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Porsche 935K3 - Full MoTeC - retain original gauges at dashboard

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G'day HP Members,

Design challenge for Forum readers!

In the advantaged stages of building a Porsche 935K3 running on "Full" - MoTeC; M190GPR, C187, PDM32 & PDM16 as the main electrical devices.

Now exploring the options to "drive" the original "Porsche" gauges out of the outputs of the M190, C187 and / or PDM's. I like to retain the "OLD SKOOL" look of the dash and avoid a double set of sensors to input MoTeC and Porsche / Stack gauges separately and having a Christmas Tree of sensors on the engine and gearbox. The sensors on the engine and gearbox / transmission will be of "Motorsport" grade while retaining the original Porsche fuel level sender and oil level sender. In addition, a set of IDIOT Lights on the Dash needs to be driven out of MoTeC as well, 5 units; Low Fuel, Alternator, Oil Pressure, Fan Belt & Launch Control "ACTIVE".


Gauge 1, Fuel Level Tank & Oil Level Tank

Gauge 2, Engine Oil Pressure & Engine Oil Temperature

Gauge 3, Tachometer

Gauge 4, Speedometer

Gauge 5, Boost (Mechanical - pressure line from manifold)

In addition the centre console will hold additional gauges (refer to attachment IMG_7020) for mock-up. The brand of gauges is "Stack" and will backdate the dial-face and hands to VDO period correct appearance at a later stage when all is working as it should. Same here, like to drive them out of MoTeC and "Motorsport" grade sensors and eliminate the original Stack sensors.

Gauge 6, Fuel Pressure

Gauge 7, Transmission Oil Pressure

Gauge 8, Transmission Oil Temperature

In case technical info is required on the Porsche Gauges and Stack Gauges, please let me know and will add the data to this post.


Attached Files

Oooooooof, that's just plain awesome. Completely agree with keeping the aesthetic look of the original gauges, however it is going to be difficult to get them all working. Are there any Factory Service Manual diagrams for the vehicle (or atleast something similar) we could have a look at?

Assuming the pressure gauges are electrical, I'd experiment with driving them with a PWM output. With their age, I think they will be a bi-metallic spring type, that deflects the needle in relation to the amount of power they dissipate.

I remember looking at porsche tachos ages ago when putting one into a mini. There were two common types from memory, those that pass the power to the original distributor through them, and pick up the inductive spike from that line, or those that tap onto the coil negative post, and pick up the inductive spike that way. If its the former type, I think you'll be stuck. If its the latter, there are a couple of options:

Often, inductively triggered tachos will have a resistor as the first element on the circuit board, which reduced the energy of the inductive spike, avoiding it damaging anything downstream. By-passing this resistor often allows the tacho to then be triggered from a low level signal (like a digital output) from an ECU. I've done this to a few tachos from 80's japanese cars, particularly Starions, and it works great. No personal experience with the porsche gauges though.

Another option is to bust open a standard automotive relay and take out the switching coil. Wire the switching coil of the relay to your ECU, getting it to fire it once for each ignition event. This will generate inductive spikes, which you can tap into to trigger the tachometer. Have to be careful that the ECU output channel is okay with this though, and has protection in place against the spikes!

Personally, what I'd do is rip all the gauges apart and replace the internals with stepper motors used for gauge applications (as used in any modern car that has an analogue gauge). If you are, or you know anyone that has an electronics and programming experience, its not overly complicated to whip up a board that could read all the data you need from CAN, and drive the gauges to where they need to be. I've done this with a mini tachometer before, and it worked great. I've been prototyping a newer design using a stepper motor driver IC which should be able to drive the pointers in a very smooth fashion. Just got back onto it in the last couple of days, its been sitting in a drawer for a couple of years!

Attached Files

Wow, quit taken by the idea to drive the whole Dash via CAN and re-build old gauges with stepper motors. New challenge, new task.

Attached "Wire Diagram....." might help.

Have also the notes in regards of the Bosch 8pin CDI boxes on the signals "Bosch 8 pin Note.....", vintage looking with modern electronics inside!

1: Coil A

2: Coil 1 (GND)

3: GND

4: Tacho Drive

5: Shift Light (Active Low)

6: Distributor Signal -

7: Distributor Signal +

8: Power 12V

For tacho have an alternative; "Stack" stepper motor Tacho.

Having two (2) engines to trow in some more juice!

Engine No.1 as per photos posted earlier - ignition coil on plug (twin plug) Engine build with std. of the shelf Porsche Parts nothing to be trilled about, estimated 600 BHP.

Engine No.2 will be more close to the 935's, 935 heads and goodies, having the Bosch Boxes and Distributor to maintain vintage look, massive KKK K27 turbos modified with ball bearings, target 800 BHP and possibly running on E85 - at least whole system is prepared for it.

Attached Files