[Davide D.] I have written this guide based on the repair of a speedometer from my 1991 940se Wagon. It will apply to many other models equipped with the Japanese YAZAKI speedo. The dash board disassembly is the same also for the VDO model.

A. How it works.

• The Sender Unit: is located on the rear differential housing. It is connected with a connector and two wires. Often the connection is oxidized. On a few units there might even be a metal wire seal (required in Sweden for taxi use).
• The Speedometer: in the dashboard instrument cluster is an electronic unit that receives the pulses from the rear sender and converts them into indicate speed. This also provides a signal to the odometer.

B. Which Speedometer: YAZAKI or VDO?

C. Fault location.

REAR SENDER TEST:

Faults are usually either in the gage itself or the rear sender.

A very effective and easy way to diagnose the rear sender is to disconnect the rear connector on the differential case and connect two long pieces of wires to the plug coming from the vehicle wiring, bring the “extension” wires inside the car, start the car, and start shorting the wires at a pace of 4-5 shorts per second. If the gage is OK you should start to read a speed over 15MPH. The faster you short, the higher the speed.

If you have a reading, then STOP HERE, and instead of tearing your dash apart, remove the rear sender unit, clean it using DeOxIt electronics cleaner (available at Radio Shack and good electronics stores) or eventually replace it (and while you are at it change your rear differential oil!).

ERRATIC READING:

If your speedometer has a reading when you start driving and then suddenly goes dead, or if it will indicate constant speeds, then it is probably the gage and this guide will help you.

On rare occasions the responsible for erratic readings has been found in the grounding bar located in the driver foot compartment (worth to check before tearing the dash apart).

D. Dash disassembly and Removing the Cluster.

Depending on your car model, see the FAQ file for instructions on how to remove the trim and cluster.

OPENING THE INSTRUMENT CLUSTER:

Remove the screws and then separate the two halves.

Once separated DO NOT LAY THE INSTRUMENTS FACING DOWN, or you risk of scratching their delicate surface.

REMOVING THE SPEEDOMETER UNIT:

Remove the screws. Pull the unit out.

E. Speedometer Surgery

SEPARATING THE IC BOARD:

Make some notes as to wiring locations and colors so you can reassemble correctly:

- Desolder the flexible contact of the odometer from the board.

- Desolder the two wires of the service reset switch.

- Desolder the 4 pins of the speed indicator gage.

- Remove the screws of the indicator gage.

At this point, if the 4 pins have been correctly desolded you should be able to push the gage with your thumb and to remove the IC board. If the thumb pressure system does not work, keep heating up the pins with the soldering gun, one at a time, until you slide out the gage.

ANALYZING THE IC BOARD:

Now you should be able to see four capacitors, the source of your speedometer fault:

C7 – C12 – C6 – C16 and their good cousin C15.

The first four electrolytic capacitors have leaked their acid content on the board causing corrosion and shorting nearby components. The C15 is their good cousin since it usually doesn't leak.

FIXING THE IC BOARD:

Sourcing New Capacitors. [Davide/Michael Melkonian] Buy new electrolytic aluminum capacitors from Digikey or a local electronics store. These are small Coke-can shaped capacitors with dimensions around 2cm long by 1cm in diameter. Here are the values you need:

• C7: 22µF 50Volt
• C12: 6.8µF 25Volt
• C6: 10µF 25Volt
• C16: 47µF 25Volt
• C15: 0.47µF 25Volt

You can use higher voltage rated capacitors, just do not go below the indicated voltage. Higher voltage means often bulkier sizes, so be careful not to buy something too large. But make sure the capacitance is the same as the original. The specifications in the Digikey list should be "General Purpose";
"Radial Leads" with spacing up to 7mm (I believe the originals were radial but you might be able to get by with axial if necessary); "Through Hole"; and
"Tolerance +/- 20% or less". The rest of the specifications are not important.

Repair Procedures:

- Desolder capacitators C7 – C12 – C6 – C16 and since we are at it also C15.

NOTE THEIR POLARITY!!! WRITE IT DOWN SINCE THEY ARE POLARIZED! The circuit board should show a small "+" and "-" where the leads are soldered but these may be obscured by solder or corrosion.

- Spray the board with electronic contact cleaner/deoxidizer such as DeOxIt and use a tooth brush to clean the mess the faulty capacitors have created.

- Re-flow all the nearby components. Pay attention to the two ICs; their pins are very close each other. Do not short them out.

USE A GOOD QUALITY SOLDERING GUN, SMALL TIP, for electronic use.

- Reinstall the new electrolytic capacitors:

• C7: 22µF 50Volt
• C12: 6.8µF 25Volt
• C6: 10µF 25Volt
• C16: 47µF 25Volt
• C15: 0.47µF 25Volt

E. Reinstalling

It's exactly the reversal of everything. Just make a test drive after you reconnected the wires, before reinstalling all the plastic moldings.

Volvo Maintenance FAQ for 7xx/9xx/90 Cars