308 Speedo Repair/Rebuild

Happy new year everyone! I thought i give you a feedback why the Type 2/3 replacement board still is not fit for DYI installation after all these years. The good news, the circuit board and software has been completed some time ago and works quite nicely. The bad news, i still haven't found a practical calibration method to turn this into a DIY installation.

Some background about this. The first 308 Speedo, "Type 1" uses a stepper motor for the ODO counter and an independent Speedo needle moving coil driver, similar to a Tacho. As the ODO motor in the Type 1 Speedo uses fixed gears based on the version, it does not need calibration, just the right amount of pulses coming in. The needle coil driver can be independently calibrated to show the correct speed. This type of Speedo has turned out to be very precise after being calibrated, both from the ODO as well as from the speedo display.

The successor "Type 2/3" uses a simpler mechanical design and a different electronics board. The redesign from Type 1 to Type 2/3 seemed to have been mostly driven by cost reduction as the Type 2/3 design removed the needle moving coil driver from the design, and uses just one motor driving both the ODO via gears, and the Speedo needle by magnetic coupling. The problem with this is that if you calibrate the needle you also change the ODO values. The unmodified Type 2/3 Speedos i tested were usually about 10-20% higher than the expected values, e.g. if your ODO shows 50.000 miles, your car actually only ran for 40.000-45.000 miles.

This makes a simple calibration quite problematic unfortunately. If you calibrate the needle/speed display to 140 kmh/90 miles (e.g. the 12'o clock position on the faceplate) and the mechanical calibration of the needle assembly is off (it was for all speedos i tested), then your ODO might have an error in display values up to 25-50%.
The correct way to do this calibration is to first calibrate the ODO for correct display, as example by sending 60 mph pulses for one minute which should result in exactly one mile on the ODO. And then to follow up by sending 90 mph pulses and mechanically calibrate the Speedo needle to the 90 mph position. With that approach, both the ODO and the Speedo are calibrated independently, ensuring precise display of speed and distance. But i don't think that this is a DYI approach where people not having a past as a watchmaker or model builder would have fun doing this.

So this is where i am right now, it has been a tad frustrating to try to design an easy DYI solution, as it seems there is no free cake with this one.


Just some cute development mules for entertainment...

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Jason,

the new circuit board already generates all the required calibration signals when you press the calibration button. Calibrating the ODO itself is also simple, although a bit tedious as each run requires you waiting for a minute. You need about 3-5 runs for a good calibration, maybe more if doing this for the first time.

I worry about the mechanical adjustment of the speedo needle as a DIY task. What i do is to have the board send the calibration signal for 140 kmh/90 mph to center the needle at 12'o clock. If the needle is at a different position (not calibrated), then i use a WIHA precision 2mm slotted screwdriver which i insert from the 11'o clock position into the magnetic coupling mechanism to fix the needle magnetic plate into position, then gently turn the needle to the correct position by turning it at the base. The trick is to not bent the axle by applying to much pressure with the screwdriver or break the plastic needle when correcting the position.

 

The calibration of the factory unmodified Type 2/3 Speedos i had access to was quite off, both in speed display as well as in alignment to each other (ODO vs Speed). When calibrating just the needle or the ODO i discovered that the Speed display is not aligned with the ODO distance. I think it is a design flaw in just using one motor for driving both a gear and a magnetic coupling. The Type 1 Speedo way more precise from my experience.

 

The good ones were 10% too high, e.g. 10% faster speed and 10% more ODO. (Keep in mind that all Ferrari Speedos are calibrated to show 5-10% higher speed, similar to modern Speedos who are set up to display ~5 kmh/miles above the actual speed). But this also means that the ODO reports 10% too high mileage.
With those instruments the speed display is less accurate than the ODO as the needle mechanism is not perfectly linear due to the needle weight and spring progression.

Anyone from Europe (to avoid customs hassle) with a broken Type2/3 Speedo who would be interested to get it fixed and calibrated for beta testing?

 

Thomas, i'm going to build a new batch of Type 1 boards the next weeks.

 

'80 US GTSI (Type 1) is available for DIY install, EU GTSI (Type 2) is available but not an easy DIY install.

Triangular side indicators or jumping ODO numbers indicate a Type 1 Speedo.
Round side indicators or flowing ODO numbers indicate a Type 2/3 Speedo.

 

Hi, just overloaded with work, sorry for not getting back. If its a technical question, please post them here so we can share with the forum, or please resend the email. Sorry for the inconvenience!

 

There are two knurled screws at the back of the Speedo, pushing an u-shaped bracket against the back of the dashboard panel. It is possible to remove those by reaching behind the quite busy dashboard wiring, but the most comfortable way is to remove the steering wheel, then the four knurled screws of the dashboard, gently pull it a bit it front and then reach behind the back to remove the two knurled screws holding the Speedo in place.

 

P.s. your board is a Type 0 board, a predecessor of the Type 1 board. The only difference for the installation is that this board has three (blue, orange, blue) instead of two wires coming from the ODO motor. The correct wiring is explained at the end of the installation sheet, "Wiring for very early Type 1 Speedos".

 

If you managed to calibrate it then the Speedo should be fully functional. Either you have no power going to the Speedo (the green wire going to the connector carries +12V) or the gearbox sender or wiring (the black/red wire going to the speedo connector) is broken.