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Speedo/tach from a VW Fox
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Guybrush
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PostPosted: Thu Jul 19, 2018 9:50 pm    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

You can't connect the LED directly to 9V, that's too much. You need a resistor. Not sure if those are 3V or 5V LEDs. The original foil uses a 470 ohm resistor.

The exciter circuit is in parallel with the LED, so it works regardless, or it should. It has its own resistor off of the 12V (it's 150 ohms if I remember correctly, 1 or 2 watts, the largest one of the bunch). You can see that if you follow the circuit on the foil. A bit tedious but you can see what goes where.


Last edited by Guybrush on Thu Jul 19, 2018 9:54 pm; edited 1 time in total
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Guybrush
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PostPosted: Thu Jul 19, 2018 9:53 pm    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

Also, some alternators are "self exciting". That's the case for the one in my Bostig conversion, not sure about the models for the WBX. I'm getting 14+ volts even without any dash cluster.
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davevickery
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PostPosted: Thu Jul 19, 2018 10:16 pm    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

Guybrush wrote:
Also, some alternators are "self exciting". That's the case for the one in my Bostig conversion, not sure about the models for the WBX. I'm getting 14+ volts even without any dash cluster.

Thanks, this all started because my subaru alternator would charge but very intermittently. I think I blew the bulb when I was swapping in the Jetta tach or earlier when I was trying to adjust the potentiometer with it running. The new NAPA alternator would not charge at all.

So you think the excite circuit should have worked regardless of the blown bulb? Mark said the same thing in another post. I'm hoping it is going to work now, but I'm less optimistic since you just said that. I pulled some bulbs out of the Jetta cluster and they seem to be compatible, so I should be able to test it in the van tomorrow. Thanks again.
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Guybrush
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PostPosted: Fri Jul 20, 2018 7:33 am    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

Yes, it should. The alternator should be receiving power directly through a resistor and the diode from +12V. The LED circuit is one that is mounted in parallel to that. Mine is an 86, I'm not sure if the foils are identical throughout the years.
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crazyvwvanman
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PostPosted: Fri Jul 20, 2018 8:18 am    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

There are quite a few different foils over the years. The alternator exciter/LED circuit is one of the areas that has evolved a bit. After 86 the single exciter bypass resistor became two in parallel, to increase the exciter current while dissipating heat.

Mark

Guybrush wrote:
..... Mine is an 86, I'm not sure if the foils are identical throughout the years.
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davevickery
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PostPosted: Fri Jul 20, 2018 8:47 am    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

The led light will come on bench testing pin 8 and 11 with a 9 volt battery, but it does not come on with key on and the alternator is not charging although I can apply power at the alternator and get it to charge.

I tested the resistors and that diode and the cluster voltage regulator and they appear to be o.k and there are no obvious problems with the foil. There aren't many threads discussing a non op alternator light, most of them seem to be on a light that doesn't go out even though it is charging. But I'll keep looking.

This all started after I swapped in the Jetta tach but maybe there is a short somewhere else or a fried electrical component.

A new foil or a new cluster might be in my future.
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crazyvwvanman
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PostPosted: Fri Jul 20, 2018 8:51 am    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

Pull round 7 pin connector junction apart and measure the voltage on the blue wire with key on. If you see 12v on the dash side of it, ground that blue wire and watch the dash led to see if it comes on while that end of the wire is grounded.

Mark
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davevickery
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PostPosted: Fri Jul 20, 2018 9:23 am    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

Thanks Mark,

I had disconnected the blue wire connector at that junction box in the engine bay to check continuity again yesterday and I forgot to plug it back in Embarassed so naturally nothing was working.

Now everything is working (well not the OX light but it wasn't working before either). Phew!!!!! :2gunfire: :2gunfire: :2gunfire:


Thanks so much for your help. And the electronics lesson.
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Guybrush
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PostPosted: Fri Jul 20, 2018 9:32 am    Post subject: Re: Speedo/tach from a VW Fox Reply with quote

Very cool Smile
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