No Tachometer

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No Tachometer

Postby P6 4000 » Mon Oct 02, 2017 4:14 am

Hi all, sorry for a huge first post, but I have made it as short as poss :)

I have a P6 with Megasquirt 1 V3, running fuel and ignition on a 4L Rover V8. Most things working now and the car ticks over well, accelerates well, drives nicely, starts well, etc. very please with the transformation from the four barrel :)

Two problems still though, 1/. a bit of hunting now and then, which I'm sure I will chase down and sort completely very soon, and 2/. The Tacho - now that is driving me nuts :(

I contacted a company who sells a device called a 'tachfix.' After conducting a few tests, like attaching the IDM to the tacho, where it made the tacho jump a little when engine turned on, and again when it turned off, nothing when running. I then disconnected the orange wire on the EDIS and brushed it against a ground with the ignition on, and the tacho jumped to 500 - 1,00rpm every time I did it. I then connected the SAW to the tacho and that worked but was reading 100rpm low, not a problem, that would do.

He gave me two options, the one I chose was to send him the ECU and he fitted a tacho output, which connected up to the Tachfix. It arrived back and I connected it up, red to ignition live, black to earth, orange to new tacho output wire and yellow to tacho. Fired the car up, the tacho burst into life but the needle just bounced up and down.

I can upload a video of the tacho with the tuner studio one next to it, if that helps.

The only other thing, which may or may not be connected is a strange reading from the "Rear bank O2 voltage," which shows the needle pinned to the end of the gauge, as in the pic attached. The car is ticking over when this is happening. I was just wondering if, somehow the output for the tacho is pointing at the O2 gauge instead? I don't know how or why, just guessing.


As I have a wire fitted to the ECU as a tacho output, a tachfix, and a Tacho which 1/. Worked with the coil set up, and 2/. a spare Tacho I have does the same and jumps, I can only assume that something is wrong with either the output wire or the tachfix. I was hoping that there is something in Tunerstudio was pointing in the wrong direction and that I could just hit a few keys and get it to work :) - now there's optimism for you :)

Is there any way, with the help of a multi meter that I can check the output wire and the tachfix?

I have done some basic testing - does this lot look right?

Ignition off, I checked from the wire at one side of the connectors to the wire at the other side, to check for any resistance. None at all for any of them, so connectors are ok.

Resistance from yellow wire on Tachfix to back of tacho - 0.04 ohms.

Black wire on tachfix to chassis - 0.04 ohms.

With the ignition on, all connected: -

Red 12.58V

Orange 0.81V Tachfix to ECU

Yellow 0.04V Tachfix to tacho

Black 0V

Engine running

Orange 0.06 to 0.13V

Yellow 13.6 to 14V

Any ideas anyone?

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P6 4000
MS/Extra Newbie
Posts: 2
Joined: Wed Dec 28, 2016 5:42 pm

Re: No Tachometer

Postby P6 4000 » Fri Oct 13, 2017 12:17 pm

I have made up a small electronic board from the manual containing 4 x 1N4004 diodes and an 18V zener diode. This works up to 1,300RPM but above that, the needle just drops to nothing. On advice from another forum I changed the zener to a 1000 ohm resistor - then it didn't work at all.

I then tried another type which worked from the installed output wire and consisted of a 1000 ohm resistor, a transistor and an old relay. Nothing, not a sausage :cry:

This, along with the fact that the tachfix doesn't work from this wire either, and that the tacho works with coils, does make me suspect the installed wire.

Unfortunately looking inside the ECU is like looking into a tin of spaghetti to me, I have no ides what i'm looking for. Can anyone tell me whether this looks right?



Would appreciate any help :)
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P6 4000
MS/Extra Newbie
Posts: 2
Joined: Wed Dec 28, 2016 5:42 pm

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