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Conversion of Early Speedo from Cable Drive to Electronic

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Old Dec 10th, 2020, 17:00   #1
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Default Conversion of Early Speedo from Cable Drive to Electronic

Hi All,

I recently bought a hybrid speedo kit from Peter to convert an early style cluster from a mechanical cable drive to electronic, which is achieved by integrating a later electronic speedo into it. I’ve written the following mini how-to guide, hopefully it’ll be of use to someone.

The reason to complete this conversion is to have a working speedo if you swap the M45 or M46 gearbox for something else that doesn’t have a mechanical speedo drive. For the car this is going into, the B21a and M45 have been swapped for a B230FK and an M90 and the rear axle will be swapped for a later one that has a speedo sensor in it.
First off, here’s what you get in the kit as well as what you’ll need

Kit contents: a milled aluminium backing plate that will hold the newer speedo in the correct location, a new faceplate for the newer speedo that looks like the old one and an electrical connector block which clips into the back of the aluminium backing plate with a corresponding connector that clips into that.

Also required: Old style instrument cluster and a newer electronic speedo

Not pictured – later axle OR sensor set up to read a custom tone ring on the axle / the propshaft bolts.

Later axle - The later axles have a sensor in the diff cover and a tone ring on the diff, the earlier axles don’t have these. Unfortunately you can’t just swap the diff cover or even the diff and cover, you’ll have to swap the whole axle. It’s not fitted as I write this, but for the car this is going into a complete later axle has been bought and will be swapped in.

Custom tone ring/sensor reading propshaft bolts – In this excellent how-to thread linked about how to use a variable resistor to calibrate the speedo, I asked if the calibration could stretch far enough to use a sensor pointing at the three bolts of the gearbox output flange or the 4 bolts of the diff input flange and the answer was that yes, the adjustment available would allow an accurate speedo needle. The odometer cannot be adjusted so would read fast or slow depending on whether the sensor was pointing at 3 or 4 bolts, but I expect for most people this won’t be a major concern. Taking this route avoids the need to replace the rear axle, but does require some minor fabrication. An example of a fabricated new tone ring with standard speed sensor reading it in the photo below.

First off, remove the speedo needle from the newer speedo by gently twisting it anti-clockwise past ‘0’ to break the glue and then continue twisting it whilst pulling gently. Then undo the two screws holding the faceplate on and remove.

The main frame of the speedo is clear plastic, with a pcb board screwed onto the side. The pcb board needs to be trimmed slightly, so unscrew the two screws that hold it and the motor on and then release the power wire pins for the motor using a safety pin to push back their locking tabs, making a note of which way round they go.

Note screws holding motor and PCB on are already removed

I took the opportunity to replace the cogs for the odometer whilst I had the speedo apart, although the original ones hadn’t broken (which was good, because it meant I didn’t have to find all the broken pieces)

Trimming the PCB
Whilst the speedo will mount inside the aluminium backing plate without trimming the pcb, the assembly won’t fit inside the instrument cluster case without trimming it.
Here’s the PCB, removed from the main speedo – it needs to be trimmed along the yellow line, keeping the solder connection points just to the right of the cut line intact.

Reattach the trimmed PCB to the clear plastic frame of the speedometer after reattaching the motor power wires. Clip the provided multi block connector into the aluminium backing plate, then cut the wires to length for soldering with the speedo unit in the correct place within the backing plate.

PCB Connections
There are 4 connections to make to the pcb, which are:
12v power
Sensor signal
Sensor/common ground
Speed signal for 2.4LH
The L-shaped connector in a later car provides three of these connections and clips onto the bottom of the PCB. The 2.4LH signal is routed out of one of the three pins in the top of the pcb and then through the main instrument cluster PCB.

I attached the wires in the positions highlighted below – ground (black highlight) has two wires soldered to it, one leading to the new multi block connector for the sensor ground and the other is routed through a hole I drilled in the side of the case and soldered to an instrument cluster ground point.
12v live (red highlight) I routed through the pcb and then soldered it to the socket for the connector pin that the pcb used to connect to the main board in the later speedometer cluster.
The speed signal for 2.4LH (green highlight) was pushed through the pcb and soldered on the track side – this part of the track on the pcb is quite weak, so be careful handling it. Blue is the speed signal sensor.

