Tacho Issues

[Holden202T]

ok so i bought a tacho off ebay (possibly the first mistake) to go on the gemini, its basically running your standard blue motor hei dizzy with its matching coil....

so the tacho is connected to the negative coil terminal and thats all good, its obviously getting a signal as it is working .... HOWEVER........

the problem is that its WAY out .... like at idle which is approx 1000rpm on my data logger its showing 2000rpm on the tacho ... this is on the 6 cylinder setting

flick it to the 4 cylinder setting and its about 1500rpm at idle ....

so what i'm posting all this shit for is to see if anyone has any suggestions etc ? tests i can do ?

i was planning on putting it onto the torana at some time this weekend to see what it does (camira ignition module) but other than that im at a loss as to what i can do ....

i'm pretty much thinking if it doesn't work on the torana then i'll have to see if i can get my money back or something cause its pretty much useless!!

[Apples]

you could try this mod I have used it a few times.

http://www.mindspring.com/~jayk3/toyota/tachmod.htm

[delcowizzid]

get in there and start tweaking trim pots if it allready has one may not of been celibrated correct when built.

[Holden202T]

i didn't think an off the shelf tacho would need something like that ..... but its easy to try cause i actually have a trimpot on the datalogger to stop it jumping (was installed when i had it on the camira module) so i can very easily run the tacho wire to the terminal on the datalogger and see if it cleans it up!!

stay tuned

[Holden202T]

SWEEEEEEEET!!!

as i said above, hooked it up to the same wire that goes into the datalogger (after the trim pot) and its spot on same numbers as my datalogger now

so i think i might have to get off my ass and finally buy a resistor to suit the trim pots position so then i can wire it into the line that feeds both and i wont need the trim pot then!!

thanks to ye all for the suggestion!

i now have a "fewly sik" tacho with a shift light mind you it is only 3.75 inch not the usual 5 inch monsters hahaha i'll be setting the shift light and i dare say i'll be watching it more than the tacho itself!!

[delcowizzid]

no time for watching it youll just end up seeing it light up out the corner of you eye.good to see its working now mustnt of liked the output of the module aye

[Holden202T]

yeah well i don't really know enough about the signal and how the pot changes it ... i can only assume it cleans up alot of the noise or something similar to resolve the issue.

[VL400]

The electronic holden ignition modules are a pain to get working with aftermarket tachs. If they are using a frequency to voltage convertor then the pot adjusts the voltage down, much the same as setting the 4-6-8 cyl switch does.

[Holden202T]

so this frequency to voltage converter i assume is internal in the tacho is it ?

the only issue i noticed once i got it working was that every now and then the tacho would blip up about 500rpm then come back down, only a quick blip .... but everytime it did this the engine sounded like it had a small miss, and its been like that forever an a day ... so im not sure why the motor does it, but i know its not the tacho thats causing it, but it is showing me that its some spark related issue ...

either way it doesn't effect the engine when you give it to it, it goes how it should !!

[VL400]

Yeah it is. There are a few ways to build a tach, a frequency to voltage convertor chip like a LM2907 or a use LM1819 air core driver (this is common in OEM applications) or the more modern way by measuring the time between pulses.