SAAS tacho reading too high

GraGra · Last edited by GraGra on Tue Sep 08, 2020 4:31 pm; edited 1 time in total

Wondering if anyone can help with my new tacho in my ‘53 ute running a 948. I’ve wired it in but it’s reading 2k rpms at idle and off the gauge just flicking the pedal.

I have the reading wire connected to the CB terminal of the coil (old Lucas coil and running on negative earth) and even tried changing the cylinder selection switch from 4 to 6 to 8. Actually reads slightly lower on 8. I know the motor has had some work but don’t think that extended to coil/dizzy.

Installation instructions mention using a resister if gauge bouncing around. Would that clear up my reading or is there something else I’m missing? Is the coil too old or do I need some other type of adapter?

Any help is much appreciated.

https://www.shopsaas.com/assets/files/install/MuscleTachSmallFitting.pdf

DON58VAN · Posted: Mon Sep 07, 2020 4:42 pm Post subject:

Hi GraGra

Sorry you are having trouble with your tacho.

Can you get hold of a 10k resistor to try? They only cost a few cents.

I can post you one if you can't easily source one.

BTW Your link below doesn't work unless the part in the square brackets at the end is deleted.

_________________
Don
1970 2-door -- on the road
Trying to get some Morry projects done, but ultra slow. ;(

GraGra · Posted: Tue Sep 08, 2020 4:43 pm Post subject:

Thanks DON58VAN

Jaycar have an 8 pack of ‘10k Ohm 0.5 Watt Metal Film Resistors’ that say they are the same as 1/4 watt. Would these work?

Otherwise I might have to take you up on your kind offer.

PS: I fixed the link.

DON58VAN · Posted: Tue Sep 08, 2020 6:11 pm Post subject:

Hi again GraGra

The 0.5 watt ones will be fine. That just means that they can handle twice as much power. They will be slightly bigger than 0.25 watt ones, but this will be of no significance in your application. Most small resistors are metal film these days--they are perfect for your application.

Most cars built in the last 20 years or so have ECUs and so the tacho is probably optimised for taking its signal from one of those rather than from an old-fashioned distributor/coil system (known as a "Kettering ignition"). The resistor probably adjusts the input signal to make it more suitable for the Kettering system.

I'm not sure whether it will solve your malfunction, but it is simple, cheap and harmless to try.

_________________
Don
1970 2-door -- on the road
Trying to get some Morry projects done, but ultra slow. ;(

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