The joys of finding a single wire…

Posted on March 28th, 2004 in Opinion by Julian Edgar

Today I spent nearly six hours trying to find a wire. Odd, you say? Wasn’t there plenty of wire available at the local electronics store…cable, even? Well there was, but this particular wire was contained in the loom of a car. And the wire that I was looking for had a very specific signal on it. It was the speedo input on a mid-Eighties BMW735i.

It had all seemed easy enough…

Part of the very exciting range of DIY automotive electronics kits I am developing with magazine is a speedo interceptor. It’s a small box of tricks that will let you adjust your speedo reading up or down, giving you the facility to not only correct errors but also make appropriate changes when the gearing of the car has been altered. The concept is easy – it’s just a frequency-altering device – but as always, actually developing the kit has been another ballgame altogether.

So the module that today I was trying to fit is actually the second prototype. The difficulties with the first design came about when it was realised that there’s a whole host of different design speed sensors, and if the kit was to have any pretence of being universal in application, it was going to need to be able to be configured to work with all those sensors. Not to mention, be happy working into the speedo or the ECU.

The first design came and went, then electronics designer John Clarke completed the second iteration. And it looks a beauty – you can digitally increase or reduce the speedo reading in 1 per cent increments, simply by turning two multi-position switches. So if you alter the diff ratio so it’s 10 per cent taller, you simply alter the speedo correction by the same 10 per cent.

And not surprisingly, John was eager to see how it worked on a car. And so was Leo Simpson, the publisher of Silicon Chip. Me? Well I’d have loved to see how well it worked… especially if someone else had been around to fit it. My reluctance wasn’t based on the difficulty of the wiring – after all, there’re only four connections: power, earth, signal in and signal out. Nothing very hard about that, is there?

But what I knew would be difficult was finding the right wires. You see, I don’t have a workshop manual for the 1985 BMW 735i, one of the cars that lives here. (And, AFAIK, no such manual exists in other than factory form.)

So why not tackle the other two cars at this address? Well, the 1988 Nissan Maxima V6 Turbo uses a mechanical speedo. Yes, like many Nissans of that period, the speedo is driven by a cable and a pick-up inside the speedo itself passes road speed information onto the ECU. So while I could alter the speed reading that the ECU was seeing, I wouldn’t be able to correct the speedo reading itself. Then there’s my 1998 Lexus LS400, but even one attempt at intercepting the speedo signal had proved that, er, at least multiple adjuster units would be needed. In the Lexus the speed is sensed both on the input and output shafts of the auto trans, and if you alter one of these speeds the car actively changes its auto trans strategy, thinking that there’s excessive slip (or not enough slip) occurring within the transmission. So, no, the Lexus isn’t really a good car to trial a new design speedo interceptor on…

Which took me back to the BMW.

I’d already had the dash apart while doing previous work on the car, so my first step was to unplug the instrument panel – that left all the wires accessible for electrical probing. I then jacked-up the car, placing it on axle stands so that the rear wheels were free to turn. Using my Fluke 123 Scopemeter (and also a more humble general purpose multimeter than can measure frequency) I laboriously probed every pin on each of the plugs, all with the car in gear and the rear wheels spinning. I worked my way through perhaps 75 pins to find – nothing. Sure I had found some frequency signals – clearly visible on the scope were injector pulses (probably being communicated through to the trip computer for fuel consumption information), and the ignition or crank trigger pulses on their way to the tacho could also be seen.

But no speed signal could be found.

I thought about this for a while then realised that it was quite likely that the speed sensor wouldn’t output a signal unless it was powered-up. Which meant plugging the instrument panel back in and then back-probing the plugs so I could access the signals with everything working. ‘Cept that was basically an impossibility unless I shrunk myself to the size of a mouse, had an equally small scope and insinuated myself into the tiny space behind the working instrument panel. In short, access to the wiring was simply impossible with the panel plugged in.

By now three hours had gone by – the jacking up of the car and the placement of axle stands, the removal of the steering wheel to give better access to the instrument panel, the removal of the panel to give better access to the wiring loom, the probing of each conductor in each plug…

However, that couldn’t be helped so I resolved to tackle the problem in another way.

I took the instrument panel inside and disassembled it. With the speedo assembly able to be isolated from the rest of the electronics, it could be seen that there were only four connections to the speedo. Whew! After fifty or seventy – or whatever it was – pins that I was used to looking at, four was a mere bagatelle. So, how to find the functions of these four connections? I carefully soldered the end of four ribbon cable wires to each of the PCB connections and then reassembled the dashboard. With the speedo back in place and plugged in, I had four thin wires coming from the assembly that I could probe with the meters. While the car was running and the instrument panel was all plugged in.

And, finally, there it was!  A nice square wave that varied in frequency with speed – I’d found the speedo input. It had taken me over five hours of hard work…

Rather than try to find my way back from the signal connection to the full wiring loom, I elected to cut that particular PCB track and solder a wire to each side – that way, I had accessible both the signal coming from the sensor and the signal connection to the speedo. Of course, join the two wires together and the speedo will work as it did before; separate them and the speedo will stop.

Except, with the wires soldered into place, the speedo reinstalled in the cluster, and the cluster reinstalled in the dash, the speedo keep working whether I open-circuited my extra wires or not! 


I had noticed another square wave frequency on another pin, so this time I repeated the whole process on that other track. (And the first track? I now think it is a speed output… maybe to the trip computer….)

And this time it all worked as it should. I could measure the square wave signal from the speed sensor on my wires and when I open-circuited them, the speedo stopped working. Finally, after more than five hours – in fact closer to six, when I think about it – it was time to connect the speedo corrector. Finding 12V and ground connections inside the instrument space took a little while, but then I could start the car (still free-wheeling in space) and see how well the speedo corrector worked. But it didn’t. Or did. Or didn’t. What was happening was that the speedo was behaving all the world like it had an intermittent connection. Sometimes the needle would shoot up, sometimes it would fall back.

I pressed on the faceplate of the instrument panel and found I could alter the speedo’s behaviour with thumb pressure.


I again pulled off the steering wheel (silly boy: I’d replaced it before making sure everything worked), undid the screws that held in the panel, and then pulled the plugs one after another. I took the instrument panel inside and (again!) disassembled it. Really close inspection showed that there were some dry solder joints around where a plug joined the PCB so I resoldered these joints, took it back down to the car, connected the plugs, reassembled… but I am sure you don’t want to hear all that again.

This time flexing the front of the panel did nothing to change the speedo reading – but the jumps in the needle movement were still there. I then bypassed the speedo corrector and they went away. Aaaa-ha! This prototype still has a few problems…

The speedo corrector problem is no big deal – that’s why we’re doing so much testing on real cars, to find just these sorts of issues. But I can’t help feeling swindled after spending all day fitting this bloody thing to a car – only to find it isn’t working as it should!

John, I’ll be on the phone tomorrow – bright and early…


We’ll be recommending that the kit – in final published form – is only fitted where the user has access to a workshop manual clearly showing the speed sensor wire. Hell, then it’ll be dead-easy!

One Response to 'The joys of finding a single wire…'

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  1. PCB said,

    on August 8th, 2010 at 6:58 pm

    I read something a while back about speed limiter chips getting installed in cars and left out on police models. Something about legal requirements to prevent people from racing? I’m not sure if its true, but it seemed a valid possibility when I read it. (I’m not much of a car guy…) Anyhow I just thought I’d mention that the mod you’re designing could potentially override a speed limiter chip by falsifying the speed read from the sensors. (of course that might be a positive effect…) It just might get you into a sticky “legal” situation though.