Testing and Repairing A.C. Mechanical Fuel Pumps

Identification, Operation, Diagnosis, Overhaul and Testing of Series A, B, M, Y and T Pumps

This article covers the five early A.C. mechanical fuel pump series commonly identified as A, B, M, Y and T. Later A.C. and AC-Delco pump types may differ in their valve retention, filter arrangement, linkage and assembly procedure.

Please note the following article is based on J.N. Queenborough, “Testing and Repairing the A.C. Fuel Pump”, in Motor Repair and Overhauling, Volume IV: Electrical and Accessory Equipment, Third Edition (Revised), George Newnes Ltd, London, c. 1940s. Some procedures reflect period workshop practice; modern safety guidelines should take precedent.

Modern Workshop Safety

Work in a well-ventilated area, away from naked flames, heaters, smoking materials, sparks and other ignition sources. Allow the engine to cool before disconnecting any fuel pipe.

Catch drained fuel in a suitable, clearly labelled container. Wear eye protection and the personal protective equipment specified in relevant fuel or cleaning-product safety data sheets.

Clean reusable metal parts in a purpose-made aqueous parts washer or enclosed recirculating cleaning unit, using a product suitable for the component materials. Do not use petrol as a cleaning fluid or clean components in an open bucket of solvent.

Do not immerse diaphragms, rubber components, cork or fibre gaskets, painted parts or bonded assemblies unless the cleaner manufacturer specifically confirms compatibility. Renew doubtful diaphragms, seals and gaskets rather than attempting to reclaim them.

Retain used fuel, cleaning fluid and contaminated absorbent material for disposal through an appropriate waste service. Do not pour fuel or cleaner into drains, onto the ground or into household waste.

Modern Fuel Compatibility Note

During overhaul, use a newly manufactured diaphragm, seals, gaskets and flexible fuel hose that the supplier confirms are compatible with the fuel to be used. Do not assume that old-stock rubber components remain serviceable simply because they are unused.

Owners uncertain about a vehicle’s compatibility with modern petrol formulations should consult the vehicle manufacturer’s information and current official fuel-compatibility guidance before selecting a fuel.

Introduction

A.C. mechanical fuel pumps steadily evolved from their original form, and any one of the five early series may be encountered by the motor mechanic or enthusiast. It is a tribute to their initial design that their essential operating principle remains unchanged.

Fig. 1 – Sectional Diagram of the latest A.C. Fuel Pump, Series T

Fig. 2 – Sectional Diagram of an early A.C. Fuel Pump, Series B

Identifying the Early A.C. Pump Series

As stated, the same basic operating principle applies to all five series, although their designs differ. They can be distinguished by the following features:

Identification must be completed before parts are ordered. Pumps that appear broadly similar may use different rocker arms, diaphragm springs, valves, gaskets and diaphragm protectors. Fitting components on appearance alone may result in insufficient rocker arm movement, excessive fuel pressure, noise or mechanical failure.

Check the Fuel System Before Condemning the Pump

Before dismantling a suspected fuel pump, examine the remainder of the fuel system. Many symptoms attributed to the pump originate elsewhere.

First, confirm that there is fuel in the tank. Then inspect the fuel pipes from the tank to the pump and from the pump to the carburettor. A bent, flattened or internally obstructed pipe restricts flow, while a cracked pipe or loose union may admit air without showing an obvious petrol leak. Pipe fittings should be checked carefully but should not be tightened so severely that the threads or castings are damaged.

Vapour Lock

Vapour lock is another possible cause of intermittent delivery. This may occur if a fuel pipe system is positioned too close to the exhaust system or if the pump receives insufficient cooling. In either case, correct the pipe position or fit a suitable heat shield where appropriate.

Filter Cover

The filter assembly must also be examined. On Series M, Y and T pumps, a loose filter-cover nut, damaged cork gasket or obstructed filter screen may allow air to enter or restrict fuel flow. On Series A and B pumps, if the glass bowl is loose the thumb-screw nut should be tightened to ensure an airtight seal. The bowl should not be tightened excessively merely to compensate for a defective gasket or distorted cover. If the bowl is secure, inspect the filter screen for any debris and clean if necessary.

Fuel Leakage Around the Diaphragm

Apparent leakage around the edge of the diaphragm should be investigated carefully. Fuel from a loose pipe fitting can run down the casting and create the impression that the diaphragm joint is leaking. Where the joint itself is loose, its screws should be tightened gradually and alternately rather than one after another around the circumference.

Flooding of the Carburettor

Carburettor flooding is not by itself proof of excessive pump pressure. Air admitted through leaking pipe joints or a defective filter gasket can aggravate the condition. The carburettor should therefore be examined and, where necessary, adjusted or cleaned before the pump is blamed.

Noisy Pump

A noisy pump may contain a worn, loose or broken component, but noises transmitted through the engine can be misleading. Confirm that the noise originates in the pump rather than elsewhere in the engine before dismantling. Do not run the engine with the pump removed unless the vehicle manufacturer’s workshop procedure specifically permits it and provides instructions for securing fuel pipes, blanking the mounting aperture and controlling oil loss.

Fig. 3 – To Dismantle the Pump

Attach the pump to an A.C. Test Stand and mark the position of the top cover relative to the body casting

Mark the Position of the Top Cover

Clean the exterior before opening the pump and attach it to the stand. The position of the top cover relative to the body casting should be marked, either by a centre-punch or a file. These marks will allow the castings to be returned to their original relative positions during reassembly. This part is important and, if forgotten, can cause loss of time when reassembling.

