tech tip 4+3 overdrive bypass Question...
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3rd Gear
Joined: Apr 2007
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tech tip 4+3 overdrive bypass Question...
Tech Tip C4 4+3 overdrive bypass
Published 2003-06-21
By Member Alias Rich B
if im reading this tech tip correctly it states
for overdrive on demand you must remove wires A7 and C8 from ECM and connect them to one another essentially removing the ECM from the circuit and allowing the overdrive request button to be the initiating device. this was written for a 1986 does anyone know if the wiring is the same for an 85? in Helm's manual it looks as if A7 is overdrive relay coil negative and C8 is ground when vehicle not in 1st gear. wired like this would seem to have the over drive constantly engaged and the drivers over drive request switch having no effect.
In order to make the overdrive on demand work in the 85 would you not also have to remove C7 and D3 from the ECM and connect them to one another? in this circuit the driver's overdrive request switch is actually working opposite the way it was intended to ( closed for od ). C7 is driver request input to EMC and D3 is supply side of 1st gear switch.
I'd be most appreciative of any input on this. I'm assuming that none of the ECM inputs in question have dual functionality.
Published 2003-06-21
By Member Alias Rich B
if im reading this tech tip correctly it states
for overdrive on demand you must remove wires A7 and C8 from ECM and connect them to one another essentially removing the ECM from the circuit and allowing the overdrive request button to be the initiating device. this was written for a 1986 does anyone know if the wiring is the same for an 85? in Helm's manual it looks as if A7 is overdrive relay coil negative and C8 is ground when vehicle not in 1st gear. wired like this would seem to have the over drive constantly engaged and the drivers over drive request switch having no effect.
In order to make the overdrive on demand work in the 85 would you not also have to remove C7 and D3 from the ECM and connect them to one another? in this circuit the driver's overdrive request switch is actually working opposite the way it was intended to ( closed for od ). C7 is driver request input to EMC and D3 is supply side of 1st gear switch.
I'd be most appreciative of any input on this. I'm assuming that none of the ECM inputs in question have dual functionality.
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Run wire from ECM C7 to A7.
Don't worry about the 1st gear switch. If the ovedrive doesn't have enough pressure it will shut itself off.
Why are you overriding it? Why not just fix it correctly? The 4th gear kickdown is sweetness.
Don't worry about the 1st gear switch. If the ovedrive doesn't have enough pressure it will shut itself off.
Why are you overriding it? Why not just fix it correctly? The 4th gear kickdown is sweetness.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
Burning Brakes
Joined: Sep 2006
Posts: 1,202
Likes: 3
From: MD
This will make the ECM ignore the gear switches, the temperature sensor, and the kickdown. I did this to mine because with my 400+ hp motor an accidental kickdown would mean the end of the OD. I need to be in total control of the OD unit. The wiring should be the same for all. I did it to my 84 and it was fine. Mine had a malfunctioning 1st gear switch as well and this cured it right up. Since you've already got the switch on the console (I think) then this is by far the simplest way to override the ECM. I cut the wires an inch from the ECM plug (in case I want to reconnect it later) and spliced them together with a bit of solder and heat-shrink tubing. Works fine.
Team Owner
Joined: May 2002
Posts: 24,337
Likes: 25
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
An accidental kickdown means you're already in overdrive though. The only computer forced OD shift in 85 is when you hit 110 mph. Not sure about 84.
Anyhow, any given year may be wired differently at the ECM. Obviously 85 is different from the tech tip. I wouldn't go blindly connecting stuff.
Anyhow, any given year may be wired differently at the ECM. Obviously 85 is different from the tech tip. I wouldn't go blindly connecting stuff.
Last edited by CentralCoaster; Apr 30, 2007 at 01:57 PM.
