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Thread: Tach output- only in reverse (swap issue)

  1. #1
    Tuner in Training V8 Supra Builder's Avatar
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    Jan 2007

    Tach output- only in reverse (swap issue)

    I've overcome all of the electrical bugs in this setup except for the tachometer only working while the car is in reverse. I can't figure out what is causing this, so I'll throw out as much known info/attempted solutions for the brain trust here:

    Here is my setup:
    1988 Toyota Supra, was an automatic, converted to 5 speed by using a '91 donor car.
    Trans is a Temec TKO600
    Motor is a 2003 LM7 5.3 out of an Avalanche
    VCM and wiring harness are from a 1998 F-body unit. The VCM was reflashed by Keith here with a stock '98 F-body tune.

    Tach is adapted using a Baker Electronics adapter, 4 wires:
    1) Power
    2) Ground
    3) Tach signal in
    4) Tach signal out

    This device is configured for an LS1 to a 6 cylinder tach (the Supra had a 3.0L I6 motor originally). It has tests OK (6 LED flashes on power up) until it is connected into the harness. When the VCM is disconnected, it also tests OK.

    I have inspected the wires from the VCM to the adapter, as well as from the adapter to the tach, all are intact with no shorts.

    I thought the issue was the dash wiring at the tach, as there were differences in the auto vs. manual Supras, but when I wired the auto dash as per the manual dash, I get the exact same result. When I do away with wires 4-6, I also get the same result.

    The auto tach has a total of 6 terminals:
    1) Ignition +
    2) Earth (ground)
    3) Ignition -
    4) Norm
    5) Power (Norm and Power correspond to indicator lights on the auto tach)
    6) Unused

    The manual tach has 5 terminals, 1-3 the same
    4) Cruise (goes to an indicator light in the tach)
    5) Power (same as above)

    The Tremec has three electrical connections:
    1) Speed sensor, passenger rear
    2) Neutral safety, driver rear
    3) Backup lamps, driver middle

    The Tremec is connected as follows:
    Speed sensor to pins (Red connector) C1-7 and C1-71. C1-7 shows as a VSS return and C1-71 shows as a VSS signal low. The Tremec had no info as to which wire went to which terminal, so I used the color coding on the T56 VSS plug I used in the install to match up to the VCM.

    The backup lamp switch is connected to the Toyota harness at the connector where the original backup lamps were connected. One leg is +12V and the other goes directly to the lamps. I also bypassed this connection under the dash and ran the wire straight to the plug in the driver kick panel that goes directly to the lamps. When I jumper these wires together the tach works and the backup lights are on.

    The Neutral safety switch is not used.

    I have verified the pinouts on the VCM as going to the proper places. The following leads are not connected due to the nature of the setup:
    C1-3 Evap canister vent solenoid valve (the Toyota evap cannister is marked Purge and tank. It is connected via tubing to the LS1 cannister purge valve control at C1-45)
    C1-14 AC status (the AC uses the Toyota controls and pressure sensors)
    C1-30 Spark retard signal
    C1-36 Fuel gauge output control
    C1-39 AC clutch relay control
    C1-40 Skip shift lamp

    C1-43 Cooling fan relay #1 control (a seperate control is used for the fan)
    C1-47 AC request signal
    C1-62 EGR pintle position control
    C1-63 AC pressure sensor ground
    C1-75 Left & right fuel tank sensor ground

    C2-7 AC pressure sensor signal
    C2-9 EGR pintle position sensor signal
    C2-10 Fuel level sensor signal
    C2-28 Cooling fan relay #2 & #3 control
    C2-30 AIR pump relay control
    C2-31 AIR solenoid relay control
    C2-34 EGR valve ground
    C2-38 EGR valve control circuit

    The tach adapter is wired to pin C2-35, Engine speed (Tach) output signal

    I bypassed the tach feed wire from the tach adapter to the dash with the same result.

    I have tested voltage at the dash, ignition + and earth show 14.2V with the motor running, this drops to 13.7 with the car in reverse. Testing the igntion + and earth, I get 7.4V in neutral and 6.9 in reverse.

