How to fix the speedometer cable connector

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brinkie
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How to fix the speedometer cable connector

Post by brinkie » Thu Jan 25, 2018 10:02 am

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 :eek:

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:

Image

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.
Robert.

Present cars: 1994 Volvo 480 GT 2.0i Vase Green, 2010 Volvo V70 2.0F Momentum Mystic Silver
Past: 1981 Talbot-Matra Murena 2.2 Rouge Mephisto, 1995 Volvo 480 GT 2.0i Paris Blue

jopa7
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Re: How to fix the speedometer cable connector

Post by jopa7 » Mon Apr 01, 2019 10:03 pm

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

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MisterH
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Location: Stockbridge

Re: How to fix the speedometer cable connector

Post by MisterH » Tue Apr 02, 2019 7:25 am

Thank you very much! I am sure to try this soon!
Current Jobs to do (31/03/19):
Fix Speedometer
New Tires
Fix Radio/Cassette
Gearbox difficulties

jifflemon
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Re: How to fix the speedometer cable connector

Post by jifflemon » Tue Apr 02, 2019 7:51 am

jopa7 wrote:
Mon Apr 01, 2019 10:03 pm
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
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Re: How to fix the speedometer cable connector

Post by AleksanderHugo » Tue Apr 02, 2019 9:04 am

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

jopa7
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Re: How to fix the speedometer cable connector

Post by jopa7 » Tue Apr 02, 2019 9:42 am

Perfect! Will solder it directly and give you feedback :)!

Many thanks for your help

jifflemon
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Re: How to fix the speedometer cable connector

Post by jifflemon » Tue Apr 02, 2019 10:23 am

AleksanderHugo wrote:
Tue Apr 02, 2019 9:04 am
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.

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brinkie
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Re: How to fix the speedometer cable connector

Post by brinkie » Tue Apr 02, 2019 11:26 am

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.
Robert.

Present cars: 1994 Volvo 480 GT 2.0i Vase Green, 2010 Volvo V70 2.0F Momentum Mystic Silver
Past: 1981 Talbot-Matra Murena 2.2 Rouge Mephisto, 1995 Volvo 480 GT 2.0i Paris Blue

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