Last Updated: 10/18/96
by Matt Blodgett
First, how does VTEC work? At a pre-designated RPM, if and only if there is enough oil
pressure, the CPU activates a solenoid. This causes a pin to lock the intake valves
together creating a sudden jump in acceleration. The stock switching point is excellent
for a stock engine. But, we obviously don't want to leave our engines stock. This is
where a VTEC controller comes in, this allows you to tune the VTEC to the engines
modifications. When used in conjunction with a dyno you can tune your engine to
My first design consisted of an Autometer Shift-Light ($75). This may be the cheapest
design. You take it apart and connect a wire to the positive lead of the light bulb. This
is your signal. When the engine reaches the RPM set on the shift-light a constant 12 volt
signal is sent through the wire. This signal should be isolated from the shift-light
through a standard Bosch relay. You now have a VTEC controller. Locate the positive lead
of the VTEC solenoid. DO NOT JUST TAP IN TO THIS WIRE!! as 12 volts would be sent back to
the computer and possibly fry it. You must cut the wire. The end from the CPU cannot just
be taped off. The computer will see this as a malfunction in the VTEC solenoid. In
non-VTEC mode the computer looks for a resistance of 14-30 ohms on this wire from the
solenoid. This is how the computer senses when something is wrong with the VTEC solenoid.
In VTEC mode the wires switches to +12 volts. Build the following circuit. Take the cut
wire from the ECU and attach it to the positive terminal of a 12 volt LED. The negative
terminal of the LED should be connected through a resistor to ground. The total resistance
of the LED and resistor should be about 22 ohms. Measure the resistance of the LED and
then use the appropriate resistor to give you 22 ohms total. All of this is of course
available at Radio Shack. When the ECU is in non-VTEC, the wire still provides 22 ohms and
when it switches to VTEC the wire won't short to ground.
Connect the remaining end to your 12 volt signal from the VTEC Controller
(shift-light). Now all you have to do is insert the chip into the shift light that
corresponds with the point you want to have the magic happen (i.e...4500, 5000, 5500).
Don't set it too high or you will lose bottom end performance. Don't set it too
low, like less than 4000RPM, or you may not have enough oil pressure to ease the pin into
the rocker arms. This could hurt the engine.
My latest design uses an RPM activated switch. These are available from Mallory, NOS,
and I think MSD. Cost is a bit higher, around $175, but you have less wiring to do and you
don't have to change shift light chips. On these units there are dials to adjust the
RPM. If I've left any thing out or you have any question just ask.
After I posted that, someone told me that you could just add a diode between the ECU
and the VTEC controller solenoid to protect the ECU from a back signal, rather than
totally eliminating the factory connection. I told them that was a good Idea. I was wrong.
You can't have the stock signal intact, because if you set the VTEC controller higher than
stock, the factory setting will kick in before the user setting.