Custom tachometer and speedometer in an old Porsche


I´m an mechanical engineer, with some basic knowledge in programming, who has found the fun world of arduino circuits. I´m currently working on a Porsche 944 from 88 and in my project of turning it into a light weight race car I´m developing my own digital gauges.

I´ve read quite many tachometer/rpm-threads on the forum now but I´m still not sure how to approach the signal pickup so I though I might ask the more experienced right away. I´ll make sure to explain as detailed as possible to help other people on the same level as me and with similar questions :slight_smile:

1) RPM-signal
"Clarks-garage" (a quite handy page for a 944-owner) explains the PRM sensor as follows:
"The 944 Engine Managment system uses two crankshaft sensors. (...) The second sensor (Speed Senors) counts the teeth on the flywheel and provides this signal in the form of pulses to the Engine Management Computer (DME) for engine speed in RPM (Revolutions Per Minute). Using the combination of the two signals, the DME can determine where the engine position is at any point in its cycle to apply the correct fuel injector cycling and correct ignition firing."

Further more:
"To check the speed sensor output, connect an oscilloscope from terminals 8 - 27 on the DME plug.
Crank the vehicle and check for a saw tooth wave output with a peak-to-peak voltage of > 2.5V."

Full text: Speed and Reference Sensors - Checking, Replacement, and Adjustment

And some more information:

I found a guy who logged the RPM signal and he plotted this:

I found a video of a guy measuring the AC-voltage:

Between pin 8 and 27, where he gets just above 1 volt and according to the crest factor of a triangle wave suggest we have a amplitude of about 2 volts?
In the schematics it looks like:

I´m concerned that me tapping into a sensor cable would cause signal degradation to the signal and affect the performance of the car. Why another aproach seems to be to connect to pin 1:
Connecting it to the ignition signal wire going from the DME to the ignition coil. It's Pin #1 on the DME wiring connector. Pin one goes to the ignition coil:

2) Speed-signal
According to the wiring schematics it is a hall sensor:

But could I possibly add noise to the signal --> alter performance? :confused:

Full wiring schematics of the car:
Full wiring schematics

What would you guys suggest in terms of approach?

Am I totally lost if or would such module be useful as a simple signal pickup for the rpm and possibly speed signals?

I will try to borrow a oscilloscope from work, but so far are there any suggestions of the simplest approach?

Link to your wiring diagram requires signing up to some account or other.

Link to your wiring diagram requires signing up to some account or other.

just dismiss that. and it'll show!

Perhaps this one is better? :wink:
Link to wiring

You will probably have noticed how this, as with many automotive wiring diagrams, have wiring disappearing inside of a shaded area box.

As a result there is no way of knowing just what goes on inside and how any modification to the existing setup may or may not affect standard operation.

One thing is well known, automotive wiring contains a lot of electrical noise which has to be addressed so good to keep that in mind.

As for your added gauges, I'd tend to keep them a seperate item.

Many circuits out there to show how to do it without getting bogged down in an unexpected side effect of a modification to original circuits.

If your vehicle is to be used on a public road, it may be a good idea to check whether you will need a modification certificate to legally operate with your mods.