Board logo

Possibly (Probably) Silly query about using donor instruments.
John P - 20/2/14 at 08:52 AM

This is probably a nonsense but I’m still trying to find a genuinely LoCost solution to getting some good looking analogue instruments and could do with understanding how modern donor instruments work.

The general consensus seems to be that you can’t use donor instruments without the associated OEM engine management but presumably inside the speedo or tacho housing is actually a stepper motor or, on older cars, a moving coil meter. Although I’m no expert on either I assume the reason newer gauges have to be used with the original ECU is that the stepper motor drive is actually in the ECU.

If this is the case then couldn’t this function be done by a separate stepper motor drive getting a pulsed input from either the ignition system (in the case of the tacho) or from a suitable sensor in the case of a speedo?

I suspect my lack of understanding means it seems simpler than it actually is but there do seem to be motor driver IC’s available reasonably so would it be possible to use these to create an interface between the ignition system / speedo sensor and the donor instruments?

I know I’d need someone to actually design the circuit but if it were possible it would potentially open up a supply of almost new instruments at low cost.

John.

MikeRJ - 20/2/14 at 10:48 AM

quote:
Originally posted by John P
The general consensus seems to be that you can’t use donor instruments without the associated OEM engine management but presumably inside the speedo or tacho housing is actually a stepper motor or, on older cars, a moving coil meter. Although I’m no expert on either I assume the reason newer gauges have to be used with the original ECU is that the stepper motor drive is actually in the ECU.

If this is the case then couldn’t this function be done by a separate stepper motor drive getting a pulsed input from either the ignition system (in the case of the tacho) or from a suitable sensor in the case of a speedo?

I suspect my lack of understanding means it seems simpler than it actually is but there do seem to be motor driver IC’s available reasonably so would it be possible to use these to create an interface between the ignition system / speedo sensor and the donor instruments?

I know I’d need someone to actually design the circuit but if it were possible it would potentially open up a supply of almost new instruments at low cost.

John.


On modern instruments, all the drive electronics for the gauges remain within the instrument cluster. However, rather than the instruments being connected to the various sensors, the sensors are connected to the ECU and the ECU sends the measured values to the instruments over a network system called CAN bus.

You could certainly strip the gauges down and drive them from a small microcontroller, though stepper motor movements are surprisingly complex to drive properly. You can get IC's that take much of the hard work away e.g. Freescale MC33977 (one of which may already be present in the gauge electronics), though you'd still need a microcontroller with an A/D converter to translate the sensor voltage/resistance etc. to a stepper motor movement.

bi22le - 20/2/14 at 11:40 AM

In my eyes its not the drive that is the issue its the calibration.

The older the dials the more chance you have of solving it.

spegru - 25/8/14 at 08:54 AM

I have two suggestions for this.

I have managed to splice together some Peugeot/Citroen clocks to a Ford engine and factory ECU. Somewhat of a surprise that it worked - esp the electronic speedo. Obviously the key is serious detective work on the wiring.
The benefit of this is that modern production car clocks are very cheap on eBay and that the latest ones are also very compact.
I can provide more info if people are interested.

The other suggestion is the use of a Bluetooth adaptor to the diagnostic can bus socket, if your ECU provides that, and use a smart mobile phone diagnostic app. I use one called Torque. It works extremely well providing calibrated measurements of engine temperature for example. Combined with a high quality mounting bracket it can provide a real alternative to physical instruments for very low cost. I suspect it ma not be fully legal as there can be no proper odometer, however I guess you cold attach a hidden one in the engine bay or something for that.
Perhaps the ultimate version of this would be to use a suitably sized cheapo android tablet built into the dash specifically for the purpose (although note that some of the really cheap ones don't have Bluetooth or GPS, which are both needed)