These values are not typically supported by off-the-shelf editors, but can be found if you know what hex values to look for.

The key here again is to convince the car it's still in first gear all the time, where od is default off. It is necessary to reduce the first gear n/v limit (255) and 1st gear od N/V limit to some value less than the normal 4th gear N/V limits (35-50 or so). This way the od will remain off unless there is as switch input, like it does in 1st gear.

Raising the lock speeds is another approach, but the switch logic will get inverted, meaning the first push of the button will turn od off, so it will require 2 pushes of the button for od to be engaged.

Therefore the 1st gear/ 1st gear od n/v limit change is a cleaner solution, and should preserve the normal switch logic and upshift/downshift capability.

The corvette clinic switch should accomplish the same thing and looks like a bargain. I may buy one just to check it out.

 

I have a $32b bin and Tuner Cat and you can set the OD default to off.
That's how I have mine set.

 

Can you explain in more detail, or do you have a special 32b definition file from tc that permits this? I'm curious how this is accomplished, without messing with the n/v limits, etc.

Thanks

 

They added that after I sent them the values for it. Have you tested it on your car yet? I can't take full credit though as another forum member a long time ago clued me in on the hex values for it.

 

Yes. After I start the car the OD stays off until I hit the OD switch.

 

Email TC and ask them for the latest $32b file "With the 4+3 fields" and that amongst a bunch of other 4+3 fields will be in there. The Default O/D on/off is a checkbox you can select.

 

Thanks for the info Scorp. TC sent me the updated definition file. I did figure it out on my own, but its good to have the updated file.

Here's how it works in a nutshell. After key on the value of 8 is loaded into the transmission mode word byte which now looks like 0000 1000 instead of 0000 0000.

Then if bit 0 (od on) is not set then the value is decremented from 8 to 7 which looks like 0000 0111 and is stored to memory. This is how od is set to default on, bit 0 being the key.

The transmission mode word looks like this (working from memory so I may have a few bits swapped but you get the idea) It is mislabeled in the data master status bits as CARS:

Bit 0 od on
Bit 1 1st gear
Bit 2 1st gear look ahead ok
Bit 3 1st od
Bit 4 4th gear
Bit 5 on request
Bit 6 off request
Bit 7 od active

The TC change prevents the decrement operation from 8 to 7 by changing the decrement operator $4A to $0A which has no real effect. By preventing the decrement operation, the od is by default off, since bit 0 is never set. Pretty cool, and accomplished only by changing only one bit.