decatted engine

[panic]

I remember when i decatted my 500 engine i had a new sport exhaust with no cats (from act), i've fixed a new bosch large (black plastic) airmeter and a new mark adams chip, there was a included a resistor (?) that had to be put in the ecu to 'bridge' the lambda signals that were gone...that was about 10 years ago and actually i dont remember much of that iussue but i well remember that the electrician made a 'flying' attachement somewhere around the ecu leads to work it out...if that attachement was faulty now, or even gone altogether, and the ecu didnt receive anymore that false lambda signal, what could happen to the engine? what kind of malfunctions could i have?
sorry for the long thread but i need to go back to that iussue now, i'm reviewing my engine faults and i think this very iussue could be one of the reasons of my pains...thanks

[spend]

I think you are talking about the 'tune resistor', there are several varieties and the one you should have installed tells the ECU to implement a map that does not use Lambda inputs. Its just a resistor in the ECU loom (along with the relays & fuses & diagnostic socket for the ECU).

Dave

[panic]

that's exactly what it is...
if that tune resistor switch is no long connected to the ecu, what are the possible malfunctions the engine could have?? thanks

[dwgriffith]

Here's the reply from mark adams taken from a forum post in 2005.
Everything you ever wanted to know about the Tune Resistor....

All 14CUX ECUs contain five sets of engine tune information. The tune to be used is selected by the choice if Tune Resistor. All 14CUX vehicles are fitted with a Tune Resistor, except certain North American Specification (NAS) vehicles. In some cases NAS vehicles may have provision for a Tune Resistor to be fitted (i.e. socket fitted to the loom in the normal place), whilst others have this resistor taped inside the wiring loom to prevent anyone interfering with it.

Always ensure that the correct Tune Resistor is fitted to the vehicle. If an interchangeable Tune Resistor is fitted, then it is located as follows. Follow the wiring loom back from the ECU plug. About twelve inches (30cm) away, a small group of connectors and relays emerge from the loom. Amongst them is a small blue plastic two-pin plug that connects to the Tune Resistor.

The Tune Resistor itself is located in a clear plastic heat-shrink tube, connected to a blue plastic plug by two wires. Tune resistors are identified by the colour of these connecting wires. There is considerable variation according to application so please check for the correct version for your vehicle. If the vehicle is modified then the supplier of the modified ECU software should have specified the correct tune resistor to use.

Colour Ohms Cat Common Application
Red 180 No Australian 3.9
Green 470 No Europe & UK 3.9 (or 3.5 Disco)
Yellow 910 No Gulf States 3.9, or Europe & UK 4.2
Blue 1800 Yes Gulf States 3.9, or Europe & UK 4.2
White 3900 Yes Europe & UK 3.9 (or 3.5 Disco)

If there is any doubt about the presence or value of a Tune Resistor then the simplest method of checking it is as follows. Disconnect the ECU plug, and measure resistance between pins 5 and 27. The table above gives a list of the possible values:

Note that certain NAS vehicles do not use Tune Resistors, even though there may be a facility to fit one. In these cases the ECU will always operate as if a White Tune Resistor were present, and Oxygen Sensors are mandatory. If a Tune Resistor is later fitted, then it will be ignored. This is a function of the software version used inside the ECU, so of course it may be modified by a software upgrade.

For those of you who wonder if there is some advantage to be gained by experimenting with the resistor value - the answer is no! It must assume one of five particular values if it does not then the system will detect a fault condition.

It is of particular importance to ensure that the choice of a cat or non-cat tune is correct.

If a non-cat tune is selected where cats are fitted, then the cats may be destroyed. This happens because a non-cat tune will exercise no control over the cruise fuel setting. If the mixture is excessively lean or too rich, this can cause very high hydrocarbon levels (unburnt fuel) in the exhaust. This in turn can be burnt in the cats, leading to severe overheating. Certain fault conditions that cause over-fuelling will be undetected, which can also lead to severe catalyst overheating (and even a fire). Any input from Oxygen/Lambda sensors will be ignored.

Where a cat tune is selected, but no oxygen sensors are fitted, the car will run terribly rich. Eventually this will cause a fault flag to be set within the ECU, thus forcing it to drop into limp-home mode.

Note that the default (or limp home) tune is a cat tune, and will therefore always cause rich running on a car not equipped with Oxygen/Lambda sensors. If no Tune Resistor is fitted then the default tune will be selected (other than NAS vehicles described above).

Should the Tune Resistor be changed or disturbed for any reason, the ECU should be reset afterwards by disconnecting it for ten seconds. Failure to do this may cause the ECU to select the default tune.

[panic]

Thanks DW, the best explanation ever found on this iussue...