Wiring the Tach to a compushift on a Defender TD5

[ashtrans]

The compushift has the ability to display the engine revs if you wire up a 'Tach' signal to put a pulse on the input wire in the loom,

We have fitted quite a few of these on Auto TDCi Defenders but never bothered with this option as you have a rev counter on the dash,

I have a couple of customers that are trying to get this working and they have connecetd the wire to 'Pin 19' on the engine ECU which puts out a tach signal but it doesn't work.

I think the problem is the ECU puts out a 5V signal and the Compushift needs a minimum of 8V.

So I think I need a simple circuit with a 12V feed, 5V pulse from this Pin 19 to output a 12V pulse which I can feed to the compushift.

Would anyone know how I can achieve this ?

Electronics are not my strong point and I know there is a wealth of knowledge on here so wondered if anyone may be able to help.

[white90]

Calling Fridgefreezer!

[MECCANO]

Something like this.

Although the values probably need tweaking. ( oops yup make the resistors 100k) done it!

[ashtrans]

That's great Meccano,

Many thanks,

I will let you know how it works out,

[FridgeFreezer]

What meccano said, although I'm not sure you really need the darlington arrangement, a single transistor should do it.

[MECCANO]

That's great Meccano,

Many thanks,

I will let you know how it works out,

It may be a good idea to put a resistor on the input to the first transistor as the ECU might not be a buffered/isolates output.

[MECCANO]

What meccano said, although I'm not sure you really need the darlington arrangement, a single transistor should do it.

Your probably right, but its not much more effort/ cost... and knowing my luck it would wouldnt work because of it

[FridgeFreezer]

I knew I had an example somewhere - this is the exact same thing but from a MegaSquirt ECU:

Apologies for size.

[western]

Superb response Guys

[TSD]

Your probably right, but its not much more effort/ cost... and knowing my luck it would wouldnt work because of it

He's nearly right, except it's not a Darlington.

Oh, and if you leave out the series resistor in the base of the first transistor, it's a bit harsh on the BC108 (which saturates at about 0.7V, and then looks a bit like a short circuit), and on the poor logic gate (or whatever it is) that's trying to drive it.

Suspect that 100k source impedance and 100n coupling cap won't give much action either, into any kind of real load, at real speeds.

[Dave W]

The compushift has the ability to display the engine revs if you wire up a 'Tach' signal to put a pulse on the input wire in the loom,

We have fitted quite a few of these on Auto TDCi Defenders but never bothered with this option as you have a rev counter on the dash,

I have a couple of customers that are trying to get this working and they have connecetd the wire to 'Pin 19' on the engine ECU which puts out a tach signal but it doesn't work.

I think the problem is the ECU puts out a 5V signal and the Compushift needs a minimum of 8V.

So I think I need a simple circuit with a 12V feed, 5V pulse from this Pin 19 to output a 12V pulse which I can feed to the compushift.

Would anyone know how I can achieve this ?

Electronics are not my strong point and I know there is a wealth of knowledge on here so wondered if anyone may be able to help.

Have you asked Patrick about this ? I'm no expert on TD5s but I gave Patrick a hand on the TD5 auto prototype and that wasn't an issue that came up and I'm pretty certain the RPM was showing on the Compushift... I seem to remember playing around with the Compushift settings to get it to read the same as the Tacho on the prototype vehicle.

The TD5 rev counter uses a 12v input so you can hook the Compushift directly up to the same feed. On the Megasquirt setup I have to use an additional circuit to convert the 5v output from the Megasquirt to be compatible with the Compushift and the TD5 rev counter I use (hence I'm pretty certain there must be a 12v pulse available on a TD5).

The circuit I use on the Megasquirt is the top one in the picture Fridge has posted above.

[ashtrans]

Hi,

thanks for the responses,

I will try Patrick in the morning to see if he got it working on the TD5, if not I will try making up a circuit,

[Dave W]

I was talking to Patrick at Maddison 4x4 about something else today and mentioned your query.

As suspected the ECU does produce a 12v signal BUT it can only drive the Compushift OR the rev counter, not both at the same time. It produces 2 pulses per revolution on pin 19. This suggests the problem is down to a current limit rather than a voltage problem, this could probably be resolved by using one of the circuits above anyway although if it is that sensitive on current you'll need to be careful not to put too much load on the ECU output.

[Dave W]

Just resurrecting/updating this thread as I have just gone through the process of installing a TD5 with Compushift again and it's fresh in my mind so hopefully it will help someone else, or me if I have to do another one in a couple of years time !

The tacho output from the TD5 ECU is a 5v pulse and, when the engine isn't running it outputs a dummy RPM signal of around 200 RPM. There was a bit of head scratching going on over the whole 5v/12v issue and it seems that the prototype/early ZF Compushifts which myself and Patrick were involved with would work off the 5v tacho signal without a problem but the production/later ones, as Dave says, don't respond to the 5v signal.

To get the tacho working on the later Compushift I used a circuit similar to the top circuit in FFs post. I used 1k resistors on the base and collector connections and substituted the 2N2222A with a 2N3904.

Another difference between the earlier and the later Compushifts is that the later ones will allow you to go into setup when the RPM is below (at a guess) 300 RPM. On the earlier (5v compatible) versions you had to disconnect the rpm signal into the Compushift to go into setup - which was a tad annoying with the TD5 ECU putting out the "dummy" 200 ish RPM signal when the engine isn't running.

The Compushift has had the "Pulses per Rev" setting replaced with a "Number of Cylinders" setting which, for the TD5, needs setting to 4. As a reference a TD5 normally idles around the 750 rpm mark.