OD Button Went Out...
Thread Starter
Heel & Toe
Joined: Dec 2007
Posts: 15
Likes: 0
From: Lanexa VA
OD Button Went Out...
I have an 87 Corvette with the 4+3. The overdrive button just went out so I cannot switch out of overdrive on command. I do not like to drive the car with the overdrive on until i get to about 45 because the engine boggs way to much. I start in 1st and when I go to shift to 2nd I can trick the computer and go back to 1st with the overdrive on which is not as big a shock to the engine as 2nd with overdrive. Does anyone know if this ok on the transmission or should i just painfully push through 2nd with overdrive on? Also I will only be doing this for a couple of weeks until I can fix the button.
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
Check out this site for information on how the OD unit operates in the 87.
Basically if you use sufficient (60%) throttle in 1st gear, the OD should not engage until you hit 4th gear. My 87 works this way although sometimes I can stay in 4th/ no OD at fairly low speeds and not have the OD kick in at all. But each time the engine stops and is re-started, you have to go thru this again.
Basically if you use sufficient (60%) throttle in 1st gear, the OD should not engage until you hit 4th gear. My 87 works this way although sometimes I can stay in 4th/ no OD at fairly low speeds and not have the OD kick in at all. But each time the engine stops and is re-started, you have to go thru this again.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Go to www.mouser.com and order the switch, its Mousers number 506MPA 106F, used to be $12.80, probably close to that now.
Melting Slicks
Joined: Aug 2003
Posts: 3,389
Likes: 391
From: Lakeville MI
The 1st gear look ahead ok flag is set with tps > 40% in non-Z51 (MH5) 87s and 88s. Z51s (MK2) are 25%.
This flag is what prevents od until 4th gear is detected if the tps exceeds the threshold while in 1st gear.
I agree that you should troubleshoot and fix the switch. Then consider custom programming to tailor the 4+3 to your driving style.
I have mine set up to be off by default and also set to turn off again following any O/D downshifts, so it will never upshift unexpectedly.
This flag is what prevents od until 4th gear is detected if the tps exceeds the threshold while in 1st gear.
I agree that you should troubleshoot and fix the switch. Then consider custom programming to tailor the 4+3 to your driving style.
I have mine set up to be off by default and also set to turn off again following any O/D downshifts, so it will never upshift unexpectedly.
Thread Starter
Heel & Toe
Joined: Dec 2007
Posts: 15
Likes: 0
From: Lanexa VA
Thanks a lot for all of your help, especially the detailed instructions on how to replace the switch. I really need to look into getting the reprogramming. How would I go about having that done?
Team Owner
Joined: Jan 2003
Posts: 20,293
Likes: 240
From: Summerland B.C. Canada
Tequilaboy set me up with the programming and I burned it onto my chip.
Thanks again, Patrick
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Burning Brakes
Joined: Jan 2002
Posts: 993
Likes: 4
From: Glen Burnie MD
Isn't that a bit of overkill for him? We 86 owners have the unique ECU-only overdrive controls, but he should be able to just disconnect and cap one wire from the transmission to get full manual control. Sadly, I can't find my link which explained what wires did what for which years (I thought it was on Ken Stapel's Vette Obsession site, but maybe not).
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