Tach does not work, but tacho sweep does.

[Guest |649|]

I have a V88 in my 1996 USDM Mazda Miata (MX5).

The ECU sees RPM, and the tacho sweep function works at startup, but the tach does not function (stays at 0) while the engine is running. I should also mention that the tacho sweep only covers 1/2 of the rpm range selected (set to sweep to 7500, but only sweeps to ~3500-3700 RPM).

To add to the mystery, I have an Autometer shift light with numerical RPM display, which is connected to the stock tach's signal wire. It (the Autometer shift light w/ RPM) works perfectly.

If I swap in my "old" ECU, the stock tach and everything else works perfectly. If I swap back to the V88, everything except the stock tach works perfectly (but the Vi-PEC's dash sweep still works as described above).

My MX5 is a 1996, which is the first year where the tach output signal comes from the ECU. In previous years, the tach output signal came from a wire on the coil packs.

Am I right in assuming that this is a setup issue? How should the tacho be setup? Though I am using a V88, and have a US-market MX5, my base configuration is based off of the MX-5 plug & play configuration.

Any help would be greatly appreciated!

- Will

[rayhall]

Will,

On the plugin ECU for the MX5 there is no output on the ECU going to pin 9. This is a tacho input pin and not an output. The tacho signal does not come from the ECU, it goes too the ECU.

Do not connect an output on the V88 to this pin, and everything should work correctly. Only down side is no tacho sweep on startup.

On the plugin ECU there is a 470 ohm resistor connected to pin 9 and pin 52, which is the permanent 12v from the battery. Only add this if you still have problems.

Ray.

[Guest |85|]

The rpm signal comes directly from the igniter located in the engine bay.

I connected the tacho o/p from the vipec directly to the tacho input wire located at the back of the instrument cluster. There are 2 connectors; 12 pin and 13 pin. RPM is at the 13-pin connector, 8th pin from right looking from wire side - should be a yellow/blue wire.

RPM works perfectly for all MX5s I worked on.

CP

[Guest |649|]

Thanks for the replies, I'll give that a try!

[Guest |649|]

Will,

On the plugin ECU for the MX5 there is no output on the ECU going to pin 9. This is a tacho input pin and not an output. The tacho signal does not come from the ECU, it goes too the ECU.

Do not connect an output on the V88 to this pin, and everything should work correctly. Only down side is no tacho sweep on startup.

On the plugin ECU there is a 470 ohm resistor connected to pin 9 and pin 52, which is the permanent 12v from the battery. Only add this if you still have problems.

Ray.

Hi Ray,

Unfortunately, none of these suggestions work. Disconnecting the V88 tacho output (aux 3) from the dash does not bring the stock tacho to life, and kills the signal that the shift light and data acquisition system were previously receiving.

It is my understanding that In the US, after 1995, the tacho signal goes from the ECU to the tachometer. The tachometer output pin that was previously on the coilpacks, are not there anymore.

From my research, it seems that the stock tachometer gauge wants to receive a 12v square wave.

I am using Aux 3 for my tacho output, and just came across this statement in the VTS online help file:

2. Only Auxiliary Outputs 5 to 8 can high side drive loads.

Does that apply to tacho outputs too? Should I be using Aux 5, 6, 7, or 8 to drive the tacho instead?

Thanks in advance,

Will

[iliasfyntanidis]

Was the mystery solved?I am planning my own i88 installation.

[Simon]

From our info it looks like the tacho should be driven direct from the coil/igniter.

However if it is ECU driven this will be no problem at worst a pullup might be required to get the required signal.