GruvyTune

Hey,
I went to dyno my car and they connected their dyno to my RPM test connector at the driver's side under the hood. For some reason, their measurements were all over the place at higher rpms. My rpms measure fine on my dashboard tach. The helms manual shows the blue test wire to come from the distributor's ignition control module directly. What could be the problem? Have other's used this wire without problems?

thanks!

Jared

Never had a problem with that wire. Maybe you have a loose connection somewhere. Try just tapping straight into your tach wire next time.

GruvyTune

where is the tach wire? Is it the blue wire at the distributor? Is there a tach wire that goes to the ecu?
I tried looking in the manual but i don't see a "tach" wire going to the ecu. just the wires coming from the ignitor to sense the TDC and crank positions.

Is there an actual rpm wire at the ecu and if so what color/position is it?

thanks again

bob01721

The "tach" wire to the ECU doesn't come from the ignition. It's the other way around. Maybe I can clarify some of the "from" and "to" connections in general and you can take it from there and apply it to your car.

With Honda PGM-FI, the crank sensor (RPM), the TDC sensor, and the Cyl sensor (where #1 piston is) all go directly to the ECU, not the ignition system. The ECU has a different wire that goes to the igniter to tell it to shut down and let the coil fire the plugs. On my car, it's a YEL/GRN wire coming into the igniter, but yours may be different. This is the signal the CAUSES the igniter to produce the tach signal, so it's behaving the same way as the tach signal will, in terms of timing and duty cycle.

Then there's the BLU wire from the igniter that actually fires the coil. (Yours may be a different color, but it sounds like we're talking about the same BLU wire here.) This is "the" tach signal. It has the primary coil current (low voltage) and it switches on and off as directed by the ECU. On my car, this signal also goes through an isolation resistor inside the coil and then (on BLU wires) to the maintenance tach connector with the little cap on it and to the to the controller for the dashboard tach.

There should also be a constant 12V coming into the igniter whenever the ignition switch is ON. On my car, it's a BLK/YEK wire, but on your car it may...

So the bottom line is, the igniter will have constant 12V on one wire, and some flavor of tach signal on the other two. Oh, and a BLK ground wire.

I don't know of any blue wire at the distributor, because my car doesn't have one. I'm not aware of any PGM-FI connections to the distributor except those big high-voltage secondary wires.

I hope this helps you sort things out.

GruvyTune

that helps alot. thanks for the clarification. Makes sense electrically.

Any ideal why my dash tach looks fine but hooking up that blue wire to an external tach was all over the place?

torin

bob01721

Loose wire? RFI? Bad connection? These are just guesses. My tach/voltmeter uses an inductive pickup around a plug wire to sense rpm, so I don't use the BLU wire.