Megajolt- Tach out -> LPG switch

[Quagmire]

As the title really... another silly MJ question (although this would apply to squirt too if using LPG)

I wanted to use the tach output from the Megajolt to provide a signal to my LPG switch that the engine is running. Normally this is taken by connecting the LPG switch to the negative of the coil, or by wrapping a wire around an HT lead a few times. Without a signal on this wire the switch will close all solenoids after being open for 3 seconds.

However, i have tried running the tach out of the MJ to the switch and this does not seem to provide the signal that the switch is looking for, the engine will not run without my old wire in place.

Is there anything that i need to add/change to get this to work, or should i just give up and wrap a wire around a lead somewhere (although i'd prefer not to)?

Thanks

[FridgeFreezer]

It would help if we knew how your tach out is built (is it a 5v logic output, 12v with pullup resistor, or what) and what the LPG kit specifies if you have that info.

You also need to make sure the tach output is running at the same rate (1 pulse per ignition event) rather than some multiple or fraction of it.

[Quagmire]

Unfortunately the manual/diagram does not state what the box is looking for, simply to connect the brown wire of the switch to the coil neagtive or wrap it around an HT lead.

From that i guess its probably looking for any kind of pulsating voltage, anything that is not a steady state...

FYI the switch is the third one down here (just incase anyone else has the same):

Switch

However, i do know that the MJ sends out a 12V pulse for every ignition event:

MEgajolt- interfacing to tachometers

All the other user outputs are shown as connecting to 12V- i.e the MJ switches them to ground, so could the tach be the same?

[ThreeSheds]

In my case the LPG controller must have been looking for the flyback pulses the same as the 14CUX and whoever fitted the gas had teed off that wire which originally came from the coil through the resistor to the ECU... When I Megajolted it, I simply used the 'four diodes and a zenner' circuit and linked the output of this into the coil wiring, therefore the LPG switch just worked without any additional thought...

[Quagmire]

Ok, so you used the Zener diode circuit shown in the installation guide and this is connected between the coil negatives and the LPG switch?

For an 8 cylinder would that just be two of those connected together in parallel?

[ThreeSheds]

Ok, so you used the Zener diode circuit shown in the installation guide and this is connected between the coil negatives and the LPG switch?

For an 8 cylinder would that just be two of those connected together in parallel?

Yep! instead of 2 diodes there are 4 - the output then goes through a zenner and then that feeds into the (white?) wire that used to run from the coil to the ECU. Wait - I'll go and have a look at the wire colour...

NO - it's the white/black one. This wire is teed to feed the gas controller which switches when above 2000rpm and revs are falling. No timer or anything. Crude but works (like me)

Sorry - a bit more - don't know if this is relevant to you at all... On my system the ECU still works and thinks it is controlling everything but the gas controller switches both the fuel pump and the injectors off when on gas using some relays.

Here is an extract from the notes I made for myself:

ECU Tach input is through four diodes and a zenner and then is fed back to the coil black/white lead. This means that there is also the standard fitment current limiting resistor still in place.

When using EDIS these wires must be isolated from the original coil and amplifier

Since there already is a 10k resistor in the circuit I used the zenner.

Edited October 2, 2009 by ThreeSheds