C4 4+3 Overdrive Bypass
4+3 Overdrive Bypass
This mod will bypass the ECM and allow
\”on-demand\” overdrive operation (in any gear, at any speed, whether
warm or cold).
Disclaimer: I accept no responsibility for any
malfunctions or damage resulting from this modification. Proceed at your
This modification assumes your Overdrive is operating
correctly, and all wires and connections, relays and switches, are present
I recommend disconnecting your battery before
proceeding. If in doubt about any of the described procedures, please
consult the Owners Manual or the CorvetteForum Archives.
need to get up under your passenger footwell. The plastic \”hush\” panel
needs to be removed (the one with the door-operated light in it). Above
this, you\’ll see a cream-colored plastic brace with two cap screws
holding it up; take the screws out, carefully push the myriad wires out of
the way, and drop the brace and ECM down. Remove the two harness
connectors from the ECM.
I have seen two types of harness
retainers: A plastic type, which you just depress the tab and the
connector pulls out of the ECM, and the type in my \’86, which uses two
metal clips that must be pried up while pulling the connector
If you look into the harness connector slots on the ECM,
you\’ll see that GM has thoughtfully labeled the connectors (but not the
harness itself). The two wires you are interested in are A7 and C8.
Carefully select the wires *on the harness connectors*, and use a fine
flat-blade jewelry screwdriver or even a paperclip to release the
connectors. Pull them right out of the harness connectors.
two wires need to be connected together to bypass the ECM. I cut the wires
about two inches up from the end, and installed a six inch jumper wire,
with insulated spade connectors on each end (M & F). This way, I can
easily & quickly re-connect the wires to the ECM if I so desire to restore
it to OEM operation.
Re-connect the harness connectors. Restore the
ECM to its holder and re-install the brace, capscrews and plastic hush
This completes the ECM bypass; but the Overdrive button will
still be momentary-contact (you\’ll only have Overdrive while you hold
the button down). So, we need to install a 1985 Overdrive switch to get
On/Off operation of the Overdrive. These switches can be ordered from
Newark Electronics (www.newark.com) for under $15.00. The part number is
61F752 On/Off 1985 Type
61F753 Momentary 1986-on
Type (in case you just want the OEM switch)
Some soldering will be
required. I recommend ordering a spare switch or two (there is a minimum
for orders anyway).
To get at the Overdrive switch, you need to
remove the shifter upper cover (the one with the leather boot). This
involves (on my car) removing the console lid; the drivers dashboard
fascia panel; the radio/AC fascia panel; the shifter upper console panel;
the gearshift knob, wedge, and Overdrive \”T\” screw. After removing all
these, you can get at the base of the shifter. (It\’s not absolutely
necessary, but I heartily recommend removing the drivers seat and side
cover for the transmission tunnel. You\’ll be amazed at what you\’ll
find down in this cavity).
Remove the switch from the shifter; it
may or may not be pinned to the white Nylon block. Jot down the wire
locations; you\’ll need to solder the wire harness to the new switch. I
recommend using a continuity tester to get the leads correct. Solder it up
and when cool, cover the switch leads that you just soldered with epoxy or
heat-gun hot-glue. The leads will flex with the operation of the shifter
and soon break off if this is not done (tip thanks to
Re-install the switch and re-assemble the shifter.
Re-connect the battery. Test the operation of the switch; the OD light on
the display should function as normal, indicating whether the OD is On or
This should complete the operation.
(At this point, I
recommend adjusting the shifter rods. This involves fabricating an
alignment tool to hold the shifter in Neutral while adjusting the rods on
the side of the transmission. My tool measures 3 1/2\” long and has a 1
3/8\” long .650\” x .100\” \”tang\” to insert into the shifter
Re-install any left-over panels, parts, or
accessories. You\’re done!
On my car, I have an after-market
shifter installed, with the OD switch re-located to the change tray. If
you desire to go this route, any convenient SPDT switch will
Credits go to CorvetteForum C4/Tech discussions between
Blake Kerkovich, David F., Scorp508, Hendej, and Sarkee back in the Spring