Signal requirements for a std XD tacho.

[Anonymous]

Am fitting an aftermarket ignition system and cannot get the std tacho to work, even after hooking the input wire to the negative of the coil.
In std form, the XD tacho ran off the negative of the coil ( bosch electronic magnetic reluctor), so I am a little perplexed as to why the tacho still won't work.
I have had the tacho tested and it's o.k.
any suggestions? What are the signal requirements other than !2 volts? the genuine ford manual doesn't say anything.

 

[addo]

Have you fitted an MSD?

 

[Anonymous]

I did have a Mallory Hyfire fitted. to get the tacho to work required a small coil and 10k resistor fitted in parallel with the tacho output line, presumably to boost and control the signal?????
The unit fitted now is a Haltech IG5 ignition computer. I have tried a couple of things so far to get the tacho to work, but with no response;
(rpm registers on the engine data page of the IG5 and the engine runs o.k)
-connected directly to the tacho output on the IG5, with and without the booster(?) coil and10k resistor as fitted to the mallory.
-connected the tacho directly to the negative of the 3 direct fire coils with diodes in each line to prevent signal crossover. Then added booster(?) coil and still no response.
-connected an ordinary VDO tacho to negative on coils and worked o.k
Wiring to tacho is o.k, and tacho has been checked as o.k as well.

Am a little bit puzzled as the std electronic system ran off the negative of the coil and the pull to ground signal should be pretty similar anyway.

P.S. rang Haltech helpline; absolutely no b....y use.

 

[addo]

Warning: I am a carpenter, not a knob jockey so this is all speculative.

My guess is that the Haltech provides sparking like an MSD and the outputs will need to be similarly buffered. Is it possible the VDO tach tried was inductively driven (and thus would work on an "aggregate" spark discharge), whereas the Ford unit had full electronics?

Seeing the waveform on a scope would help. You could determine peak voltage, and the frequency between pulses (assuming it is that type of system). Then you can clip the signal and the fuzziness, just using the remaining blunted signal peak to trigger a transistor driving the tacho.

Or, look for an MSD tach adaptor to see if that does it for you...

 

[Anonymous]

thanks addo, I will get hold of an oscilloscope from a friend and let you know.
thanks for the input.

 

[addo]

Yet another toy I'd love to have room for on the bench. You lucky bugger.

 

[Anonymous]

Addo; this one's for you!
Got the XD tacho working. seems the voltage from the haltech tacho output and the negative of the coils with diodes is insufficient in it's signal strength.

Went to Jaycar and bought a 3.5 watt pre-amplifier (1 inch square and potted; $18 approx) and put it in the line from the tacho output. tacho started working immediately. checked with engine data page and readings correct.

Ah the joys of having a tacho at redline again.

Thanks Addo for your input.

 

[addo]

Good old Jaycar. Thanks for the feedback. I wonder if that's all that's in the MSD tach adaptor?

 

[Anonymous]

Jaycar has amplifiers of this nature in even stronger wattages ie 12 and 18 watts so it should be possible to play around a bit.
wouldn't surprise me if that is all that is required. worth a try for under 20 bucks. bet the msd item is multiples of this to buy.