Lucas RD 953066... what the heck is if for?

[headdamage]

I've had an ignition problem with my 1987 NAS Range Rover Classic. At this point I have replaced the distributor, ignition amp, cap, rotor, wires, condenser, and coil. The truck seems to be running well now but there is one odd thing that is bugging me. When I took it all apart there was a little black thing with male spade connectors on each end of it that was between the positive wire to the ignition coil and the positive terminal on the coil. It says "Lucas RD 953066" on it. It measures about 6.8k ohms resistance. If I put it back in place the truck with not run, no spark. If I leave it out and just hook the positive wire strait to the positive terminal on the coil the truck runs fine.

What the heck is "Lucas RD 953066" for and is there now a problem that I've left it out? I should also add that I had to change from a two terminal ignition amp to a three terminal ignition amp.

Thanks for any help.

[David Sparkes]

I Googled - lucas 953066 -

Reading this thread it appears to be an engine speed sense resistor.

http://www.landroversonly.com/forums/f8/tune-resistor-31283/

I've plucked the 'best post' from the thread.

From a great article on the 14CUX system (Rover 14CUX Hot Wire Mass Flow EFI: Service and Troubleshooting) here is what a tune resistor is:

Tune Resistor

A Tune Resistor was originally specified in the wire harness so that one ECU part number could serve multiple vehicle markets. (Multiple computer programs could be stored within the ECU, and the ECU could "decide" which one to use based on what resistance it sensed.) One leg of the Tune Resister was wired to terminal 5 of the ECU, and the other leg was spliced to ground.

At some point during 14CUX production, Rover apparently decided to eliminate the Tune Resistor. (According to Rover service literature, this started with VIN number LA451517.) It isn't clear from the literature whether any US-market 14CUX systems would be effected by having an incorrect or damaged tune resister. The example system we photographed and described for this article NEVER HAD A TUNE RESISTOR!

The following information on Tune Resistors is based on Rover service literature.

Visual inspection of wire color codes will allow you to quickly differentiate between resistors.

White 3900 Ohms USA and European vehicles with catalytic converters

Green 470 Ohms UK and European vehicles without catalytic converters

Yellow 910 Ohms Saudi vehicles (without catalytic converters.)

Red 180 Ohms Australia and "the rest of the world."

Note: all four types of Rover Tune Resistors were rated for 0.5 Watts.

When fitted, the Tune Resistor was connected to the ECU through terminal position 5, and it was also connected to terminal position 27 through a splice. If you wish to check the resistor, first disconnect power to the system, second disconnect the EFI Cable Harness Assembly from the ECU, and finally simply measure resistance (using an Ohmmeter) from pin 5 to pin 27 on the main ECU connector. (Resistance isn't polarity dependent.)

Here is the resistor I think Ian may have been talking about near the ECU:

When visually inspecting the EFI Cable Harness Assembly, one unusual component that's particularly conspicuous is the resistor that Rover included on the Engine Speed sense circuit. This resistor should NOT be confused with the Tune Resistor. This circuit is the only connection between the ignition system and the fuel injection system. A damaged or loosely connected resistor would likely result in an open circuit, leaving the ECU wondering about engine speed.

In our example system this resistor is marked "Lucas", "RD953066", "83630A", and "892Q". The resistor should provide about 6.8k Ohm resistance.

I suspect the resistor is there to limit the current flow under fault conditions, however, if the resistor OR ITS CONNECTIONS go High Resistance the Signal may be degraded so much that it is effectively lost.

It might be worthwhile to check the resistance of the wires that go to and from the resistor, one could be broken. (Your cure reads as though you have bypassed these wires with your fix).

HTH

[Pete The Biker]

This was discussed in a recent thread on another forum:-

http://www.lro.com/forum3/viewtopic.php?f=65&t=66634

Pete

[headdamage]

In my case it was fitted in a different location (positive side of the coil) and why will the truck now not run with it in place but runs great without it?

I have to think it has to do with the change from a 2 terminal ignition module to a 3 pin but I'd like to know for sure and should I be using a different resistor or some thing to do what ever this one had been doing?

[p76rangie]

There are basically two types of coils used in the older vehicles. One uses a resistor and one does not. So what coil did you buy? Does it say "use only with resistor" on it somewhere or does the model of coil have an "R" at the end of it. If so, it needs a resistor and you will burn it out if you run it without one. Basically the coil is designed to run with a power supply of around 9 volts. The resistor drops the cars 12 volts down to this 9 volts. The reason they did this is that there should be a second power lead going to the coil from the starter motor. When you are starting the car it provides a full 12 volts to the coil which causes the coil to put out a stronger spark so the motor will start easier. But if you run the coil at 12 volts all the time it will overheat. So they have a second wire going to the coil that only provides 9 volts for once the car is running. Land Rover tended to use a resistor wire rather than a resistor to drop the voltage. But these wires overheat over the years and people replace them with a 12 volt ignition sourced power lead and use a resistor. If you need a resistor you can go to any autoparts store and purchase a generic coil resistor. They are usually a white ceramic looking thing about 1 1/2 inches long and has a coiled up wire running between the terminals at either end.

So don't worry about the resistance in the resistor you have, just see what voltage comes out the coil side with the ignition on but not in the "start" position. Then measure the voltage to the coil with the key in the start position. The first should be around 9 volts and the second above 12 volts. If you are not getting the correct voltage on start, check the small wires down on the starter motor.

[Roverbo]

Now, let´s please keep these two subjects apart: The 6.8 k Ohm resistor and the Coil resistor. They are not related to each other in any way. p76rangie, you are probably right about the coil/resistor stuff, and this is maybe a thing to investigate, but the Lucas RD.. is a low-wattage, high resistant component and its only purpose is "to tell the ECU if the engine is running or not". The value of 6.8 kOhm is problably quite random, I guess the only point here is to secure the ignition circuit in case of a short in the loom, as the input on the ECU side is so high impedanced.

Headdamage, as I read your description, the resistor is connected to the ECU loom in one end, and to the positive on the coil in the other end. This will not give any signal to the ECU at all, as the positive is continiously "on". The ECU thinks that your engine is stalling, it shuts off everything. Try to connect to the "amplifier side" of the coil instead, this is where to get a pulsating signal. This is what the original schematic shows:

Regards,

Bo

[p76rangie]

Now, let´s please keep these two subjects apart: The 6.8 k Ohm resistor and the Coil resistor. They are not related to each other in any way. p76rangie, you are probably right about the coil/resistor stuff, and this is maybe a thing to investigate, but the Lucas RD.. is a low-wattage, high resistant component and its only purpose is "to tell the ECU if the engine is running or not". The value of 6.8 kOhm is problably quite random, I guess the only point here is to secure the ignition circuit in case of a short in the loom, as the input on the ECU side is so high impedanced.

Headdamage, as I read your description, the resistor is connected to the ECU loom in one end, and to the positive on the coil in the other end. This will not give any signal to the ECU at all, as the positive is continiously "on". The ECU thinks that your engine is stalling, it shuts off everything. Try to connect to the "amplifier side" of the coil instead, this is where to get a pulsating signal. This is what the original schematic shows:

Regards,

Bo

Maybe the question is whether he has changed anything in the coil wiring since it last ran. Maybe someone has just connected it to the positive side instead of the negative.

[SteveRK]

It will probably still work if fitted to the + side of the coil as i have checked mine when investigating an ongoing rough running problem with an oscilloscope and you get pulses inducting across to the positive supply. Obviously best to adhere to the correct connection though