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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Sat Jul 09, 2011 8:22 pm Post subject: |
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Peak coil primary voltages can reach 350 volts or more, so I'd recommend a 600 volt diode or higher. That's 1N4005 or better. I'd advise 1N4007 (1000 volts). _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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SGKent Samba Member
Joined: October 30, 2007 Posts: 41031 Location: Citrus Heights CA (Near Sacramento)
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Posted: Sat Jul 09, 2011 11:17 pm Post subject: |
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Telford - have you ever scoped the problem to see what the waveform looks like and why it is requiring a diode to work sometimes? _________________ “Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Sun Jul 10, 2011 3:30 pm Post subject: |
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I have, and it's fairly nasty. This one's about 225 volts (50 volts / div), measured from the green points wire to ground.
Varies quite a bit with the condition of the plugs and wires, as one would expect. The poorer the wires and plugs, the bigger the voltage spikes. No idea why the diode trick works (for some people - didn't work at all for me). Only way to find out is to reverse engineer a tach. Unfortunately, this requires opening it up by uncrimping the front trim ring - which sorta means the end of the tach, at least cosmetically
On the other hand, my request for someone to donate a dead tach for inspection still stands. I'll repair it (assuming parts or substitutes are available) and return it, no charge. You'll have to deal with the trim ring, of course... _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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Vince Waldon Samba Member
Joined: November 07, 2010 Posts: 451 Location: Edmonton, Alberta
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Posted: Sun Jul 10, 2011 8:10 pm Post subject: |
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telford dorr wrote: |
I have, and it's fairly nasty. This one's about 225 volts (50 volts / div), measured from the green points wire to ground.
Varies quite a bit with the condition of the plugs and wires, as one would expect. The poorer the wires and plugs, the bigger the voltage spikes. No idea why the diode trick works (for some people - didn't work at all for me). |
Hmmm... last car I scoped had a peak of about 35V... but as you say there's a wide variation to be expected... different coils, condenser condition etc.
Did you ever try a bit of a pull-down resistor along with a diode? 10K or so?
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My guess is that some tachs are more sensitive than others to the initial ringing... particularly after the 10-15 foot trip to the front of the bus. Diode at the tach rather than the coil perhaps? |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Mon Jul 11, 2011 9:16 am Post subject: |
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35 volts? Seems a bit low. Assuming a 100:1 VW coil, that means only 3500 volts at the coil secondary. Must have been a much higher turns ratio coil, say 400:1 (= 14,000 volts)...
Edit: did you take the probe attenuation factor (10:1 typically) into account? That would make it 350 volts at the probe tip.
Haven't tried a pull-down resistor, but it's easy enough to do so. My concern is that if anything attenuated the spike directly, it also attenuates the coil secondary voltage (via transformer action). And you're right about sending the coil primary voltage all the way to the front of the bus - not a great idea. Think of it as a really long radiating antenna. Hence my solution, located in the engine compartment (from the gallery):
_________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)
Last edited by telford dorr on Mon Jul 11, 2011 10:37 am; edited 4 times in total |
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rustbus Samba Member
Joined: June 18, 2009 Posts: 2078 Location: alberta
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Posted: Mon Jul 11, 2011 9:28 am Post subject: |
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telford dorr wrote: |
Haven't tried a pull-down resistor, but it's easy enough to do so. My concern is that if anything attenuated the spike directly, it also attenuates the coil secondary voltage (via transformer action). And you're right about sending the coil primary voltage all the way to the front of the bus - not a great idea. Think of it as a really long radiating antenna. Hence my solution, located in the engine compartment (from the gallery):
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excuse my ignorance - but im very interested in this stuff.
If the diode is placed at the gauge, and we have a long radiating antenna running to the front, what are the negative consequences? does this affect performance to any degree? or would the radio get mad?
Also - yo mentioned that big voltage spikes are indicative of a less than stellar spark plugs and wires. should a voltage spike as you've shown suggest that plugs and wires need replacing?
I have an old scope but havent sat down to learn how to use it yet _________________ May of '72 Deluxe. 2.0L L-Jet CS & 091 trans conversion
my Bus thread |
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SGKent Samba Member
Joined: October 30, 2007 Posts: 41031 Location: Citrus Heights CA (Near Sacramento)
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Posted: Mon Jul 11, 2011 9:44 am Post subject: |
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telford dorr wrote: |
35 volts? Seems a bit low. Assuming a 100:1 VW coil, that means only 3500 volts at the coil secondary. Must have been a much higher turns ratio coil, say 400:1 (= 14,000 volts)...
Haven't tried a pull-down resistor, but it's easy enough to do so. My concern is that if anything attenuated the spike directly, it also attenuates the coil secondary voltage (via transformer action). And you're right about sending the coil primary voltage all the way to the front of the bus - not a great idea. Think of it as a really long radiating antenna. Hence my solution, located in the engine compartment (from the gallery):
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one could have coil packs that use greater windings. I would think that a higher winding ratio on a single coil would limit the RPM substantially as it would saturate sooner. My guess is that with a higher winding ratio the collapse would be dirtier also. Telfprd - is that circuit you posted the most current circuit for the VDO Dampening? If so I would like permission to build one and try it. The VDO light dimmer works really well BTW. _________________ “Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Mon Jul 11, 2011 9:54 am Post subject: |
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Quote: |
If the diode is placed at the gauge, and we have a long radiating antenna running to the front, what are the negative consequences? Does this affect performance to any degree? or would the radio get mad? |
Sound system noise would indeed be my first concern.
