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Thread: 14CUX and Tune Resistors

  1. #1
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    14CUX and Tune Resistors

    I have been doing some reading on tuning the 14CUX and couldn't really find the information I am wanting on the tune resistors fitted to these ECU's.

    There are 4 different resistors fitted depending on the market the car is destined for and then one without a tune resistor for USA. I can find the values and the markets for each resistor, but what I can't find is the effect and result of each value of resistance.

    The two resistors that I am most interested in are ours and the Saudi one. There is quite a big difference in values for each market, and I was wondering what the difference in performance was between the Saudi tune and our tune. Our resistor is red, and as you go to the green (Europe and UK 3.9L no cat) and yellow (Saudi 3.9L and Europe and UK 4.2L no cat), the resistance increases. I'm thinking the fuel increases as the resistance goes up as the tune it defaults to if no resistor is present is the highest value cat tune.

    Another thing I was thinking of doing to get a bit more adjustability out of the ECU is to fit a couple of O2 sensors and use a resistor to suit so that the ECU can trim the fuel more finely. At the moment I am running a 3.9L hotwire system on my 89 RRC 3.5L EFI. I have changed over the whole intake and am using the injectors and harness from the 94 RRC 3.9L. So basically I want the ECU to choose the map for sensors but no cats. Apparently I can wire in the O2 sensors and select the appropriate tune.

    Does anyone have any info on how each resistor affects the tune? I know that the cat tune runs a bit richer, but that's about it.

  2. #2
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    AFAIK, the resistor is what selects a tune in the 14CUX, hence the name "tune resistor". It doesn't have a direct effect on fuelling, only which map the 14CUX reads, and this depends on the emission requirements of the particular market.

    AFAIR I fitted a UK tune +cat to mine when I fitted the O2 sensors as again AFAIR this tune resistor activates the input from the O2 sensors.

    Regards Philip A

    .

  3. #3
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    Yes. that is correct, although from what I have read, the resistor used does affect fuelling. A cat tune runs richer than a non cat tune to prevent leaning out too much at cruise so the cats don't melt. Mine runs pretty rich now, the fact that it came from a 94 RRC 3.9L that wasn't running cats or O2 sensors, it should be the red resistor. I haven't actually looked yet, but if it is a green or yellow one, then it tells me it was an imported car. Some things that I used off that car weren't quite right according to what it should have been for the Australian market.

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  5. #5
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    I know what I need to do to tune the ECU, that's not my question. I want to know what the tune resistors actually do. I think I have an idea, but would like someone who actually knows to confirm it.

  6. #6
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    Quote Originally Posted by Not_An_Abba_Fan View Post
    I know what I need to do to tune the ECU, that's not my question. I want to know what the tune resistors actually do. I think I have an idea, but would like someone who actually knows to confirm it.
    Not sure if the following is of any use to you:

    Below is a description of the three possible scenarios of CAT and Lambda sensors and which Tune resistor to use in each case which is relevant to the Hotwire fuel injection system only.

    Lambda sensors with CATs
    A white tune resistor is required to run Lambda sensors with a CAT. This will enable the ECU to make adjustments to its fuel trim (be adaptive) and also to improve its diagnostic ability, as it is able to see what effect changes are having on the running of the engine.

    Lambda sensors options
    There are two options here, and generally option one is the best to go with.

    Option 1 - For the ECU to be able to make adjustments to its fuel trim (be adaptive) you still need to run Lambda sensors even if you do not have a CAT in your exhaust system, this means you require the White tune resistor. Without this Lambda feedback to the ECU the self diagnostic capability of the ECU is seriously compromised.

    Option 2 - If you have a vehicle that has an auto gearbox and a high lift cam, then it is sometimes best to run without the Lambda sensors, which means you need the Green tune resistor. The reason for this is that under these circumstances the engine will struggle to idle if it is trying to adjust itself
    from the Lambda sensors at idle. However, this is very rare, so it is always best to try running with the Lambda first.

    Early EFi system with no CAT or Lambda sensors.
    The Green tune resistor is needed here otherwise the ECU will show a fault code indicating faulty Lambda sensors, because it would be looking for them. Unfortunately, in this case the ECU loses its ability to be adaptive (to make adjustments to its fuel trim), and its diagnostic system will be compromised due to the inability to see what effect adjustments are having on the engine.

    If your wiring loom is the early style that does not have Lambda sensors, then it is possible to add these, which is a relatively easy modification.

  7. #7
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    I have read all this and know what each tune resistor is for, but I was really wanting to know what effect the tune resistor has. IE: what is the difference in each of these tunes? Not what each tune does, but what values does adding more resistance actually change?

    I know that playing with the resistance is not effective as the ECU needs to read a certain value to pick the tune, the resistance value of each one has to be correct.

    I think to put it plainly, has anyone read each tune and compared them to see what is different? Has anyone compared the fuel and spark tables?

    There are 2 tunes that are used without sensors and cats, one for us and one for the Arab states. Does anyone know what the difference is between the 2?

    I may have to either get some tuning hardware to look myself, or try the other resistor and see how it affects the car.

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    Saudi Vs Oz

    Did anyone give an answer to how the gulf states non-cat fuel map (yellow) compares with Oz (red) and UK (green)?

  9. #9
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    Saudi vs Oz vs UK

    Did anyone find out how the Gulf states fuel programme (yellow) compares with Oz (red) and UK (green)?

  10. #10
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    could it be an outside temp change?
    "How long since you've visited The Good Oil?"

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