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NickandNeil

Rev counter wiring

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I have a pinto engined Exmo.  I am trying to wire up an old style cortina rev counter on a sierra loom.  I know this topic has come up before and after reading the comments several times I seem to have more questions than answers.  The comments on the previous forum mentioned  1 terminal being a switched feed, and the other a constant feed.  What is the difference? 

When I wire up the tacho it either cuts out the engine or does nothing at all.  Trying different permitations with the wiring sometime I can get the engine to fire and the tacho needle move, ony to have it to, again cut the engine out imeadiately..

Is there a difference in voltage with the switched feed over the constant feed?

Could there be a cable in the loom with a resistance in to reduce the voltage?

Also the coil has been replace with a modern one as the old one was obsolete (see pic)  It is 12 a volt one.

I have attached a picture of the tach, front and rear.  Could someone advise me which connection is which/  I am pulling my hair out here.

coil_pic_.jpg

Tacho_front_pic.jpg

Taco_rear_pic.jpg

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I have a newer rev counter and I only have a 12V from ignition  Neg and a signal connection to the Neg side of coil

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1 hour ago, geordie40 said:

I have a newer rev counter and I only have a 12V from ignition  Neg and a signal connection to the Neg side of coil

That's right, your tacho will sense the voltage change at the coil neg, as the points open and close.This earlier tacho (it looks like "RVI" hiding underneath the tacho needle in the photo) uses the current in the ignition LT circuit to drive it.

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11 hours ago, RedTrev said:

That's right, your tacho will sense the voltage change at the coil neg, as the points open and close.This earlier tacho (it looks like "RVI" hiding underneath the tacho needle in the photo) uses the current in the ignition LT circuit to drive it.

Didnt see RVI in photo as viewed it on phone last night, yeah the pdf info you sent should work. I know Nick is desperate to get the car IVA'd and on the road as when I met them last summer they were a little bit gutted they had missed last summers events

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Here is another question.  What do you men by  RVI?  Also I have  told the eureka wire might need tp b removed   I have no idea what a ureka wire is.?

And we have not got the old points system on but the later one with a module about 4" x 3".

Does that make a difference?

By the way for those that have been following our IVA build thread we have put in for it.  They said they will start the tests fron the 22 June, and to expect at least a three mount wait for ours. So there is another summer gone.  Totally gutted.

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Thanks for the link Red Trev, I did not realise there were so many variation of smiths tacho.

As  i said on the previous post, I think it has electronic ignition.  I have attached a picture of the 'Module' 

Is that classed as eletronic ignition? and does that mean my tacho is not going to work?

Module_for_kit_car.jpg

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If the +12v ignition feed that goes to your coil is fed through the tacho as per the circuit diagram (on page 14), then it should still work. The electronic ignition module, that you have shown, replaces the points, but you still have a coil with LT wiring.

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image.thumb.png.bda5e467f904a2adcdf25bde2cdd91ab.png

 

(Ignore the wire colours btw)

Edited by RedTrev

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Thanks for that Red Trev, I will try that tomorrow.  I have not been wiring to the + terminal on the coil.

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1 hour ago, NickandNeil said:

I have not been wiring to the + terminal on the coil.

Be sure the existing wire feeding +12v to the + teminal on the coil is removed (else no current will flow through the tacho and it will show no reading). The only wire connecting to the + coil terminal should be the one from the tacho (the green wire in my picture in the post above). And don't forget to ground the case of the tacho (the black wire in the picture).

Then, if your engine runs but the tacho still doesn't show a reading, swap the two wires on the bullet connectors, so the circuit is then like this:

 

image.png

Edited by RedTrev

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I have got the needle to move now, but it is irratic and bounces uo and down.

I did this as you said by linking 1 and three, and 2 and three,  You said the coil should go to the electronic module, but as there babout eight wires i m not sure which wire to attqch it to.

 

Will that cause the needle to bounce?  And werdly when i short out  the two feeds that brake lights come on ?????

 I have attached the wiring diagram.

Tacho wiring diagram 7.jpg

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Your circuit looks ok for a points ignition system.  If you're using the motorcraft module then it needs to be connected.

Here's my circuit when I ran with the module, before changing to Megajolt:

image.thumb.png.1c05aa2278d42f359685ce253fc8f54c.png

 

6 wires to the module:

B = Black to +12v ignition

N = Brown to ground

BR = Black/Red to Inductive trigger

GY = Green/Yellow to Inductive trigger

BG = Black/Green to cranking signal

G = Green to coil neg 

My rev counter is not a Smith's RVI, so is wired differently to yours, but the module wire functions are the same.

 

Your brake lights coming on??? Not sure about that, but I think you need to double check all the wires you are using really are what you think they are.

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Thanks again.  The motorcraft module is already wired in tothe circui.  I thought you were saying i needed to add another wire to it.

I will check the brake light wiring, but it did not do it until I changed the wiring on the coil from negative feed to the tach to posative feed.

In that case what would cause the needle to bounce violently.  I think the tacho has been stored for years, could it simply be no good, or do you have to stabalize the voltage feed in some way?

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2 hours ago, NickandNeil said:

 

 

2 hours ago, NickandNeil said:

 

In that case what would cause the needle to bounce violently.  I think the tacho has been stored for years, could it simply be no good, or do you have to stabalize the voltage feed in some way?

I know i have a more modern Rev counter but my needle bounced all over the place above 2000 rpm and it was down to the coil, dont know what was wrong with it as it was brand new and worked fine with my Chinese rev counter but wouldn't work correctly with my new ETB one. Changed coil to a Valeo 245025 and hey presto worked a treat.

I looked on google and I did try putting a resistor on the feed but that stopped the rev counter working (maybe wrong value)

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