Stuck with Speedometer repair

[Stephan]

Hi!

I'm trying to repair the speedometer of my old Volvo 940, the speedometers are incredibly rare, so I have no option but to repair it. I've already spent some time narrowing it down to the speedometer, I do get a pulse from the speed sensor. Neither speedometer nor odometer work. I've already changed the capacitors, didn't help.

So the construction is based around 3 ICs I can't find a datasheet of, it's a Toshiba TB9213P (DIP-16), a Toshiba TD6903S (Sip-9) and a BA6713N (SIP-7) .

Here is what it looks like:


The black bits on the PCB all have electric contact, double checked that.

And here is the circuit diagram of it, as far as I could trace it, very crude, sorry:


So the 6 pin connector that connects from the dashboard to the unit has 6 pins, 3 of which I considered relevant for testing (1,3 and 4):

1 - This is the minus from the Speed signal sensor (S-)
2 - goes to some unconnected bits, probably functions not used by my car
3 - +12V
4 - The Plus of the Speed signal sensor (S+)
5 - goes to a non-existent warning light bulb of the dashboard, again unused functions
6 - The speed signal, I presume output, found a drawing that says it goes to Powertrain control module, and cruise control and such things.

The 4 pins of the actual speedometer spindle:

1X               3X

2X               4X

the wire between 1 and 4 have connectivity of 172 Ohm, and 2 and 3 too, also 172 Ohm. No other connectivity besides that between the 4 pins or the metal cover.

I've used a cheap function generator that spits out a sine wave in the range I saw coming in from the speed sensor (S+ /  S-) of something around 50 to 150Hz or so while driving the car (measured it with my multimeter, don't have an oscilloscope so am just assuming it's probably a sine wave coming in). When the speedometer spindle isn't soldered in I can see the following going on, when I crank up and down my fake speed signal:

Between 1 and 4 I get a stable 15.62kHz, but the voltage goes up and down between roughly 0.5V and 1.2V
Between 2 and 3 I see the frequency change between 7kHz and 15.6kHz, but the Voltage stays stable at 6.5V

As soon as I solder the speedometer spindle back in I get none of these Voltages anymore, I can hear the spindle buzzing though, and when I turn the dial with my hand while having a fake high speed coming from the generator, it can hold itself there for a bit until I go down with the speed. But it doesn't move by itself.

I have no clue what to search for to find a speedometer like this explained, so that I can figure out whether anything I measure is right or wrong. Also I'm not sure of what to make of my diagram. What happens to the 4 pins of the speedometer seems to make some sort of sense, on the other side the speedometer spindle seems completely intact. Also nevermind the speedometer, the odometer also doesn't do a thing.

So am stuck, since I don't know what a working one would do as opposed to what I observe, and I'm not good enough with electronics to deduct what should happen from the diagram.

Could anyone help me? Maybe even with a search term or a link that explains how this thing should work if it works?

Thanks a lot!!!

[Rerouter]

The 2 capacitors have leaked, replace them,

Use acetone or similar and scrub at the black traces, usually the trace will have been eaten through by the electrolyte near the diode,

Repair the trace, I usually use stranded ground wire for this, but it doesnt need much thickness

Clear and refill each solder joint, e.g. braid away the old solder then add fresh, I know how stupid it sounds, but that electrolyte damage makes the solder not last more than about 6 months after the repair (becomes brittle really easily)

Any joints where you see the solder jump away from the pin when heated, spend extra time cleaning, you may need up to 3 passes for pads near the capacitros.

You may also need your signal to be +-5V or more to get the speedo to reliably respond, I usually just go for a +-15V output for most speedometers,

The diagrams do exist, but even as a qualified repair company at the time they where released, they did not want to release them.

[Rerouter]

The IC in the middle is the sine/cosine driver for the coil, the chip near Z2 is the odometer driver, and the chip to the right of it would be the divider / output driver.

[Stephan]

Thanks for the replies! You guys are quick!!!

I've replaced the 3 electrolytic caps already. I can scratch away the black stuff, but underneath it's all good, just double checked. Also checked with the multimeter that they all have connectivity. R2 didn't so I've resoldered it, but the rest is all having connectivity.

I didn't dare to put +-15V on, didn't want to fry something. When I measured the incoming signal while driving it was also something around 4 or 5 V, so that's roughly what my function generator spits out as well (is a cheap XR2206 based kit). Was hoping that is good enough? I have nothing else to increase that ...

So this sine/cosine driver, does what I measured look like what it is supposed to do?

[Rerouter]

Went looking.


http://cleanflametrap.com/speedo7.html

1993 Speedometer

Your frequency source should be enough, just saying what I use,

Seriously if you have not reworked every single joint on that PCB, it will be the fault. without fail. I can count 6 bad joints on your original photo alone.

Don't have measurements to compare your chip, but its rare they fail

[Stephan]

Wow amazing! Was looking all over for something like that! Will try out what you suggest as soon as I get my hands on some acetone, and report back 

[Rerouter]

Other solvents can be used, e.g. isopropyl alcohol, (the non gel hand sanitizer), but if you use the hand sanitizer stuff, it takes longer, and you will need to dry the board afterwards as it contains water.

[themadhippy]

as soon as I get my hands on some acetone

nail varnish remover is a readily available source,for example https://www.wilko.com/en-uk/wilko-nail-polish-remover-with-acetone-250ml/p/0296233

[Stephan]

It worked! Speedometer works again! I used isopropanol and a fibreglass brush I still had to repair the damage:


Thanks for all the help, you're amazing! Wish I had thought of asking here earlier!

[robca]

There seems to be a place where the trace repair you did gets very close to another trace, I put two red dots between the points of concern

With car vibrations, that wire might have just enough flex to cause problems

I would personally try to space them more, and if needed I'd add some nail polish or liquid tape to prevent contact

[Stephan]

ok thanks, will have a look, will need to get it out once more anyways because the engine temperature needle always shows half way, even on a cold engine, so when I look at that will have another look at that spot you mentioned. Thanks!

[Ovi4]

Perhaps there are some tips here:

https://www.volvoclub.org.uk/faq/ElectricalSpeedometerRepair.htm

and here:

http://cleanflametrap.com/speedo7.html