Map switching resitor - wrong value! Have I hurt anything?

TwoSheds · Post by **TwoSheds** » Sun Jul 27, 2008 6:23 am

Hi,
I am just setting up my Range Rover 3.9 V8 dual-fuel with MJLJ v3 using MAP option and dual maps. My design for map switching takes it's signal from the lead that controls the LPG relay and hence switches entirely automatically from the petrol map to the gas map even when using auto-switching mode on the LPG controller.

However there is one point at which (due to wiring convenience) I have deviated from the suggested circuit and one point (due to a senior moment) at which I made a mistake:

Deviation
The aforementioned wire from the LPG control unit to the LPG relay shows +12v when the relay is on and 0v (and 5ohms to earth) when it's off. I have taken a single wire from this to the Option SW pin (through a resistor) and that is the sum of my wiring for the option switching. This appears to work ok on the bench - in fact the map switches as soon as the "Option SW" pin loses the +12v without needing it to be grounded. Is this OK?

Mistake
I have realised that I have used a 10Kohm resistor instead of the 1Kohm that is described as being critical!

, and I don't have a 1Kohm resistor (I am trying to avoid a 15 mile trip to buy one resistor, and the associated rewire)

So my questions are - Am I likely to cause any problems with my 'one wire' approach and will it be ok with a 10Kohm resistor instead of the 1Kohm one?

Sorry to be a pain - I would normally try to work these things out for myself, but I can't seem to get my head around how this switching works and why everyone (in other postings) talks of needing 5 volts, but then the suggested circuit uses 12...

Cheers,

TwoSheds
(One of which now sounds like a real V8 should - thanks to MJLJ!)

david jenkins · Post by **david jenkins** » Sun Jul 27, 2008 9:35 am

I seriously doubt that you have caused any damage - I believe that the resistor is only there to provide a current-limited voltage, and you are limiting it even more! the only effect may be that it doesn't switch reliably due to the input pin getting to a state where it is neither one voltage nor the other.

I know what you mean about buying 1 resistor though - a few pennies/cents for the resistor, a big wad of Pounds/Dollars for delivery (or petrol to get to the shop).

Post by **brentp** » Sun Jul 27, 2008 5:16 pm

As david says- the resistor you used is fine. It's more critical that the resistor actually be present rather than have that precise value. (it should be at least 1K)

It's recommended you switch the other pole to ground, otherwise the option input will be left 'floating' and may rapidly switch back and forth between states.

There are other approaches for wiring the option switch- if you only have a single pole switch you could add an additional pull-down resistor on the MJLJ side of the option switch:

Code: Select all

                         ---
                          |    SWITCH
                       --------                 
                                                1K
OPTION INPUT---------- *      *-------------/\/\/\/\----------+12V
                       |
                       |
                       |
                       |                    10K
                       |------------------/\/\/\/\----------- GND

ASCII art is fun!

TwoSheds · Post by **TwoSheds** » Sun Jul 27, 2008 5:53 pm

Ahh, but this (like wot I ave done) is simpler:

(gaining confidence now - am I about to fall?

)

Code: Select all

                                
                      10K       
 OPTION SW   -------/\/\/\/\----*
                                |         | SWITCH
                                |       -----
LPG SOLENOID -------------------*-------*   *-------- +12V
     |
     |
    GND

ASCII art is fun!

Hmm - Like coding in assembler... No thanks

TwoSheds

(One of which now runs like a dream
and the other is a diseasal)

910esprit · Post by **910esprit** » Tue Aug 12, 2008 5:51 pm

I have been using the MAP switch without a ground for Map 1 however i have an issue with changing the outputs in Map1, thinking this could be the reason i want to add the ground to the input for Map 1.

I have a single pole switch and do not really want to use a relay and the switch is perfect in the dash, so can anyone confirm that if a put a 10k resistor between the input and 0v (as shown in the picture) will this work.

Please ignore the wire replacing the track this was due to a previous error where i put 12v with out the 1k resistor onto the input

An experience i do not want to repeat hence the question.

david jenkins · Post by **david jenkins** » Tue Aug 12, 2008 7:16 pm

Mine goes like this:

Code: Select all

 
OPTION INPUT ----------------/\/\/\/\/--------------------- 12v
                         |
                         |
                         |   |
                             |
                             |----|
                             |
                         |   |
                         |
                        CHASSIS

The idea is that with the switch open the input is high, and with it shut it's low. I can't remember what value the resistor is - 2.2k rings a bell, but even when connected across from 12v to chassis the power dissipated is quite low.

910esprit · Post by **910esprit** » Tue Aug 12, 2008 8:35 pm

I have used the 12v from the TPS ref pin by putting a 1K resistor in L1.

I think my theory will work but wanted a second opinion before causing too much damage

910esprit · Post by **910esprit** » Sat Aug 16, 2008 5:38 pm

910esprit wrote:I have used the 12v from the TPS ref pin by putting a 1K resistor in L1.

I think my theory will work but wanted a second opinion before causing too much damage

I can now confirm that installing a 10k resistor between the option switch and 0V allows you to use a single pole switch