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The ALDL (Assembly Line Diagnostic Link) is a 10-way socket which allows communication with the ECU.

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ALDL connector

Locating the ALDL

LHD Cars

For the standard LHD car, the ALDL can be found plugged into a square black female plastic housing. This housing is under the dashboard within the passenger's side footwell (right side). Similarly for those LHD with aircon.

RHD Cars

For the standard RHD, the ALDL is in a similar housing again located in the passenger's footwell (left side). It can easily be seen under the glovebox - here's a view looking under the glovebox of a non-aircon UK car. From the Service Notes drawing, it appears that it will be best found by opening the passenger glove box past the metal stop, exposing the wiring behind it however there is no need. The RHD with aircon location is slightly higher up and probably needs the glove box swung down however.

Diagnostic mode

If the diagnostic terminal (B) of the ALDL connector is grounded (terminal A) using the Lotus bridging tool T000T0909 {or similar - a paper clip works just as well and is cheaper and probably not on backorder) with the ignition on and the engine stopped, the ECU will enter the diagnostic mode:

(a) Display code 12 by flashing the CEL light on the dashboard (1 short flash, short pause and two short flashes). Code 12 continues to be displayed if no errors are stored in the ECU.

(b) If the ECU has trouble codes stored, code 12 will be displayed three times, followed by any Fault Codes and finally a code 12.

(c) All ECM controlled relays and solenoids except the fuel pump relay are energised.

(d) The Idle Air Control IAC valve is fully extended.

Diagnostic mode is also used for setting the CO pot and setting the base timing.

There is a practical demonstration on YouTube which takes you through the whole process and demonstrates how the code is displayed.

Communicating with the ECU

The dealer's diagnostic scanner (called a "Tech 1") can be plugged into the ALDL for digital fault code readout as well as "real time" diagnostics and sensor readings.

The good news is that now there is an alternative: ElanScan. This is a free program that does the same stuff as a Tech 1 with a laptop PC and an ALDL interface to connect the PC to the car. The program and information on making an interface can be found at

Ready made interfaces are available for either serial or USB ports at The USB opto interface is optically coupled to remove all electrical connections between the laptop and the car, thereby reducing the possibility of damage to either the laptop or ECU and allows the interface to be left permanently connected to the car safely. There is also a cheaper direct coupled USB interface available and USB to serial converters for those who no longer have serial ports (usually after upgrading their laptop).

See Also