Under the Bonnet – Fall 1990

Welcome to Under the Bonnet, our quarterly technical column dealing with the basic maintenance and repair of your British car. We’ll be covering topics here that have been the cause of recurrent problems and questions by customers as well as our own staff members. While much of this information may be rudimentary to old-time mechanics, we’ll be exploring various short-cuts as well as talking about tricks-of-the-trade not mentioned in manuals. If you’d like us to cover a particular topic, please write to: Under The Bonnet, 400 Rutherford St., Goleta, CA 93117.

(In this edition of Under The Bonnet. Eric Wilhelm, Research & Development ace, discusses the answers to some often-asked tech questions.)

Ballast Resistor Ignition Systems

Conventional ignition coils suffer the disadvantage of being designed to operate best at about 12 volts. Unfortunately, a 12 volt battery often produces as little as 7 volts when “run down” because of excess starter operation, especially in extreme cold. To produce optimum coil performance (and hence nice fat sparks at the spark plugs) under such adverse conditions, the “ballast resistor” or “ballasted coil” system was developed.

This system uses a coil which is designed to be most efficient at about 8 volts. For starting, full battery voltage is supplied! This makes this system as efficient at low battery voltage as a “conventional” coil is with the battery supplying a full 12 volts. (For any battery voltage above the coil’s design voltage, it’s even better—an “overboost” condition.)

However, an eight volt coil cannot be run continuously at 12 volts without over heating and falling. As soon as the starter switch is released, the coil no longer receives full battery voltage. It is then powered through the ballast resistor, which reduces the 12 volts (the generating system is now working) to the coil’s design voltage.

“The Holes Don’t Line Up…”

The above phrase is a complaint we often hear about parts that mount with multiple screws or bolts, such as fuel tank sending units, differential crown gears, oil pans and timing chain covers. This apparent problem can almost always be solved by first holding the part in place with one or two fasteners fitted loosely, allowing the part to move slightly. Fit the rest of the fasteners in like fashion. Only after all of the fasteners are in place should they be tightened.

Front Axle Ball Bearing Installation

Dealing with obsolete technology is something we as a society quickly forget how to do as a result, in the light of more recent developments, we often tend to overly complicate procedures that are really very simple. An example of this, which we often encounter, regards front axle ball bearings as found on MGT-series. MGA, early big Healeys, Sprite-Midgets and others: “Since the workshop manual doesn’t have any instructions on the proper procedure for tightening the axle nut, how can I tell if the bearings are too tight or too loose?” This question no doubt arises from familiarity with the later tapered roller bearings that require critical adjustment for which detailed instructions are required.

Since there is a spacer between the centers of the inner and outer ball bearing assemblies, and the outer races are fitted against shoulders in the bearing hub, there is no end-float adjustment possible. Simply assemble everything and tighten the axle nut to the next nearest cotter pin hole before inserting and locking the cotter pin. It’s really that easy.

Some front axle ball bearing assemblies are asymmetrical, and have one face marked “Thrust”. It is imperative that this side be installed toward the spacer. If the bearing is symmetrical and unmarked, it may be installed either way.

Overdrive Solenoid Operating Lever
Applicable for TR2-TR6 (thru 1972), all Big Healeys. and Jaguars (thru 1964).

Overdrive solenoids will suffer “meltdown” if the operating lever is not adjusted correctly. Refer to your workshop manual for details of the correct procedure for adjustment.

When the overdrive switch is engaged, both solenoid coils are energized, causing the plunger to be pulled sharply into the solenoid. When it hits the top of the solenoid bore, the plunger opens switch “A”, disconnecting the closing coil, leaving the holding coil to keep the plunger in the upward position.

If the overdrive unit’s operating lever is maladjusted so that the solenoid plunger cannot reach the switch plunger, both coils will remain energized as long as overdrive is selected. The closing coil will soon overheat to the point of failure, as it was designed to be in operation for only the fraction of a second it takes for the plunger to open the switch and remove the closing coil from the circuit.