'85L 4+3 OD Bypass Question
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Joined: Dec 2006
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From: Austin Texas
'85L 4+3 OD Bypass Question
A failing harness and ECM have caused me to install a F.A.S.T EZ-EFI system in my '85 Lingenfelter. I'm very impressed with the new system's self-tuning but I'm still sorting how to get my 4+3 OD working again without an operating ECM.
I have searched the Forum and elsewhere for 4+3 overdrive bypass techniques and found the Tech Tip but it relates to an '86 and most threads and an article in 101 Projects for Your Corvette, 1984-1996 relate to '84's. An '07 thread did specifically apply to '85's but unfortunately no one answered the poster's question and I've emailed him but received no response. The model differences are: 84-85E = On/Off switch on the Center Console; 85L = On/Off switch on the shifter; and 86-88 = Momentary switch on the shifter. My car is an 85L with the On/Off switch (not momentary) on the shifter.
I'd be VERY appreciative if someone can suggest how to bypass the ECM in a late '85.
Many thanks!
John
I have searched the Forum and elsewhere for 4+3 overdrive bypass techniques and found the Tech Tip but it relates to an '86 and most threads and an article in 101 Projects for Your Corvette, 1984-1996 relate to '84's. An '07 thread did specifically apply to '85's but unfortunately no one answered the poster's question and I've emailed him but received no response. The model differences are: 84-85E = On/Off switch on the Center Console; 85L = On/Off switch on the shifter; and 86-88 = Momentary switch on the shifter. My car is an 85L with the On/Off switch (not momentary) on the shifter.
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.
Many thanks!
John
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Joined: Nov 2006
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From: Carmichael ca
On the overdrive relay there should be 2 tan and black wires. The one that goes to the ECM, B terminal on the relay is the one you want. Cut the wire, connect it to a switch. The other side of the switch to ground.
Drifting
Joined: May 2006
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From: Chewelah Wa
I have done this mod and it is a favorite of mine...it is nice to know that when you hit that switch, your overdrive will engage without the ECM telling you that you can't for some dumb reason...
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Joined: Dec 2006
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From: Austin Texas
You guys are really amazing: both the answer and the testimonial! I apologize for not getting back earlier with my thanks but ran into two problems: first, the fuel pump relay had been (mis)placed in front of the overdrive relay so I spent a lot of time looking at the wrong relay connections ("black and tan"? All I see is red!) until I figured out what was what.
But second and a question back to you both: is cut and splicing the wire at the B Terminal all that's required? I ask because my overdrive relay works fine when I apply a 12V source to Terminals 2/3 (the equivalent of B/C on the male). But when I plug the Pack-Con male connector with the lead to Terminal B now running to ground and turn on the ignition the voltage at B/C drops from 12V to about 7V and that's not enough to fire the relay. So am I missing something?
Again many, MANY thanks. I hope I can overcome this last hurdle.
John
But second and a question back to you both: is cut and splicing the wire at the B Terminal all that's required? I ask because my overdrive relay works fine when I apply a 12V source to Terminals 2/3 (the equivalent of B/C on the male). But when I plug the Pack-Con male connector with the lead to Terminal B now running to ground and turn on the ignition the voltage at B/C drops from 12V to about 7V and that's not enough to fire the relay. So am I missing something?
Again many, MANY thanks. I hope I can overcome this last hurdle.
John
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Joined: Dec 2006
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From: Austin Texas
Thanks, Paul. I did a slightly different work-around clipping the Terminal A and E wires at the relay, running wires into the cockpit and using a small rocker switch on the left corner of the shift plate. My first thought on the latter was to use the the original switch on the top of the shift **** but as I read Forum posts so many folks had problems with that switch it seemed better to eliminate one problem now.
Thanks again.
John
Thanks again.
John
