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stock tachometer use with the 350


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i've searched, honestly.

The aftermarket tach in our '72 is junk.  Is it possible to make the OEM tach work with the V8?  I see some "adapters" out there.  The 350 has an HEI dizzy.  The OEM tach is still sitting in the nacelle and it would be so nice just to make it work with the V8, ya know?

Something like this?

https://www.ebay.com/itm/UNIVERSAL-TACH-ADAPTER-CONVERTER-COP-1-2-3-4-5-6-8-10-cyl-to-any-4-6-8-Tach/141931549970?hash=item210bc79512:g:wqgAAOSwNX1b~MNz:rk:1:pf:0

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I did see the dakota one too.  I contacted the "widget man" ebay one linked above, he responded immediately and said it would work but only if I also added a widget to increase the voltage: https://www.ebay.com/itm/TACHOMETER-SIGNAL-BOOSTER-3-16V-Input-48-63V-Output-compare-to-MSD-8920/142171581596?epid=3012319916&hash=item211a162c9c:g:d9kAAOSwjoFb~Mao

 

So that's about $80 in his stuff, whereas if the Dakota works, there ya go.

Can't really get a whole tach for that anyway, let alone one that already fits the car and looks correct.  I'm leaning this route but I should probably try to see if the stock one still reads, it won't be correct but it should read something, which means another few hours crammed under the dash trying to decipher the rat's nest nightmare of wires 40 years of hackers have built up, it's something else, my turn!

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Here's what he says, I hope doesn't mind the re-post.  He seems knowledgeable and quick to respond, that's two ++ already.

Yes, but you'll also need one of our voltage boosters to drive the tacho (see our listing# 142171581596). Tachos on all pre-300 Z's were back-EMF types that needed the "spike" from a coil discharge to trigger. The 12V square wave from this BTA adaptor won't drive them, so you'll need to add the BTAC2 booster on the adaptor's output. The adaptor and booster can mount together and share the same power and ground (most of our customers use piggyback fastons). For an HEI distributor, you'll probably also want a filtering resistor (free, but you'll need to request one with purchase). Lastly, since the tacho has been out of use, we recommend you test it first to make sure it's working. If the HEI is an OEM type, the HEI tach output should trigger it (albeit with a wrong reading), since OEM HEI outputs use an internal connection to coil-negative. If it's an aftermarket HEI with digital output, you can test using a different vehicle with an external coil by jumpering signal and ground between the vehicles. We do bundle shipping, just pay full postage at checkout and we'll refund the excess to your PayPal when the items ship. In this case you'll pay $10.50 with $3.50 refunded. Thanks for your interest!

Best Regards,
Gene K

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I've never had a problem running the stock tach with a GM HEI or the stock Chevy distributor.  Typically, some additional resistance is required in the signal feed line; but no other changes required IIRC.

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47 minutes ago, jhm said:

I've never had a problem running the stock tach with a GM HEI or the stock Chevy distributor.  Typically, some additional resistance is required in the signal feed line; but no other changes required IIRC.

So all you do is add a resistor and the 6 cylinder tach reads correct for the 8 cylinder?

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9 minutes ago, jhm said:

Yep, I've done with two different cars in the last few years.  It's discussed in the JTR manual, if you happen to have a copy handy.

This one?

https://www.amazon.com/Datsun-Z-V-8-Conversion-Manual/dp/B0006EVOAK/ref=sr_1_2?keywords=jtr+manual&qid=1548178839&sr=8-2-spell

 

$263.84 (I've seen it for $40ish elsewhere)

Care to share what you did?

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Per JTR page 10-2, the 240z tach will not work with an HEI distributor.  The 260z and 280Z tach will work with the addition of a 15k 1/2 watt resister and some wiring changes.

 

 

The guts of the 260Z/280z tach have to be swapped into the 240Z tach housing because the 240Z/280Z tachs mount differently.

 

 

 

 

Edited by Miles
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alright, thanks, I think I might just chase the signal converter then, I should consider getting a copy of that book, although it's not my car (my kid's) and he may or may not be in it for the long haul, I 'should' just buy it from him, I freak'n love this thing.  I"ll be along in a week or so with rear suspension questions, maybe later today.

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My bad....I completely missed the fact that you have a 240.  My apologies.

 

A 260 or 280 tach should be relatively easy to come by, if you decide to go that route.  The JTR manual is available for $39:  https://jagsthatrun.com/products/copy-of-v-8-conversion-manual-for-volvo-200-series

 

(Yes, I know the hyperlink says Volvo; but it should work and get you to the correct manual on the JTR website.)

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We fired her up and the old Drag'n Tach actually still works.  I had shorted out the bulb holder in it during an instrument lighting upgrade (LEDs).  So for now I'm going to try to resurrect this abmonination.  I tore it all down and a trace on the PCB was burned, I'll repair that and if it all still works and lights up then we'll live with it for now. 

It's one of these from the 90's (as is the whole car so I guess it's "period correct".

 

BTW

Does anyone have extra heater control cable clamps to spare?  We're missing one.

 

Drag'ntach.jpg

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