Using a potentiometer for tacho gauge

Evening Chaps

Anybody done this to convert a tacho for using on a Landy alternator? I installed a dual boost and rev counter to keep an eye on my dwindling turbo after replacing the boost diaphragm only to now realise that "universal" tachos are universal for petrols!! Never mind - figured if I just put a potentiometer in the line and reduce the voltage by about 1/4 should work out about right? If not Ill just disconnect tacho wire and have it as a boost monitor only. Do i need a specifically rated one?

Any advice or guesses if this is a workaround idea?

Cheers

I don't think tachos work the way you think they do- rather than measuring a varying voltage, they count the number of pulses from the ignition system on a petrol. For example on a 4-cylinder, you will have two pulses per revolution, and it works out the engine speed from that. You won't be able to adjust the gauge by a simple potentiometer. You'll need a gauge which is meant to be driven from the alternator pickup, which will be fully adjustable to match the vehicle.

confused here, presumably its over-reading??
If anything its vice versa you can use a petrol rev on a doozil you just dont get full range, doozil ones only usually go up to about 6K, petrol 10-12

There should be a way of adjusting it somwhere, as all cars vary on the alternator pulse to actual revs, it all depends on pulley sizes.

double check the connections, and any adjustment, after that id think it is possibly a faulty gauge.

Hi guys

Ive got to grips with how they work now, its a frequency divider Im after not a potentiometer. Like this one: -

DAKOTA DIGITAL UNIVERSAL DIESEL TACH ADAPTER ALTERNATOR INTERFACE DSL-1 NEW | eBay

I dont need this all singing and dancing model (not at that price) but cant find anyone else in the world who make a similar product?! If I could figure out what chips / divider was in the above Id make one myself for pence.......

Cheers

After much reading last night it appears many have fallen into the same mistake with Tacho installation, few have tried to have a go but no answers that I could find. As it stands the revs are sitting at 2200 on idle, when adjusted to the lowest setting, and increase at what appears to be 4x quicker then they should. If I can put a 4x divider in the line from the alternator then this should in theory set it to about right. Does that make sense? Ive seen lots of clock dividers for next to nothing but Im unsure as to which would work for the amp/voltage in the line? Many of the dividers work up to 2GHZ which I would expect to be more than enough.

Its in, wired up and useful for the boost gauge so if I can figure a simple inexpensive divider then I may as well have a dabble at the tacho side!!

Tim - the diesel ones work from alternator, the petrol from ignition so the division is wrong no matter which setting I put it on. Highest idles at 6k and lowest around 2200.

Cheers