Here are the wires soldered in – green, red and black highlighting as above

I initially soldered the signal wire for the sensor on as shown below, but managed to pull the copper track off the circuit board just handing items, so I removed it from that position and soldered it onto the leg of the resistor on the other side of the pcb for more strength.

I also had to move this transistor to the other side of the board, so there was room within the aluminium case - you can see it in its new position on the bottom right of the photo above.

With the wires soldered on the speedo can be screwed into the aluminium case and that part of the conversion is complete.

Trimming Main Cluster Board
First disassemble the cluster, removing the old mechanical speedo and removing the main PCB completely.
The main cluster printed circuit board needs to be trimmed slightly to be able to fit the new speedo assembly. I cut it down using a small junior hacksaw with a new blade from this, to this.

As one of the tracks no longer has a complete circuit and the one next to it was now very thin, I soldered jumper wires across to complete the original circuits (the white wires).

Trip Reset Button
In the later speedo cluster the reset button is positioned lower down, meaning that the original speedo reset button assembly won’t fit through the square hole for it in the early cluster. This hole in the speedo frame is in the correct position to line up with the instrument cluster square hole, so I came up with an assembly to move the reset button.

First I tapped the front half of the speedo button to M4, then looked in my box of spares for RC planes and found this, it’s a bracket for a aileron torque rod I think – anyway, any similar sort of hook assembly will work. I tapped the mounting hole in the plastic bracket to M4, cut the other side off it and screwed it down to the bottom of an M4 screw. In position in the speedo with the front half of the reset button attached it comes through the hole in the speedo followed by the square hole in the early cluster and works perfectly.

Speedo face, Needle and Calibration
In the meantime I had been painting the speedo needle with white satin finish paint to match the other needles in the early cluster.
The new face screws on with the same screws the original later face was held on with. If you connect a 12v AC supply to the speed sensor connections and 12v DC to the pcb 12v connection and the same sensor ground that the AC supply is connected to, the speedo will read about 20MPH. Luckily enough I had a photo of another speedo hooked up to this exact power supply for testing and it was reading exactly 20MPH, so I calibrated this speedo to read the same.
Note – calibrate the speedo with it held vertically as it would be in the car. I calibrated it horizontally first and then when held vertically the weight of the needle meant it was reading a slightly lower speed.

I put a tiny dab of glue on the speedo needle to hold it in place before adjusting it to read 20MPH and then left it to dry overnight.
As the car in question doesn’t have standard wheels and tyres, what I should have done at this point is follow the excellent instructions in the link above about calibration and add a variable resistor to the pcb board to allow speedo calibration. I’ll do this before the cluster is installed.

And here it is, complete and ready to fit.

I hope that was useful.
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Old Dec 10th, 2020, 22:09   #2
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Super useful, thanks for that
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Old Oct 17th, 2022, 13:49   #3
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where can I find this peter ie the supplier of above stuff?
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Old Oct 17th, 2022, 21:30   #4
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The words ‘hybrid speedo kit’ in the first sentence of my article above are a link to Peters page.

I think the affordability of gps units that turn a motor a certain speed to drive a speedo cable speedometer have come down in price since I wrote the article, so I’d investigate that option first.

Things like this

What kind of gearbox are you thinking of using?

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Old Oct 18th, 2022, 09:56   #5
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The gearbox will be a M90 in this case.

A volvo friend sugested using the below ALi gps unit together with matching face for the old instrument from Dave Barton.

On Ali X "Speedometer Odometer 85mm Speed Gauge 120km/h 200km/h With GPS Antenna For Marine Boat Car ATV Truck Red Backlight" for 38 GBP.
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Old Oct 18th, 2022, 13:40   #6
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I’d definitely give something like that a go first- although the one you suggest would still require modifying the speedo cluster.

The gps units that drive a motor just plug into the back of the original speedo, meaning they’re much easier to fit.

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