How to fix the speedometer cable connector

[brinkie]

Though a Volvo 480 instrument cluster definitely would benefit from "a holiday abroad", some that have been rebuilt by me, still don't show speed. Sometimes they do, and then you switch gears or hit a speed bump and the needle drops. Or the displayed speed is proportional to the engine revving.

What happens is that the connector which connects the speedometer sensor to the main wiring loom has too much resistance. The speedometer signal is very, very sensitive because the wiring is actually part of the sensor circuit; if the resistance is too high, the oscillator in the speedometer ceases to function. Also, interference from the engine's ignition will be picked up, which is what has been observed by many, including me

The repair is quite straightforward and requires a soldering iron, a small cable cutter and some insulating tape or heat shrink tubing.
It's an easier job to do if you remove the air filter box, but if your car doesn't have ABS you could reach the connector with the air box in place (otherwise the ABS unit is blocking your view).

Underneath the air filter box there is a connector from the main cable loom to the wire that leads to the speedometer sensor in the gearbox. The speedometer sensor cable can be identified with a brown and yellow wire leaving the connector before they are covered in insulation.

Take off the connector and measure with an ohmmeter if there is little resistance in the sensor, it should be no more than a couple of ohms. If you measure high reistance or open circuit, the sensor is broken (very, very rare!)

I don't have a good picture of the connector "in situ" but you should be looking for this one:



Cut off the connector from both the wire loom and the main loom. Strip a bit off the insulation. Clean the copper by stripping/scratching the oxydation off it (otherwise the solder won't hold). Intertwine the copper of the ends, and solder the yellow to the yellow and the brown to the brown cable. Insulate the exposed wiring properly and put the air filter back (if it was removed in the first place).

Make a test drive. You should have a proper speedometer reading now.

[jopa7]

Dear Brinkie, first off all thank you for the excellent tips based on your experience. Indeed it happens like you say. Sometimes speedometer stops working when passing over speed bumps and restart to work the same way. Already located my connector. Need your help to confirm if I am understanding it right. Basically you are telling to take out both parts of the connector and solder wires directly? Kindly confirm.

Many thanks and kind regards

Joao

[MisterH]

Thank you very much! I am sure to try this soon!

[jifflemon]

Dear Brinkie, first off all thank you for the excellent tips based on your experience. Indeed it happens like you say. Sometimes speedometer stops working when passing over speed bumps and restart to work the same way. Already located my connector. Need your help to confirm if I am understanding it right. Basically you are telling to take out both parts of the connector and solder wires directly? Kindly confirm.

Many thanks and kind regards

Joao

Exactly that - cut out the connector and re-solder wires.

[AleksanderHugo]

Wouldn’t it be better to install some better connector instead? Removing it completely seems like a no-return option

[jopa7]

Perfect! Will solder it directly and give you feedback !

Many thanks for your help

[jifflemon]

Wouldn’t it be better to install some better connector instead? Removing it completely seems like a no-return option

Depends how you look at it.... How often do you remove your speedo sensor? Clutch change? Ok, unbolt speedo sensor, remove gearbox.

[brinkie]

Connectors are for swift installation of pre-made wiring looms in the factory. The speedometer sensor is passive, there are no parts in it that wear. Only the wiring itself can break.

The speedometer circuit is made out of an oscillator (approx. 100 kHz), the oscillator itself is located on a small circuit board which sits next to the speedometer. For an oscillator to function, you basically need a capacitor and a coil in parallel. The capacitor is located on the circuit board, the coil is the speedometer sensor. When a tooth of a gear inside the gearbox passes the sensor, it will change the self-inductance of the coil and subsequently will cause the oscillator to stop for a moment. After some filtering, this is forming the speedometer signal: a pulse train representing the speed.

I will not go deep into electronics theory, it is sufficient to say that too much resistance (10 ohm is enough) in series with the sensor coil will cause the oscillator to stop and subsquently the circuit to fail. From the speedometer, the signal goes through a connector to the circuit board, through a connector to the wiring loom, through a connector to the sensor cable. The latter is located inside the engine bay, where the conditions are pretty harsh, causing corrosion on the contacts.

