VTEC

VTS and VTM connectors

Most VTEC engines in the 90’s have a VTEC system which consists of one solenoid and often one oil pressure feedback sensor. The connectors typically have this pin configuration:

Nomenclature:
PG – Ground circuit, usually connected to chassis at G101 at the thermostate housing
VTM – Oil pressure sensor signal to ECU
VTS – VTEC solenoid, VTEC is activated by applying 12V

The following explains the basic working of what I call ‘classic’ VTEC in D, B, C, F and H-series engines. Probably I miss some engine types. Later engine series like K-series have a more complicated system which is not described here. Also there have been double VTEC systems which work similarly.

Very simplisticly put, VTEC is a system which changes the cam lobe profile used at various rpm and other engine load conditions to have a wider range of torque and / or good fuel economy which is not possible with a single cam profile. Some systems only apply different cam lobes on the intake side (most SOHC D-series with VTEC as example), others apply different cam lobes at both intake and exhaust side (most if not all B-series with VTEC) and some only switch between having a cam lobe or none at all to switch between 12 valve and 16 valve operating modes (VTEC-e).

This switching is always performed by applying oil pressure to specific channels in the rocker arm assembly which pushes small barrels around, locking otherwise floating rockers with each other and causing valve movement to be further and longer in duration.

VTEC oil pressure is engaged by the ECU which applies 12V to VTS. Current flows through the solenoid to the engine which functions as ground. This enforces the solenoid to open the oil channel to the rocker arm assembly for VTEC.

Some engine models only have VTS, some have VTS and VTM. VTM is a sensor mounted in the oil channel towards the rocker arms after the VTS solenoid. It provides feedback to the ECU whether the oil channel is pressurized (VTM signals 12V) or not pressurized (VTM signals 0V).

The sensor is a switch that is in a open or closed state:
Closed / low rpm – It is closed without VTEC engagement / no oil pressure, then VTM is shorted to ground (via PG) and reads 0V. Open / high rpm – The sensor is open when pressurized / with VTEC engaged. The VTM circuit is broken and the ECU pulls it up to 12V, hence you’ll read 12V in that case.

The PG wire for ground can be joined with PG at the ECU. The official position in 5th gen Civic wireloom is a soldered joint in the middle of the loom near the distributor junction. In 6th gen Civics the joint is made in a splicer under the intake manifold (that is one of those two dead-end connectors that reside there). For EJ9 chassis I am not sure if there is an empty slot, but in the case of EJ6 / D16Y7 I am 100% certain since I used it once for a such a PG wire.

Sometimes you may have a hybrid setup where only the VTS solenoid is available while the ECU expects both VTS and VTM to be there. Normally this will lead to a OBD1 code 22 error or alike, but this can be circumvented. My advice would always be to make sure the engine and ECU match or VTM is disabled in the ECU software, however there is the following trick:

Short circuit the VTM pin at the ECU to VTS. This simulates a working VTM sensor signal; 0V at VTM in low rpm / no VTEC and 12 V at VTM in high rpm / VTEC regimes. I have done this in the past once and it worked, but it did threw me a code 22 once. So its not guaranteed failsafe.

Dodo Bizar