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.