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Digital Rev limiter fitting to 202 with Scorcher HEI


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#1 RallyRed

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Posted 20 October 2018 - 03:58 PM

Hi,

Going to let my son have a go in a track day.
Thought I'd throw a rev limiter on the track car to manage things as he gets up to speed.
Bought a Pertronix 600 digital rev limter, after checking with Scorcher, that it was compatible with their Electronic dist. and old school coil. Yes, compatible.
So today I wired it up roughly to confirm it will work, before making it pretty.
All good, rev limit is now adjustable by 100s and 1000s settings.
My situation is this - the DRL had a "tacho output" and it is indicated that you take the factory black wire off the coil -ve terminal,( the one that goes to the factory tacho ) and connect it to this "tacho output".
The LJ factory tach doesnt work in this configuration.The DRL still works fine.

If I return the factory tacho wire to the coil -ve, the tacho works fine and as per normal.... and the tacho reads in accordance with the DRL setting when you are on the digital limit setting. i.e DRL setting 2200rpm, tacho reads 2200rpm as the DRL cuts in.
I am assuming the DRL just gets the pusle train signal that the distributor makes as it switches the coil ( and tacho) . When the pulse train frequency equals the switch setting, then the DRL randomly holds the pulses down to ground, and thus kills the spark from being made.
The DRL then reproduces an isolated copy of this pulse train, and puts it out on the "tacho output" wire.
The factory tacho doesnt like this signal.


My question is this - does anyone think that if I leave the wiring as above, so DRL AND tacho both work...then I will cause a long term issue?
I cant think why it would...but dont want to let the smoke out.
Thanks

#2 RallyRed

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Posted 20 October 2018 - 04:02 PM

.

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#3 RallyRed

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Posted 20 October 2018 - 04:18 PM

just to be clear...you can see the LHS drawing shows the Orange wire looped to the Tacho wire and both of these connected to the
HEI. My question revolves around the fact my distributor looks like a normal old school one ( but with electronic internals ) and I have an old school looking coil. Scorcher advised to wire it up as per the LHS/HEI drawing, but my physical installation looks like the RHS/non hei drawing.
Reckon its all good..but happy to take advice.

#4 VDO

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Posted 20 October 2018 - 04:37 PM

Do you know what type of signal the DRL is reproducing for the tacho, and what voltage it is..?



#5 RallyRed

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Posted 20 October 2018 - 05:19 PM

No mate ( and thanks for the reply) , there is little/no info available from what I can find.

The doco just says words  like ""tacho output, - suitable for most modern tachos""   ( admittedly, 1972 isnt real modern).

 

Dont have a CRO or a Fluke DDM to look either.

 

My wiring is as per the LHS drawing, the only thing that worries me is the way they show a dizzy with inbuilt HEI coil...I guess it all boils down to me not wanting to corrupt the rev limiter switching, by having a tacho  hanging off the same wire...


Edited by RallyRed, 20 October 2018 - 05:26 PM.


#6 VDO

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Posted 20 October 2018 - 05:29 PM

I would stick with what you've done.  It can't harm anything.

 

The reproduced signal for the tacho is probably lower voltage than it needs.

 

The signal can be either sine wave or square wave for your VDO Tacho, square wave being preferable, but the voltage needs to be at least 10-12V to get it going.

 

This system, like many nowdays is most likely somewhere in the 5-7V range, which just isn't enough for your type of tacho movement..

 

Normally with a low tach signal, I would recommend a "pull up" circuit, but in this case it could corrupt the rev limiter and possibly the running of the engine too.

 

Hope this helps.


Edited by VDO, 20 October 2018 - 05:30 PM.


#7 RallyRed

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Posted 20 October 2018 - 06:38 PM

Ledgend...thanks mate.
I'll tidy it up tomorrow and take it for a squirt and confirm a good.
Unless, its still peeing down here like it is right now...

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#8 rodomo

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Posted 20 October 2018 - 08:30 PM

Just put standard valve springs in it...…………...4500rpm governed

#9 RallyRed

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Posted 20 October 2018 - 08:36 PM

ha ha...you nailed it..my LC is a bit like that




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