Remove the filter cover, its securing screw or nut, the gasket and the filter. Keep these components in their correct order. Remove the six screws joining the upper and lower castings and separate the two halves.

Fig. 5 – To Dismantle the Pump

Detach the top casting by taking out the six

fixing screws holding it to the body

Fig. 6 – To Dismantle the Pump

Remove the valve retainer screws, then lift out

the retainer, valves and springs

To detach the diaphragm, press the diaphragm and pull rod assembly down against its spring and rotate it approximately a quarter-turn to disengage the flattened end of the pull rod from the slot in the connecting link. Detach the pump from the stand and remove the spring clip from one end of the rocker arm pin and drive the pin out using a drift rod. This releases the rocker arm, connecting link, rocker arm spring and, where fitted, the two spacing washers.

Fig. 8 – To Dismantle the Pump

Detach pump from test stand, take off spring clip from one end of

rocker arm pin, and dive out the pin by means of a drift rod. This will

release the rocker arm, link, rocker arm spring and (if fitted) two spacing washers

Clean and Examine All Parts

Once clean and dry, inspect every component for cracks, damage, distortion, corrosion and excessive wear. Renew all damaged or badly worn parts, together with all gaskets disturbed during dismantling.

Fig. 9 – To Assemble Rocker Arm and Link Assembly

Gather together the parts shown

A temporary drift rod makes assembly easier. Insert it through one side of the rocker arm-pin hole, fit the first spacing washer where one is used, and position the connecting link. Whilst still holding the drift rod, fit the rocker arm spring, then introduce the rocker arm between the faces of the link. The locating projection, or pip, on the rocker arm must engage correctly with the end of the spring.

Fig. 11 – To Assemble Rocker Arm and Link Assembly

Position the rocker arm spring

Fig. 12 – To Assemble Rocker Arm and Link Assembly

Fit the rocker arm and second spacing washer to the drift rod,

then drive out the rod by means of the rocker arm pin. Fit a

retaining clip to each end of the pin

Fig. 13 – Order of Replacement of the Valve Assembly Parts

Fig. 14 – To Assemble Valve Parts

Fit valve-retainer plate with its gasket underneath

and secure it with the three countersunk screws

Gumminess – A Possible Cause of Valve Trouble

A sticky or “gummy” valve does not necessarily indicate mechanical wear. Some cases are caused by deposits from impure fuel or certain upper-cylinder lubricants. If gummy deposits are found, investigate the fuel or any additives being used so that the condition does not recur.

Fig. 15 – To Assemble Filter Unit

Assemble the parts in the order shown, taking care not to damage

the gasket

Fig. 16 – Completing the Assembly of the Pump

Attach the pump body to the test stand, fit the diaphragm

spring and refit the diaphragm-and-pull-rod assembly by

pressing it down and turning it through a quarter turn

Fig. 17 – Completing Assembly of the Pump

Fit the top cover and screw in the six cover screws finger tight.

Hold the rocker arm in the direction specified in the table and

tighten the screws alternately to pre-flex the diaphragm

The correct direction of rocker arm movement is model-specific:

Fig. 18 – Testing the Pump

The pump connected to the test stand

Pressure Table

The figures below are reproduced from period service information. Confirm the applicable pressure against the relevant vehicle or pump manufacturer’s workshop information before testing or adjusting.

Additional Checks for Series A and B Pumps

On Series A and B pumps, check the upper cover for distortion by placing the filter bowl loosely in position without its gasket checking whether the bowl rocks. If there is any distortion the cover should be renewed.

Damaged glass bowls should always be replaced. Metal bowls were available as an alternative for some applications.

These checks supplement the normal overhaul procedure and should not automatically be applied to other pump series.

Routine Maintenance

A.C. mechanical pumps require comparatively little routine attention, although the sediment chamber should be inspected and cleaned periodically.

Where a glass bowl is fitted, sediment can be seen directly. Stop the engine before removing the bowl or cover. Remove the filter gauze carefully so that accumulated grit is not allowed to fall into the pump chamber.

On pumps with the sediment chamber beneath the top cover, the small drain screw below the inlet connection can be removed to release the fuel. Remove any remaining residue with a clean, lint-free cloth secured to a suitable non-metallic tool, taking care not to leave fibres or debris in the chamber. Replace the drain screw, cleaned gauze and cover, using a sound cork gasket. Fit a new gasket whenever the cover or sediment bowl is removed.

Conclusion

The A.C. mechanical fuel pump is a straightforward unit, but it is not tolerant of careless assembly. Most unsuccessful repairs can be traced to one of a small number of errors. These include condemning the pump before checking the rest of the fuel system, fitting parts intended for another series or model, allowing dirt into the valves, reversing a valve, reusing damaged gaskets, installing the wrong spring or failing to flex the diaphragm before tightening the body screws.

Correct identification, absolute cleanliness, correct valve assembly, model-specific diaphragm flexing and a final pressure-retention test are the essentials of a reliable overhaul.

Troubleshooting Summary

Please note

All articles and guides are provided only for the interest of classic and vintage car owners. Much of the information presented will be old and it may be that since the period source material was created, better solutions have been found. A certain level of mechanical and/or electrical knowledge will be required in undertaking work as described in these articles, and anyone unsure of their abilities is advised to seek professional assistance. 
Vintage Supplies Ltd cannot be held responsible for any breakages and injuries that may occur, while working on a vehicle following any guides provided. With older vehicles being exempt from MOT checks, responsibility for vehicle safety and legality rests entirely with the owner.