Getting my tacho to work with 4 coils in wasted spark mode

[else]

Hi Everyone, got a nice little problem for you clever people. Forget all the boring arguements and insults and get your teeth into something constructive.

I have fitted a Motec ECU in place of the original Lucas crap and everything is fine except for driving the original tacho. I am using two Ford Zetec twin double ended coils in wasted spark mode.

The tacho used to take its impulses from the -ve coil connection so i have tried driving it direct from an output from the motec configured as a tacho output but no joy. I've tried biasing the signal from that tacho high and low using a 1k resistor on advice from Motec but still no joy.

Tonight i have tried a different approach. I have taken a line from the -ve of each coil (4 off) and connected them to the tach through diodes to stop the feedback messing things up. The tacho only reads with one connected and but at a lower reading as expected. Connect two and the reading increases (at idle) but is unsteady and drops to zero as soon as i increase the revs a little. If i connect three or all four i get no reading at all even if i build the revs up. I have tried using the same 1k resistor to bias the signal up to 12 volts or down to ground but no joy. If i take the feed without a diode to the tacho i get a good steady reading but obviously i cant connect more that one together without a diode.

My conclusion is that the tacho only works with the flyback that occurs when the coil is turned off hence the output from the ECU not working. I also think that the diodes are clipping this flyback somehow but i don't know how or what to do to overcome this.

Has anybody had this problem and found a solution. I am hoping Peter (greenv8s) or Joo can help, i know they have played the same game in the past. Maybe i am missing something obvious.

Andy

[dnb]

You are spot on with your diagnosis.

If your ECU has a TTL level RPM output then you can most likely use this to drive an inductive pullup to control the tacho. You can buy the bits to make this from Farnell or Maplin for a few pence. (a couple of diodes, a resistor and small inductor)

Or of course you could buy a tacho level converter for £50 or so...

[else]

dnb said:

You are spot on with your diagnosis.

If your ECU has a TTL level RPM output then you can most likely use this to drive an inductive pullup to control the tacho. You can buy the bits to make this from Farnell or Maplin for a few pence. (a couple of diodes, a resistor and small inductor)

Or of course you could buy a tacho level converter for £50 or so...

Thanks for the reply dnb,
I quite fancy the farnell or maplin route, much more interesting but please tell me how. can you give me a basic circuit diagram.

Andy

[dnb]

First of all, make sure your ECU output can handle the (approx 0.4A peak, 0.15A average) current of the inductive pullup.

You need a 600V 1A to 2A diode (ER2J or ES2J is fine) to protect the output. An additional 200..400V bidirectional transient suppressor provides clamping.

ER2J cathode (marked with a stripe) connected to ECU output
ER2J anode connected to inductor
inductor connected to 22 Ohm 2W resistor
22 Ohm resistor connected to 1A fuse (just to be on the safe side)
1A fuse connected to +12V
bidirectional transient suppressor diode one end to resistor (the leg close to +12V)
other end to inductor (where the ER2J diode anode is also connected)

Sorry - I don't have a circuit diagram to hand at the moment, but the description is what I used on my VEMS setup before I took the car apart...

[markh]

Had the same problem with mine after fitting the Autronic ECU, problem cured by installing an MSD control box, mine came from Real Steel (Middlesex, I think)

Best of luck

[else]

dnb said:

First of all, make sure your ECU output can handle the (approx 0.4A peak, 0.15A average) current of the inductive pullup.

You need a 600V 1A to 2A diode (ER2J or ES2J is fine) to protect the output. An additional 200..400V bidirectional transient suppressor provides clamping.

ER2J cathode (marked with a stripe) connected to ECU output
ER2J anode connected to inductor
inductor connected to 22 Ohm 2W resistor
22 Ohm resistor connected to 1A fuse (just to be on the safe side)
1A fuse connected to +12V
bidirectional transient suppressor diode one end to resistor (the leg close to +12V)
other end to inductor (where the ER2J diode anode is also connected)

Sorry - I don't have a circuit diagram to hand at the moment, but the description is what I used on my VEMS setup before I took the car apart...

Thanks dnb,

But you don't mention any details for the inductor and bidirectional transient suppressor. I've looked and there are so many to choose from.

Sorry but i am not an electronics wiz, just a keen amateur.

Everything else is crystal.

Andy

[dnb]

TBH it doesn't make much difference (within reason) what values you use, as long as they don't die in use - all you're after is a series of damn great voltage spikes. The actual voltage matters very little. Just be careful not to get yourself near it as 400v shocks tend to sting a little.

Inductor is 220uH. I don't remember the part number for the transient diode, but as long as it's rated at 30V you should be OK. I'll look in my box of bits tonight to see if I can find the part nos.

[else]

Hi dnb,

Got round it this weekend using a plug in relay similar to that used for the fuel pump. Also made a nice neat package once installed.

Thanks for your help, it pointed me in the right direction.

Andy