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  Electronic Ignition

My vehicle has the factory fitted electronic ignition system. It replaces the old fashioned distributor's contact breaker points with an electronic switch, which doesn't burn and so doesn't alter the timing as the points wear.

The distributor is similar to the old mechanical type, with an upper and lower shaft coupled by a centrifugal "bob-weight" mechanism. Mounted on the upper shaft is a reluctor with a lobe for each cylinder. The air-gap between the reluctor and the pick-up is critical (see the manual for tolerances) and must be checked with a non-ferrous (eg brass) feeler-gauge because of the permanent magnet.

As the reluctor rotates past the permanently magnetised pick-up sensor it produces a series of pulses which are used to trigger the amplifier module, which is mounted on the left hand inner wing below the coil. The amplifier switches the current through the coil primary, in the same way as does the conventional contact breaker, recognising the sudden voltage drop (red line) as the trigger point.

The amplifier is a Lucas 2.C.E constant energy unit, otherwise known as a Lucas DAB104.

Inside are a number of components:-

black module centre left - electronic module bearing a label which states "CONTAINS BERYLLIA DO NOT OPEN" for obvious reasons
under the label is the General Motors logo and part number 117 5005

(aftermarket spares for this module are available)

silver cylinder bottom right - suppression capacitor

white plastic inverted "V" bottom centre - coil or transformer for tachometer wiring (not used in the Range Rover, but I have no knowledge as to whether it can safely be removed)

round thing in top left corner - possibly a suppressor or diode for the tachometer circuit

The pictures are of the original unit fitted to my car, which bears the date code "2385", indicating that it was manufactured in week 23 (early June) 1985, only a few days before the vehicle was assembled.

The spark is timed to occur when the rotor arm is opposite the appropriate plug lead terminal in the distributor cap. Because the ignition timing advances and retards with varying conditions of the engine and can also be adjusted by rotation of the distributor body on the engine, it is necessary for this to be possible through a small arc of rotation, as shown in the diagram. Of course the arc needs to be small enough to prevent cross-firing with the wrong cylinder.

With increasing engine speed, the centrifugal "bob-weights" provide dynamic advance by rotating the upper shaft clockwise on the lower. During this process, the relationship of the reluctor and the rotor arm remains constant, so the rotor arm continues to align with the plug lead terminal in the distributor cap when the spark occurs.

There is a vacuum advance module which, with increased throttle opening, rotates the mounting of the pick-up anti-clockwise on the base plate, generating the spark at an earlier position of the rotor arm relative to the cap. This requires that the point of the rotor arm be wide enough to remain in contact with the cap terminal.

In the case of my 17D series "detoxed" engine there is also a vacuum retard (not fitted to later engines), which comes into play on over-run against a closed throttle. This has the opposite effect by rotating the pick-up mounting clockwise and generating the spark at a later position of the rotor arm relative to the cap terminal. Again, the rotor arm tip has to be wide enough to remain in contact with the cap terminal.

So as a guide when setting up the distributor on the engine at cylinder 1 TDC (read the manual to see how to do this), the rotor tip should align more or less exactly with the cap terminal.


An ignition failure after 29 years was initially attributed to a problem in the amplifier, but extensive testing showed no fault. Even so, the amplifier was replaced and all seemed well.

A second failure occurred one year later, when I found that the original distributor to amplifier link-lead (part number PRC4503) had become badly burnt as a result of having been factory-fitted under the alternator (mine is mounted on the left side of the engine, because of the factory-fitted aircon) and near the exhaust manifold. Apparently this is a well known fault.

It is a twin-core screened lead with a molded-on connector at the distributor end and non-reversible spade-connectors at the amplifier end. It is an easy fix to replace and re-route over the top of the alternator.


On later Range Rovers the amplifier module was miniaturised and built onto the side of the distributor body. This type of distributor has superceded the original pattern on the spares market. The later distributor uses a different link-lead to connect the distributor directly to the coil (red wire to coil +ve and ignition live, black wire to coil -ve). Remember to order the "O" ring separately, as it doesn't come with the distributor.

 
     

 early type ETC4715

  late type ECT4715P
note that it is supplied without the "O" ring seal

However when I attempted to fit the later type distributor to my car I found that the amplifier module fouls against the PAS pump mountings, which restrict rotation of the distributor body and thus prevent the correct timing being set. I have not found a fix for this yet. It is not a problem on vehicles with the later type PAS pump, which uses different mountings.


the top alternator bracket has been removed temporarily for access

H&H Ignition Solutions can move the later type amplifier to fit between the vacuum module and the 6 o'clock cap clip, but that would not help in my case because the amplifier hangs down too far to clear the water pump.

They can recondition the early type distributor, including replacement of the pick-up sensor.


After having a new pick-up sensor fitted, I could not get the engine to run properly. It was possible to correctly set the timing statically, in which position (with the engine stopped at TDC) the rotor arm pointed midway between plug terminals 1 and 8. Also, the distributor body was rotated clockwise (ie retarded) from the original position.

As soon as the vacuum retard pipe was connected the engine would barely run and then only very roughly. Attempts to adjust the ignition timing by rotation of the distributor body proved only to make matters worse. Connecting the vacuum advance pipe made no difference.

Further investigation showed that the seal on the retard side of the vacuum module had failed, so it was replaced. There was no improvement.

Much deliberation and wide consultation failed to come up with either a cause or solution. The link-lead was new and both the amplifier and coil were obviously generating the spark. All the indications were of a problem within the distributor, but a further complete strip and rebuild showed nothing amiss. The wiring of the new pick-up module was shown to be physically correct when compared with the original.

After many days (and sleepless nights!) of thought, I came up with the hypothesis that the pick-up leads might have been wrongly connected from a functional perspective. Testing by using break-out leads at the amplifier end of the link-lead proved the hypothesis, meaning that the new pick-up was wired the other way round internally.

In this situation the spark is still triggered by the voltage drop generated by the reluctor and pick-up. However, because the waveform is inverted this occurs prematurely, ie the ignition becomes significantly advanced.

This can be overcome at the static timing point by an alternative position of the distributor body (as observed). The engine will run with the vacuum pipes disconnected because the spark actually occurs when the rotor arm is within the functional arc of the plug cap terminal, albeit at the most clockwise (retarded) extreme.

Once the vacuum retard pipe is connected, the spark occurs when the rotor arm has moved further clockwise beyond that arc and can no longer connect to the correct spark plug lead. Adjusting the advance/retard position of the distributor body on the engine cannot overcome this and only serves to move the spark to a timimg point where the engine will no longer run at all.

Rewiring of the new pick-up sensor lead connections has solved the problem. I hope this sad story may be helpful to anyone else who is unfortunate enough to encounter the same difficulties.


This ignition system is similar to the Mobelec Magnum electronic ignition system which I retrofitted to one of my earlier cars. The basic kit retained the original contact breaker points.

Click on the picture below if you want to read the fitting instructions.

 

The optional fitting kit (this one is the 144 B4, for an anti-clockwise rotation Ducellier distributor) replaced the contact breaker points with a magnetic pick-up, similar to the Range Rover system.