My other concern is that if anything messes with that wire going all the way to the front of the bus, it would have a negative effect on engine operation. Of course, you could fix that in an emergency by going back to the engine and disconnecting the tach wire, but in my case, I have an AC compressor in the way, so it would not be any fun...
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Also - yo mentioned that big voltage spikes are indicative of a less than stellar spark plugs and wires. should a voltage spike as you've shown suggest that plugs and wires need replacing? |
Mine are all new, so I assume that the waveform is normal. It's the same on all four cylinders. I also get the same waveform bench testing with a new solid spark plug wire and single plug (abit set with a way bigger gap, as it's not in a cylinder under compression...)
You're correct - a defective wire and or plug usually requires more voltage to fire, hence the bigger primary voltage spike.
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I have an old scope but haven't sat down to learn how to use it yet |
Do so - it's a wonderful tool for diagnosing ignition and FI system wierdnesses. Just make sure you have a suitable (voltage, risetime) and appropriate matching probe (eBay, if you don't). _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)
Last edited by telford dorr on Mon Jul 11, 2011 10:53 am; edited 7 times in total |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Mon Jul 11, 2011 10:08 am Post subject: |
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SGKent wrote: |
one could have coil packs that use greater windings. I would think that a higher winding ratio on a single coil would limit the RPM substantially as it would saturate sooner. My guess is that with a higher winding ratio the collapse would be dirtier also. |
It depends on the driving circuitry. Higher turns ratio implies lower primary turns, higher primary current, and (typically) electronics in the middle somewhere. Hard (for me) to draw any performance conclusions without bench testing.
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Telford - is that circuit you posted the most current circuit for the VDO Dampening? |
Yes - it works well for me so far. May have too slow a risetime at greater than 6000 rev/min, so smaller capacitors may be needed (not a concern in my bus...)
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If so I would like permission to build one and try it. |
Of course! Anything I post is for others to play with. Now if you plan on going into mass production, call me...
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The VDO light dimmer works really well BTW. |
Glad to hear it. Working on posting a stand-alone switching dimmer circuit for interior camper lights (when I get a round 'tuit)... _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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pittwagen Samba Member
Joined: November 08, 2005 Posts: 763 Location: North of the 49th parallel
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Posted: Mon Jul 11, 2011 7:43 pm Post subject: |
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I have an 8000rpm VDO cockpit 80mm tach with metal case. Date of manufacture was 11/87. There is no part number on the case. There are no dip switches on the case either. It has never been installed in a vehicle as far as I know.
Would a diode be required for this if it was installed in my 79 Bus given the 12 ft or so of wire required?
Thanks. |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Tue Jul 12, 2011 9:40 am Post subject: |
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No way to tell without trying it. Would recommend a temporary connection direct to the engine to check for any issues. If none, then run the wire and install in the cockpit, then check again. _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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pittwagen Samba Member
Joined: November 08, 2005 Posts: 763 Location: North of the 49th parallel
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Posted: Tue Jul 12, 2011 9:57 am Post subject: |
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Any particular issues with the VW electronic ignition? |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Tue Jul 12, 2011 11:49 am Post subject: |
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Should be fine - just make sure that the sense wire to the coil terminal (#1) NEVER shorts to anything. Hard on the (expensive) ignition module. _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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pittwagen Samba Member
Joined: November 08, 2005 Posts: 763 Location: North of the 49th parallel
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Posted: Tue Jul 12, 2011 1:19 pm Post subject: |
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Thanks for that. Maybe I'll try it on the Beetle first. |
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Vince Waldon Samba Member
Joined: November 07, 2010 Posts: 451 Location: Edmonton, Alberta
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Posted: Tue Jul 12, 2011 1:26 pm Post subject: |
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Neat signal conditioning telford dorr... I especially like the LED monitor on the one-shot and the "old skool" resistors... I've got a couple of boxes of resistors from my ham radio days that are same old brown colour.
Rather than do the math I'll just ask you what approximate output pulse width you've ended up using?
Last edited by Vince Waldon on Tue Jul 12, 2011 1:39 pm; edited 1 time in total |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Tue Jul 12, 2011 1:31 pm Post subject: |
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Should be the inverse of the dwell angle. All the circuit does is remove high frequency components, clamp to 12 volts and ground, and invert (tach doesn't seem to care). LM555 acts as a Schmitt trigger / buffer, not a one-shot. Triggers at 1/3 and 2/3 of Vcc. _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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Vince Waldon Samba Member
Joined: November 07, 2010 Posts: 451 Location: Edmonton, Alberta
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Posted: Tue Jul 12, 2011 1:40 pm Post subject: |
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Got it... thanks for the clarification... now that you mention it there's no R/C chain on the 555... shoulda noticed that. |
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Wed Jul 13, 2011 9:26 am Post subject: |
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It's there (R2, C2) - just no feedback from pin 7 or 3... _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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kuleinc Samba Member
Joined: August 10, 2007 Posts: 1604 Location: East Bay Area, California
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telford dorr Samba Member
Joined: March 11, 2009 Posts: 3547 Location: San Diego (Encinitas)
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Posted: Mon Sep 12, 2011 10:09 am Post subject: |
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Hard to say, as the VDO tach innards are still unknown. Changing the condensor may have an effect - wouldn't hurt to try. Diode may work (didn't work for me).
Obviously, something in your hand-held tach is loading the ignition primary circuit enough to eliminate whatever it is in the waveform that is driving the VDO tach nuts, but without measurement, can't say what it is. _________________ '71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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