The reason why replacing the speedometer sensor worked in so many cases, there was nothing wrong with the sensor, but pulling the connector and pushing a new cable back in removed the corrosion, making the circuit work again.

Modern speedometer circuits involve active sensors (i.e. the speedometer signal is generated in the sensor) and don't have this problem.

[Anadinolin]

has anyone got an up to date picture of the sensor? i might need to replace mine, but not 100% sure on the auto box location on a turbo.

[bunjiweb]

I would also appreciate a working photo of the correct connector I'm looking for on this. I just bought a Volvo 480 and the speedo isn't working. The info centre also seems not to be working correctly but that's for another thread...

Ben

[jifflemon]

It'll be the speedo cog thats gone, drop me a PM and I can sort you one out.

[bunjiweb]

It'll be the speedo cog thats gone, drop me a PM and I can sort you one out.

Thanks, will do! I only suspected the sensor as the previous owner said it worked intermittently for a while before breaking.

Ben

[bunjiweb]

So I went to investigate the connector in question. Removed the air box and the battery tray.

I may be incorrect but I'm assuming this is the Speedo sensor:



The cable from this runs along under the airbox and terminates in this connector:



Doesn't appear to match the description on this thread, and the connectors appear okay, though I haven't yet tested for resistance etc.

Thoughts?

[arthuy]

These posts might help you.

The green plug is just the reverse light switch.

Panic over it is wiring which is faulty.

I followed my own old advice, 'go to bed and look at it in the morning'.

Armed with the diagram I found the 2 wires, they are quite obvious in the loom being green and black in a separate sleeve from the other wires going to the GRAY plug. PIN 6 and 12 are next to each other.

I first bridged the plug in the engine bay and probed at the plug but no continuity. Next up I wanted a length of wire to attach at the plug and test the wires individually.



I am at times amazed with the parts I pick up, I put my hands on this twin cable I bought for wiring in a highlevel brake light a year or so back which just so happened to have the same connector as the VSS so plugged straight on to the loom and had more than enough cable to reach the dash board.

A very quick probe with my meter proved that the Black wire was broken. I double checked by bridging the grey plug pins and test at the engine bay. Same results but I looked closer at the cable and the black did look some what flat or stretched. Stripping off the plastic it was pretty obvious.



Just to be 100% sure I strapped the broken wire up and hey presto I have a speedo again.



Looking at how the wire has failed explained the symptoms from the previous owner, it was working intermittently, probably a few strands kept the signal going until it finally gave up.

Seeing as I have a suitable piece of wire I will run fit that and eliminate the original. It is tempting to cut it back and hook it up but that would just leave an unknown.

Unfortunately I dont have a spare plug for it though I am sure I have a packet hiding in the garage somewhere, but they are easy to obtain and hopefully the ones I have ordered will turn up later in the week.

Moral of the story is check the basics properly.

Need to crack on with getting the brakes back on although I am still waiting for the brake tubing to turn up.

[bunjiweb]

Managed to fix mine, it was a very degraded circuit board. The car came with a spare cluster that I believe to be the original.

After taking both apart I found a broken odometer cog in one and a dry circuit board in the other, so was able to make one good one from the two bad. Now all the info centre and speedo functions work perfectly.

Ben

[brinkie]

Even a sorry looking board may work fine and a board that looks like it's been produced yesterday may be faulty. You should check and re-solder the joints of the connectors (blue/yellow, yellow/red or green/grey depending on model year), because that is the #1 cause of failing speedometers.
A "second hand" cog may fail the next day, they break their teeth because the plastic degrades with time. There are 3D-printed cog wheels available from Jeff (or buy them at the source in Hungary through eBay) which will outlast the 480.

[Macaroon]

Great tip. Thank you!

Are we talking about resistance to earth (i.e. between the connector and battery negative)?

If so, mine is ~430k Strangely, the speedo was more or less behaving itself today