Electronics > Projects, Designs, and Technical Stuff
Modifying mechanical speedometer to electronical.
Refrigerator:
I'm restoring restomodding a 1972 Volga 24 currently and i need to modify the original mechanical speedo to be controlled electronically.
The original engine and gearbox are shagged so i'm swapping a SAAB B204 with a different gearbox, which is why the speedo needs to be modified.
Also i want to preserve the original look, which is why i'm not going to put a screen in place of the speedo.
It's a strip speedometer, which presents it's own challenges.
I'm thinking there's two options right now.
First one is an external speed controlled motor, which would spin the cable input in the speedometer. I think this would be the least intrusive method but it might be a bit noisy depending on how it's done. Also i would be relying on the original mechanism of the speedometer to work and be accurate.
Second would be to tear the speedo apart and connect a stepper motor directly to the mechanism that moves the needle but this would be most intrusive. Of course this way the speedometer needle would be directly coupled to the stepper so i think reliability and accuracy would be better.
Also there are other gauges on the speedometer cluster, such as oil pressure, battery charge/discharge and fuel level, which complicates things, because none of the original electrical system will be left in the car and it would be nice to have these gauges work.
All of these are regular gauges and can be driven directly with PWM from a microcontroller but then i would need to interface remote sensors and i start diving down a bit of a rabbit hole full of CAN and diff pairs.
Would be nice to have some input before i blow this project too far out of proportion with feature creep. ^-^
pardo-bsso:
That's a nice project.
Do you know how different are the ratios of the differential and the new gearbox to the speedometer input? If they are close enough you might be able to use it as it is and it will still be within the expected tolerance/error of the meter and law regulations.
As for the rest of the oil and water temp gauges, can you use the old sensors (or new but compatible) in the new engine and wire them directly to the cluster? If the threads are different it's a short job with plumbing parts and perhaps a bit with a lathe. Same with fuel level.
Refrigerator:
--- Quote from: pardo-bsso on December 28, 2020, 06:27:17 pm ---That's a nice project.
Do you know how different are the ratios of the differential and the new gearbox to the speedometer input? If they are close enough you might be able to use it as it is and it will still be within the expected tolerance/error of the meter and law regulations.
As for the rest of the oil and water temp gauges, can you use the old sensors (or new but compatible) in the new engine and wire them directly to the cluster? If the threads are different it's a short job with plumbing parts and perhaps a bit with a lathe. Same with fuel level.
--- End quote ---
The new gearbox does not have a cable output at all actually.
I don't think the new oil pressure sensor will be compatible with the old gauges. This is a bit of a tricky situation because even if the gauge works with some new sensor there's no guarantee that it will be accurate.
I will have an OBDII port hidden to interface to the ECU so maybe i could add some CAN interface to the gauge cluster to read oil pressure etc.
But then again i might go for a standalone ECU because the original Trionic5 might throw a fit when i try to run it standalone. Anyways that's beyond the scope of this project.
Zero999:
The external motor spinning the cable is probably the better solution. Use a DC motor, with the appropriate gearing, if necessary. Assuming the load doesn't vary much, the speed is roughly proportional to the voltage. You could power the motor off a regulated power supply and vary the speed with PWM.
Refrigerator:
--- Quote from: Zero999 on December 28, 2020, 07:19:04 pm ---The external motor spinning the cable is probably the better solution. Use a DC motor, with the appropriate gearing, if necessary. Assuming the load doesn't vary much, the speed is roughly proportional to the voltage. You could power the motor off a regulated power supply and vary the speed with PWM.
--- End quote ---
If i did that i would most definitely add an optical speed sensor for closed loop control because i don't trust the DC motor speed to be stable especially in an automotive environment (temperature, vibration etc.). Also i think the cable rotates at a pretty low RPM so if i needed a gearbox for the motor that would also add more noise.
Cars are inherently very noisy but i find noise made by DC motors inside a car to be very noticeable and very annoying. Not to mention the Volga 24 is a massive rattle can, so if any vibration from the drive motor got coupled to the stamped steel panels it would amplify.
Steppers sound pretty cool when they make "cnc" noises so i kinda lean the more intrusive way of modifying the speedo. Also i do have a reasonably small 1.8 deg/step stepper, which is pretty much silent with micro stepping.
And let's not forget that this speedo was made in 1972 and it probably needs a good servicing by now, not to mention the bug that found it's way inside and died on the front glass >:(
I haven't yet looked inside the speedometer mechanism so i can't say whether it will be easy or hard to modify it but i'll look inside, because right now it seems like the biggest factor is my preference, rather than the feasibility of such mod (second option).
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