Toyota Rev Counter

[ajstars]

I am trying to repair a Toyota Surf (1996) Tacho. It has been intermittent for a while but has now finally given out. Through testing on another vehicle it has been narrowed down to just the tacho unit which is a simple affair of a number of surface mount diodes, resistors and a couple of capacitors. All joints appear good and there is continuity between all of them. My knowledge of further testing has run out, but I am suspecting one of the capacitors which obviously I can't test myself without the correct testing equipment, has failed. I know its a long shot but does anyone know of someone who could look at this properly in the Gatwick area, or should I just replace the capacitors and see if that fixes it?
Not really knowing the most likely item to fail I don't want to start de-soldering it for no reason!
Thanks

 

[davenn]

welcome to the forums

how about showing a couple of sharp and well lit pics of each side of the circuit board
then some one may be able to offer some advice

cheers
Dave

 

[Jagtech]

Check initially for tiny cracks in the solder connectins on the board. But most likely cause is a failed electrolytic capacitor.

 

[KrisBlueNZ]

I wouldn't suspect electrolytic capacitors initially, as they tend to fail gradually, not become intermittent. But as Jagtech suggests, inspect the solder connections on the SMT components very carefully. In a high-vibration environment they can crack, and sometimes the cracks are very difficult, or even impossible, to see. You can also try gently pressing down on each end of each component with something non-conductive like the end of a ball-point pen with the tip retracted, to see whether it springs to life. Or just resolder every joint - be quick, and don't flow both ends of the same component at the same time, or it may move!

Photos are always helpful.

 

[ajstars]

Photos attached. I think we can rule out the capacitors too! - I have replaced them but still no joy. The last couple of photos show the new capacitors, as you can see we couldn't match one of them up so have joined two in series.
Just in case I have made a mistake, the capacitors were as follows:
Brown, 25v 100uF(m) 105 degrees. Changed for 35v 100uF(m) 85 degrees
Blue, 25v 6.8uF 85 degrees. Changed for 50v 10uF 85 degrees and 50v 22uF 85 degrees positive leg soldered to negative leg as per photo.
I can' see any dry joints, and there certainly seems to be continuity between them all but I will try re-soldering all of them briefly.

 

[ajstars]

Fixed
Thank you for the suggestions and help. It was indeed a dry joint, invisible to the naked eye or under a magnifying glass (although oddly - looking at the photo it looks quite obvious!) As soon as I touched the left side of the component marked 153 it flew off the circuit board! A bit of superglue on the end of a paperclip enabled me to hold it back in place and solder it on. It now works perfectly again.
Maybe, someone could just verify that the new capacitor configuration is OK for me please as I am unsure if the brown capacitor replacement with lower temperature (but higher voltage) may cause problems?
Thank you.

 

[KrisBlueNZ]

Yes, 10 µF and 22 µF in series works out to about 6.8 µF, but the fact that they chose 6.8 µF instead of a common, "close enough" value like 4.7 µF or 10 µF could imply that it's a fairly critical value, and normal "close enough" electrolytics may not be accurate enough.

Without being able to see the part number on the IC, I can't guess whether the value of that capacitor is important to the circuit or not.

If you have the original capacitors, I think you should put them back in the board. Otherwise, do you know the manufacturer and the series (usually a 2-letter or 3-letter sequence) for the original 6.8 µF capacitor?

If in doubt, you could use a film capacitor with a tolerance of 10% or 5%. Any of these will be suitable, if there's enough room: http://www.digikey.com/product-sear...1434002a,1434002d,14340036&stock=1&quantity=1

I would also resolder all of the SMT components. If you've had one failure like that, there's a reasonable chance you'll have another. Do the right hand ends of the components first, then rotate the board and do the other ends. Heat the joint and suck the old solder off using a solder sucker, then add new solder to the same level as the old solder.

 

[ajstars]

The 6.8uF is a Rubycon with markings:
CE W
M4070

The brown capacitor is marked CE04W and N593 for your info.

I re-soldered all the joints as you describe.

 

[davenn]

looking at the photo it looks quite obvious!) As soon as I touched the left side of the component marked 153 it flew off the circuit board!

yup a very obvious crack in the solder

well done on getting it fixed

cheers
Dave

 

[ajstars]

I put the old capacitors back in and it didn't work - so either I had a bad joint or one of the capacitors is bust too! New ones are back in again - seems less hassle, as each component change and test requires a rebuild of the dash unit! Hopefully the capacitor configuration won't cause any problems down the line.