[Davide D.] I have written this guide based on the repair of a speedometer from my 1991 940se Wagon with a 140mph Yazaki speedometer. 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. [Editor] Your speedometer or circuit board may vary from the diagrams shown for the 940; the parts needed may also vary.

Replacement Parts: [Paul Ramdial] You can replace discrete components (including the capacitors which are the most likely failure mode) using available commercial parts. The Yazaki integrated circuits in their speedometers are probably proprietary and may no longer be available, although one does show up on Google. I was told that Volvo (NA) made a buyout of remaining inventory from Yazaki about nine years ago and after that's depleted ..well it's the end as far as availability is concerned.

Tools Needed: [Randy Starkie] Invest in a good screwdriver set capable of giving a good fit to the small phillips screws you will encounter. Husky (Home Depot) makes one that has four different sized inserts- slot on one end and Phillips on the other. [Editor] You will also need a soldering iron with a small tip adequate to solder small cable and capacitor ends along with rosin core electronics solder, a solder wick to soak up excess solder, and some small pliers to hold small parts.

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. [Editor] In my instance, the odometer worked intermittently, only when the a/c was on. The instructions below solved the problem.

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. First mark the perimeter screws and holes with one color permanent marker, then the cadmium plated silver screws below the speedometer connector. Mark the larger brass screws in another color. Place the screws in separate containers so you don't mix them up when reassembling.

Once separated DO NOT LAY THE INSTRUMENTS FACING DOWN, or you risk of scratching their delicate surface or breaking off the small zero-position pins.

REMOVING THE SPEEDOMETER UNIT:

Remove the screws. Pull the unit out.

E. Speedometer Surgery

PREVENT DAMAGE ON THE BENCH:

[Paul Ramdial] Buy styrofoam sheet of sufficient thickness to allow the speedo dial to rest on it with the reset shaft inserted into a hole in the sheet. Be VERY careful about not damaging the fragile zero position pin which breaks off in a heartbeat All this will allow work on the speedo without having to remove the dial.

MINOR CLEANING:

[Editor] Inspect the board and in particular the IC leads around the capacitors noted below. If only minor corrosion is present, clean it up as noted without disassembling the IC board from the unit: that sufficed in my case. You will eventually have to replace the capacitors that are leaking which will require disassembly.

SEPARATING THE IC BOARD:

[Method 1 Requiring Desoldering the Cables: Davide] Make some notes as to wiring locations and colors so you can reassemble correctly (see photos of different model PCBs below):

- 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.

PCB from 91 940

PCB from 93 940. The red circles are the joints that must be de-soldered, the yellow circles are optional. Arrows are the two screws that need to be removed. Note that the 4 resolder joints next to the screws for the speedo pins are very difficult.
They tend to slide down in the white plastic speedo mount as you try to line them up in the circuit board holes. See the tips below.

[Desoldering Tips: Paul Ramdial] As I am somewhat proficient with soldering and such I felt more comfortable working with the PCB off the unit. Removing the ribbon is not really a big deal once you have the proper equipment such as a suction desoldering tool. I do not recommend desoldering braid . Once most of the solder is removed the ribbon still will not separate freely as any residual solder would still hold the ribbon in place. To remove it, just reheat the soldering points one at a time and lift with a tooth pick and before you know it the ribbon is off.
It is hard to reinstall the ribbon incorrectly . Once aligned just one solder point will keep the ribbon true then you can resolder. The trick with the four pins from the speedo movement is, once most of the solder is removed, to lift the pcb gently while heating the pins. Do one at a time and once the the board is separated widen the four holes a bit so reinstallation is easy. If the holes are not wide and clean then when you try to reinstall the pcb you will end up pushing the pins down. If you do, use small pliers and gently lift them up a bit. The pins should just protrude out so you can have a good soldering point.

[Method 2-Simpler: Randy Starkie] Davide's disassembly above calls for no less than 11 unsoldering points (5 on a ribbon with multiple joints) to disassemble the speedometer. While I am comfortable unsoldering/resoldering joints on a board I find working with ribbon fraught with chances of error.

I was able to reduce the number of unsolderings to just two. Those would be the blue wires that connect the micro switch to the board. After disconnecting those from the board the remaining disassembly was only involved with removing the needle and some screws. Begin by removing the needle. Gently rotate the needle by hand to it stopping point and note (or mark) this point which is more or less the “m” in the mph at the bottom of the face. The needle is then twisted off by very gently turning it and lifting straight up after you carefully lift it over the zero peg. The thin “zero pegs” on all these gauges are very fragile and easy to break off so be careful … Yes, scary stuff, but would you rather unsolder and resolder the remaining 9 solder points as noted above? Pick your poison.

Continue on the other side by removing the four small Phillips head screws near the center of the board. Only two of them really need to be removed but it is hard to tell which ones they are until you have taken the board loose.

At that point the board will come loose but still be attached by the ribbon cable on the motor. With the board loose you have access to both screws that hold the odometer motor in place. Remove those two screws and the odometer motor lifts free and stays with the board. You now have access to the capacitors that are referenced in Davide’s tutorial below.

You can further disassemble the remaining portion by using a knife to separate the face from the clear plastic structure. Gentle prying of the face all around will pull the adhesive points loose and the face will come off. The adhesive remains tacky and simply press the face on for reassembly. The removal of the face will reveal two small Phillips head screws that when removed will leave the mechanical portion of the odometers free of attachments.

Reassembly [Randy Starkie's Method]: Assembly is straightforward based on the disassembly above. Care needs to be taken to see that the pin is rotated all the way clockwise to its stop before pushing the needle on at the stop point you found before taking it off.

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/Matt Lietzke] Buy new electrolytic aluminum capacitors from Digikey, Mouser 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 (Mouser p/n: USR1H220MDD)
• C12: 6.8µF 25Volt (Mouser p/n: UVR1H6R8MDD)
• C6: 10µF 25Volt (Mouser p/n: UVR1E100MDD)
• C16: 47µF 25Volt (Mouser p/n: UVR1E470MDD)
• C15: 0.47µF 25Volt, bipolar (Mouser p/n: UEP1HR47MDD)

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" except for the one bipolar capacitor; "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. When using Digikey, search for the parts by filtering sequentially "capacitor" then "aluminum" then the capacitance value then the other values.

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! (except C15 which may be installed either way). The circuit board should show a small "+" and "-" where the leads are soldered but these may be obscured by solder or corrosion. NOTE AS WELL whether they are soldered on both sides of the board or just one so that you can replicate the original soldering and not err on a multilayer board.

- 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. [Editor] In my case, only a small amount of corrosion was present which was removed using DeOxIt on a toothpick between the IC leads and on a foam electronics cleaner Q-tip from Radio Shack. I did not have the capacitors at hand, so reinstalled it awaiting them and the system worked.

- 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 paying close attention to +/- polarity (except C15):

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

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.

F. Potential Continuing Problems

Some respondents have reported that after this repair, their speedometers do not read greater than 45 mph. This may be an artifact of IC damage due to shorts across the corroded pins on the board. In one case in which the needle pegged at 40mph regardless of speed, it was caused by a loose ground wire.

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