4+3 shifter (again!)

[omega1940]

I received an email from Joseph Arnone, whom I think is connected with Chris Petris or Corvette Fever. The email gave instructions on how to rewire the ECM harness to give a driver complete control over the 4+3 from the shifter button.

Earlier, I had put a jumper on the transmission O/D switch, and ended up with my 88 L-98 starting out without OD through the first 3 gears then dumping in to OD (assuming the right tempeture of 127 degrees).

Jospeh said to cut the A-7 tan/black wire 3 inches from the connector. Also, cut the C-7 tan/black wire in the same manner. I was not clear on which side of the wires to reconnect, so I first tried connecting the wires that came out of the plastic connector. I test drove and ended up with no overdrive at all, just 4 straight gears. The shifter button did nothing.

I then went back and connected the two cut wires that came down from the wiring harness. I left the two cut wires at the connector disconnected. I test drove the car and discovered that I had no overdrive on any gear until I depressed the shifter button. I thought EUREKA! until I disocvered that I had to hold the shifter button down or it immediately popped out of OD.

I then tried connecting the ends of the two wires at both the harness side and the connector side and it was the same as the situation just above. (Holding down the button to stay in overdrive.

I'm sorry to go on and on, but I've tried everything. I had put a jumper on the OD connector on the transmission (not the overdrive unit as one person said). This came pretty close to perfect because when starting the car it said out of overdrive until 4th gear then dumped down into OD automatically. I'm not certain just what to expect, but I have to believe someone out there has done this right and ended up with "complete control of the 4+3 from the shifter button.

Help.

Mike

[EricVonHa]

Simple question--- is that pushbutton momentary or latching?

[scorp508]

Ok you are almost there. A-7 and C-7 are the Overdrive request and the Overdrive solenoid 12v leads on your ECM. What you need to now do is retrofit your shifter button with a 1985 style button which is a toggle on/off switch instead of the 86-88 momentary on switch.

[omega1940]

Thanks very much. It makes perfect sense. I didn't think about the "on/off" version of the older switch. I actually replaced the switch in this 88 when I first bought it because the former owner didn't mind driving without overdrive. (I guess)

Can you tell me the answer to two additional questions. Once I cut (which I already did) the two wires, which ends to I tie together? I tried tying the two wires together that go into the connector, and the OD switch didn't work at all. I then tied the two wires coming down from the harness and that's when the switch worked only when constantly depressed. I assume those are the two wires I tie together and I "cap" the others from the connector.

Secondly, when this is all done, is it correct that the car will start without overdrive (even after heating to 127 degrees) and it will go through all 4 gears without engaging OD. If I wanted to engage OD in 3rd or any other gear it would just take a push from the button. It would then stay in OD as I shifted through the gears until I hit the button again.

Does that sound right?

Thanks very much again.

Mike

[scorp508]

You've got it right. You want to tie together the two wires coming out of the harness and not the ECM. You are essentially using the switch on the shifter now to directly give the overdrive relay the 12v signal it needs to engage the overdrive solenoid.

Normally what happens is when you press the button it sends a 12v signal to the ECM. The ECM says "Oh hey... driver wants OD" and then it will send a solid 12v signal out to the relay as long as the conditions are ok for OD to engage. You are bypassing the ECM's logic completely.

Yes, OD will be fully manual from this point on. As 8388 has stated before, I do not recomment using OD lower than 40mph as the fluid pump in the unit will not have enough pressure built up to fully lubricate and cool all of the parts.

[scorp508]

p.s.


A cheap source of an '85 style switch can be bought at www.newark.com

Part Number: 61F752
Price: $5.45

The 86-88 style switch is 61F753 and is $7.71. You will have to solder your harness to the new switch from your existing one and remember to use hot glue all over the contacts and about 1/2" from them to make sure the constant rowing of the shifter does not break the solders.

[omega1940]

Scorp508, you are a gem. I can't tell you how much I appreciate your help on this frustrating topic. Clearly the problem is the changes in the shifter button mechanism over the years.

After reading your last post about the "low pressure" in the OD unit, I have decided to stay with the "jumper" switch. The only irritating thing in the past was going into OD so quickly, and the jumper keeps me out of OD until 4th gear and I'm usually well over 40 mph by then. I'm afraid I might be tempted to use OD in a lower gear if I had the option. The General's engineers are as stupid as a lot of us think, but it takes a long time to realize that there are always "ramifications" to these things.

Again, thanks very much.

Mike

[scorp508]

No problemo!