2 potentiometers, one circuit

I took some high res pics of the board for you. They are quite large for detail so I'll link them instead of embed. Over all view https://img.photobucket.com/albums/v446/micrors4racer/DSC00057.jpg Top part https://img.photobucket.com/albums/v446/micrors4racer/DSC00062.jpg Bottom https://img.photobucket.com/albums/v446/micrors4racer/DSC00061.jpg Left side with the pot wires https://img.photobucket.com/albums/v446/micrors4racer/DSC00060.jpg Detail of left side https://img.photobucket.com/albums/v446/micrors4racer/DSC00059.jpg There seems to be some potting compound applied to the board to prevent components from vibrating loose inside the tachomter it was install in.

You may be able to do it with a SPDT relay as suggested.  However, paralleling a second potentiometer across the original one may cause unwanted irregularities in the control unit. I was suggesting the 3 pole device so there will be only one pot connected to the unit at any given time.

How would it be wired up with a standard relay just incase I can't find a tpdt? I'm going to look for one now but I need a back up incase I can't find one. Have a track day on Sunday that I want to test this out in

You could use 3 separate SPDT relays to make a Triple Pole Double Throw device.  If you want to try it with a single relay, connect the left most terminal of the second pot to the leftmost terminal of the original pot. Connect the rightmost connections as well.  Cut the center wire between the pot and device.  Connect the device to terminal 30 of the relay.  Connect the original pots center tap to terminal 87A or the relay.  Connect the center terminal of the second pot to terminal 87 of the relay.  Connect power to terminal 86 of the relay.  Connect terminal 85 to the parking brake wire.

With the parking brake off, the pot that is connected to terminal 87A will be the one in use.  When the parking brake is engaged, the control that is connected to terminal 87 is going to be the active one.

To use the 3 relays to make the triple pole device, connect it as follows.  One relay per wire color.  Terminal 30 to device.  Terminal 87A to the pot that is active when the brake is off.  Terminal 87 to the pot that is to be active when the brake is engaged.  Power to terminal 86 of all 3 relays.  Parking brake wire to 85 of all 3 relays.

i am an idiot wrote: ...paralleling a second potentiometer across the original one may cause unwanted irregularities in the control unit...

Sorry Idiot, but I strongly disagree (given common liklihoods). Any relay should have similar irregularities through noise etc. The impacts of three break before make contacts for a control pot? But a single SPST is 1 or 2 wires as opposed 3 or 6. And there is no break before make issue. And what am I missing? I thought a second pot/resistance was to be switched in somehow for the second RPM cut setting. Hence either changing over to another pot (your method?) else changing the pot setting (my method). Not that I've looked at the pics yet.... Nor drawn a simple pic.... This is the quickie I'm having before picking up my girlfriend (after I vote at our federal elections). I love our freedom of speach & democracy - voting is compulsory (or else!).

Ok - downloaded the pics (NICE!). Can you post one of the trackside? Just one like DSC00057.jpg should do (but from the opposite side!). The CS289 is just a tacho chip. The LM358 is a (quad) Op-Amp that will be doing the threshold comparisons.... Gotta love these primitive discrete circuits (as in no PICs etc!)

Here is the underside, dark outside now sorry. https://img.photobucket.com/albums/v446/micrors4racer/DSC00065.jpg

What is the value of the original potentiometer?  Paralleling a second one of the same value will put twice the load on the device.  If you can find 2 pots of around twiice the resistance as the original, there will be absolutely no channce of any damage.  Both pots paralleled will yield the same stress on internal components.  The pot is used as a potentiometer and not a variable resistor, so operation will be the same regardless of value.  I have not looked at the pictures or pulled any datasheets.  But this could be the same as running a 4 ohm capable amp at 2 ohms.

The pot is a B10K

i am an idiot wrote: Paralleling a second one {.....} will ... load .. the device

Stop and think... It depend where it is connected. If between the yellow & green (low voltage sweep) it has no effect (since this is a 3-wire pot, not 2). And that was my thought. However, looking at the tracks, it seem the pot adjusts the tacho chip and not the op-amps - hence it seems like a scale adjustment (but that would effect the meter out of used). As a rule, it doesn't matter what the 2nd parallel pot does - it merely provides the same resistance that the original pot can be adjusted to.