RPM on a digital dash

[Lewis Cobley]

Hi, I've tried to find answers on vehicle specific forums but had no luck so I'm hoping some specialist auto electrician can help.

i have a 1994 Mazda MX-5 1.8 and I'm trying to connect a Race Technology DASH2PRO.

I'm having some trouble with THE RPM readout. I know it used to work when I first fired up the MX-5 engine to test the wiring.

Now the wiring is in the car the RPM doesn't work at all. The way the tacho signal comes is there is a black/white wire coming from each of the two coil packs which merge into one wire. Then it splits back into three, one going to a pin on the ECU, another to the diagnostics port and the other to drive the tacho. When ignition 2 and the engine is running around 12v is being output on the wire to the tacho, however when I connect this to the DASH2PRO RPM input (yellow wire on connector B) the signal drops to around 6v and no RPM is displayed. I've even disconnected all other wires from the DASH2 so its just power and RPM.

Connecting the original Mazda dash back up shows the RPM working and the tacho wire staying at 12v.

Ive traced the wires and they look ok, no breaks or shorts. And I've had the DASH2PRO unit tested by the maker today and it is working fine.

Here is the Mazda wiring:

And the DASH2PRO wiring:

I am am connecting instrument cluster tach from the Mazda to yellow low rpm on the dash.



Why when adding the dash to the circuit would the voltage be pulled down by half?

Thanks

 

[plugsandsparks]

There are generally two types of tacho inputs available. low level and high level. Low level refers (generally) to a tacho input derived from ccts inside the ECU and is usually a 5V square wave. High level tacho inputs generally refer to when you use the ignition coils neg switching side to use as a tacho input signal. From your MX diagram you are wanting to use a high level input but you have connected it to the low level input. You need to establish from Racetech which output wire is the high level tacho input and use that. There may be options in the software that you also have to configure. You need racetechs installation manuals and config manual to be able to set up the tacho correctly.
cheers
P&S

 

[Lewis Cobley]

Thanks for the reply. I've been testing with the oscilloscope this morning and found the following.

The blue line, CH1, is from the engine tacho signal connected to the DASH2PRO low RPM (during the testing with the DASH2 not plugged in the voltage never exceeded 12v so I don't think it is a high signal). It mainly reads 6v apart from on the pulse where it drops to around 1v. The DASH2 reads no rpm with this.


The orange line is an Arduino which set to turn on its voltage for 1ms and then turn off the voltage for 10ms. When the DASH2 low rpm is connected to this signal it reads it at around 3000 rpm.

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Here you can see the engine tach signal with the engine on without the DASH2 reading 12v.

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Maybe the DASH2 pulling the signal down is not an issue if it can read the Arduino signal which is peaking at only 5v?

 

[plugsandsparks]

High level is actually the 12V signal. It is simply the 12V derived from the battery / alternator which is switched across the ignition coils. As the coils do induce higher transient voltages when switching at high RPM it is referred to as a high level signal. If you scope the neg side of the ignition pack at 4000 rpm you will see what I mean on the trace. When testing static or low rpm the transient reduces, its not possible to measure this signal properly with a voltmeter but if you have a scope, happy days

Oh cannot see the pics

 

[Lewis Cobley]

Sorry, can't see how to edit the above post.

Thanks for the reply. I've been testing with the oscilloscope this morning and found the following.

The blue line, CH1, is from the engine tacho signal connected to the DASH2PRO low RPM (during the testing with the DASH2 not plugged in the voltage never exceeded 12v so I don't think it is a high signal). It mainly reads 6v apart from on the pulse where it drops to around 1v. The DASH2 reads no rpm with this.


The orange line is an Arduino which set to turn on its voltage for 1ms and then turn off the voltage for 10ms. When the DASH2 low rpm is connected to this signal it reads it at around 3000 rpm.

Here you can see the engine tach signal with the engine on without the DASH2 reading.

​

​

Maybe the DASH2 pulling the signal down is not an issue if it can read the Arduino signal which is peaking at only 5v?

 

[Lewis Cobley]

High level is actually the 12V signal. It is simply the 12V derived from the battery / alternator which is switched across the ignition coils. As the coils do induce higher transient voltages when switching at high RPM it is referred to as a high level signal. If you scope the neg side of the ignition pack at 4000 rpm you will see what I mean on the trace. When testing static or low rpm the transient reduces, its not possible to measure this signal properly with a voltmeter but if you have a scope, happy days

Oh cannot see the pics

From what I see on the scope, when reving the engine the height of the square waves stay constant but they get closer together the more you rev.

 

[uksparks]

A couple of cars I had years ago with analogue clocks, I took them apart before MOTs and woun the clock back a fair bit to take some mileage off, I think I took 20k off one. I bet they are long gone and scrapped now, they were old when I had them!

 

[uksparks]

Sorry, my point was you can't doc things like that with all this digital stuff!

 

[plugsandsparks]

Sorry, my point was you can't doc things like that with all this digital stuff!

Its easier nowadays, no need to take anything apart. lol

 

[plugsandsparks]

From what I see on the scope, when reving the engine the height of the square waves stay constant but they get closer together the more you rev.

You may need to set the scope so that you can see just one pulse and voltage so you get a full screen. The transient is very fast so may appear a faint trace.

 

[Lewis Cobley]

You may need to set the scope so that you can see just one pulse and voltage so you get a full screen. The transient is very fast so may appear a faint trace.

Thanks. I'll take a look at the scope settings.

 

[Lewis Cobley]

Sorry, my point was you can't doc things like that with all this digital stuff!

I'm not trying to clock anything. This is a brand new kit car so the digital dash starts from 0 miles anyway. My problem is the rev counter no longer works.

 

[plugsandsparks]

Like it. I too have a motorsport hobby. Built Megasquirt ECU on a 6 cyl BMW, one with wasted spark one using a single coil. I too had to derive a decent tacho feed for the existing BMW dash. As mentioned high level always refers to a signal off the ignition coils, whereas low level is a clear signal derived from the ECU using the crank sensor. Unless you use the correct one, you wont get a reliable signal. Looks like you are nearly there, cheers P&S

 

[Lewis Cobley]

Well I tapped the dash into a wire running from one of the coils to the ECU and it picks up the RPM fine now! On the scope it was reading around a 4v pulse so I connected this to the low RPM feed.

Like it. I too have a motorsport hobby. Built Megasquirt ECU on a 6 cyl BMW, one with wasted spark one using a single coil. I too had to derive a decent tacho feed for the existing BMW dash. As mentioned high level always refers to a signal off the ignition coils, whereas low level is a clear signal derived from the ECU using the crank sensor. Unless you use the correct one, you wont get a reliable signal. Looks like you are nearly there, cheers P&S

How do you get on with the MegaSquirt? I've seen Mazdas with them on but the Westfield factory recommend Omex instead.

Maybe I should add a MegaSquirt to my E46 330

 

[plugsandsparks]

It was over 5 years ago I built the MS2, I know they have brought more out and its getting cleverer and harder to build yourself. For a N/A simple car like the BMW E30 I have its more than adequate. My main reason for going down the MS2 route was cost. Its dirt cheap and a great way to get into ECU builds and mapping. The great thing about MS2 is there is an awful lot of it about its totally open source with a great backup. The realtime output screen of the engine parameters is really useful and well laid out.