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Electrical guides and mods

Roadster Speedo Dead

Modification Details

The speedo in the Roadster is beginning to suffer from a new common fault.

Much like the rest of the Roadster, the speedo electrics are a weak point for a few reasons.

Straight After A Battery Reconnect

The story usually starts the same way. After reconnecting the battery, the speedo no longer works properly.

Apart from a few warning lights coming up, nothing works. No speedo, no rev counter, no LCD, no backlights.
The biggest problem is usually the car won't start either.

Initially, this started happening when people inexplicably connected their battery the wrong way around.
1 of 2 things happen. Either the ECU gets fried or the speedo gets fried.

I Didn't Connect The Battery Incorrectly

I believe you. As time went on, the weakness of the Roadster's aging speedo increased to
the point where any voltage spike will damage something on the printed circuit board.

Why Does This Happen?

As with the other Roadster electrics, it's mainly a problem with water ingress.

Obviously it's unlikely that water drips directly into the speedo like it does with the SAM unit.
The position of the speedo doesn't help. Sitting right in the windscreen, the high and low
temperature fluctuations it encounters allows water vapour to condense on the PCB.

Condensation droplets are tiny so are help in place by surface tension.

As the speedo gets hot again, the water evaporates which leaves behind mineral deposits.
These can corrode the tracks but they also attract more water.

Eventually, you get corrosion on the board. Normally around the through holes of the circuit board as seen here.

The water also makes its way into the PCB material causing it to swell and delaminate the layers.
The through hole plating can't handle any movement so you get a break in the track.

Water can also collect under components, corrode connections, corrode pins and even get inside the chips.

Why Does It Suddenly Happen?

As tracks corrode, there is less and less metal conducting the electricity to the components.
The less metal there is, the higher the resistance it creates. A spike in current is
enough to create enough sudden heat to finally destroy what is left of the track.

It could also be that the condensation is the only thing left conducting the current between 2 points.
The current causes it to evaporate and your connection is gone.

Why Do Some Warning Lights Still Work?

Some of the warning lights have no real connection to the speedo, they are just a separate power in from one
of the other electrical systems and they are connected to a common earth in the speedo. The earth rarely fails.

Why Won't The Car Start?

The Speedo is an integral part of the CAN Bus. A network of data connections that allows all of the
main controllers to talk to each other. If any of the controllers can't be reached, the car won't start.

This is to save any possible damage that may occur by running a car without full electrical control.

How Can I Fix It?

Your best option is to get a 2nd hand speedo and clone your existing speedo to the replacement.

Alternatively, if you have time, effort and a multimeter, you could trace the power tracks across the
board and try to determine where the break is. If you can find the break, you can add a bridging
wire from the good side to the bad side to shunt power over the break, as Theo L has done here.

The Fix


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