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 How to fix a dead tach (rev counter)

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GPW
Hiace Master
Hiace Master
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Number of posts : 728
Home City : Cambridge, UK
Model and year : Model: KD-KZH100G-MRPGT
Year: 1996
Colour: 4K1
Trim: FN42

Registration date : 2016-07-16

PostSubject: How to fix a dead tach (rev counter)   Fri Jun 01, 2018 10:38 am

Recently my 1996 tacho failed, then it sprang into life again, then it failed again. I had thought perhaps a loose connection to the instrument cluster was to blame but having removed the tacho and powered it up and fed in a 12V pulse at 200 Hz it was still dead.

I changed the 6.8 and 100uf 25V caps despite them looking fine and no difference. Then I traced the pulse signal and found that a dry joint on a link bridge was causing the issue:



Each blue component is a 0 Ohm resistor, i.e. a metal bridge used to hop over tracks so they can keep using a (cheaper!) single sided PCB for this application. Ground is at the bottom, top left is B+ (Ign), Pulse in (Pin) at top right. Regulation on this gauge appears to be a simple resistor + zener. B+ and Pin are protected with diodes so whatever you do to the van this tacho should be very difficult to kill.

So it just needs all slightly matt looking joints reflowing with a soldering iron (ideally with a dab of flux) to fix. I later discovered an entire thread about exactly this fault on the same model tacho here:

https://www.electronicspoint.com/threads/toyota-rev-counter.268575/

I did this by putting a pinhead spot of flux on each joint I wanted to reflow and then touched it with a soldering iron briefly. After this all the joints were new and shiny, but the reading of the gauge was now slightly different, I assume it had drifted with age as the solder joints aged so I'm looking to recalibrate it before it goes back in permanently.

So if your Toyota rev counter stops working you may also find that you can fix it like this too.
Looks like I'm going to be checking the speedo before I stick the cluster back in Very Happy
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