    I tried turning on the headlights to add a load and this did nothing.

    What I can't figure out is if the problem is the VCM, it has no way of knowing the car is in reverse. If it is the car wiring, the bypasses I tried should have allowed it to work.

    The only gray area in the wiring is the speed sensor. If these wires were reversed, would this cause a tach output problem? I don't see any correlation here. I did unplug the speed sensor plug and this made no difference.

    Is this possibly something simpler like adjusting the tach output resolution? I know on some swaps this needs to be done, but my tach adapter is advertised as being setup for an LS1 tach output. My tach resolution is currently the default 6 and 6, I have not adjusted this setting.

    Thanks for any help or ideas.
    My prior projects include Chevy Gen 1 V8 into an FB RX-7, Gen 3 V8s into an FC RX-7, a MK3 Supra, a BMW E34, and an LT1 into a 280Z. Still have the Datsun, a 383 LT4 Trans Am, and some bikes.

  2. #2
    Tuner in Training V8 Supra Builder's Avatar
    Join Date
    Jan 2007

    1998 tach wiring to/from VCM

    I have attached a GM wiring diagram of the tach section of the 1998 VCM wiring. It looks like this signal is obtained from either the cam or crank sensor and output to the tach and optional ABS.

  3. #3
    Advanced Tuner
    Join Date
    May 2006
    Upstate NY
    Do you have a wiring diagram for the reverse lamps and switch? It sounds like you're either pulling power or ground from that circuit and that is why it only works when in reverse of the switch is bypassed.

    GM World Class Technician
    ASE Master Auto Technician
    L1 Certification
    2001 Silverado 6.0L Swap

  4. #4
    Tuner in Training V8 Supra Builder's Avatar
    Join Date
    Jan 2007
    Thanks for the reply.

    Wiring diagrams for the Supra are rather involved, I'm currently trying to figure out if the added resistance on the power circuit may be the cause of the tach problem.

    In a nutshell, the power side of the lamps comes from the gauge fuse via a junction box that also supplies power to the neutral start switch (no longer present), the auto trans indicator switch (no longer present) and the AC water valve. The AT stuff was removed downstream from the place where I connected the lamps power supply.

    This junction box also on this same circuit supplies power to most of the indicator lamps, the other gauges, and the rear defog timer. There is no grounding involved in the backup lamp circuit until you get to the lamps themselves.

    At this point, I am going to try a potentinomiter (spelling?) on the tach and see if this makes it work. The 280Z I have has an LT1 which also has a VCM tach output wire, as I recall, there needs to be some type of resistor in the tach line for it to work properly. The two backup bulbs are about 21 watts each, for a combined load at 12 volts of 3.5 amps.

  5. #5
    Tuner in Training
    Join Date
    Apr 2007
    i make small tacho boosters and i use them on 90% of toyotas

    when i do conversion wire ups like 2js twin turbos into supras etc i use these tacho boosters

    i also have black boxes that will make the 4 cyl tach signal from the ls1
    compatible to the 6 cyl of the supra

    so if u get stuck give us a yell

    but make try this

    get the 12v power wire from the untis and connect straight to battery

    see if this cures yr prob then rewire these power to a well known in source

    i have wired up many toyota import engines into many different cars so theres not much i dont know about toyotas

  6. #6
    Tuner in Training V8 Supra Builder's Avatar
    Join Date
    Jan 2007


    Thank you sideshow.

    I'll dig this one up for the benefit of future searchers since I recently found out what to do (thanks to posters at LS1Tech):

    1. Buy a 680 ohm 1/2 watt resistor (.99 for 5 at Radio Shack)
    2. Wire one end to switched +12V
    3. The other end tees into the tach signal line

    While I'm here, changing the tach resolution in HP Tuners from 6/6 to 4/4 eliminates the need for a converter in the line.

    I did the above & the tach in my 1988 Supra is within 25~50 RPM of what is displayed in HP Tuners when I scan it.