Volvo Owners Club Forum - How to keep you distributor in good nick

HOW TO CARRY OUT A SMALL SERVICE ON A B18 AND B20 DISTRIBUTOR

Here’s how to keep your Amazon or 1800 distributor in good health

The standard system in these cars is completely similar to any other ‘60’s and ‘70’s car with Bosch ignition. Service parts are readily available and are inexpensive. The system will be instantly recognised and easily worked on by any technician. It is easy to work on, but does require a lot more maintenance than modern car ignition systems. When in tune it works extremely well. When it’s not you quickly notice poor running, a drop in performance and increased fuel consumption. It is easy to consider faults with other components or spend unnecessary money and time when the cause is normal wear and tear on replaceable components. At times, complete ignition systems are upgraded or replaced unnecessarily

This is a basic explanation of the components of a Bosch distributor and the photos are of a reproduction air-cooled VW distributor. Please note that the distributor fitted to fuel injection cars is different to the type fitted to carburettor cars as described here

Cap

There are 5 posts on the exterior of the cap. The middle post is connected to the coil with an HT lead. The 4 outer posts are connected to the spark plugs by HT leads. On the inside of the cap are 4 pins and a central, spring loaded carbon brush

It is important to keep the inside of the cap clean and can be cleaned with most solvent based cleaners including electrical connector cleaner, carburettor cleaner and even brake cleaner, but must be completely dry before replacing and put into operation

http://i869.photobucket.com/albums/a...ervice/020.jpg

A replacement cap is extremely inexpensive and it is surprising how a small amount of wear or build up of dirt on the pins can cause problems. It is the same cap as used on most Bosch distributors. Check for cracks and deterioration in the plastic as even the smallest of imperfections can let in moisture

Underneath the cap might still exist a black disc which is a further condensation trap. It was fitted to some Bosch distributors, but quickly becomes brittle and bits can end up inside the distributor so has usually been discarded and not replaced. The distributor is well situated that it does not frequently suffer from getting water logged

It is impossible to put the cap on incorrectly as there is a notch that aligns with an indent in the distributor body

http://i869.photobucket.com/albums/a...ervice/022.jpg

Rotor Arm

Under the cap is the rotor arm. This is attached to the distributor shaft which is driven off of the crankshaft and runs at half the engine speed. The rotor arm distributes the spark to each of the spark plugs. It is connected to the cap by the spring loaded carbon brush. On the outer edge of the rotor is a metal tip which nearly touches the 4 pins on the inside of the cap as it rotates

Rotor arms don’t wear out quickly, but can cause misfiring and uneven running when there is a build up of corrosion or dirt on the terminal. It can be cleaned up easily, but is inexpensive to replace and is the same as used on most Bosch distributors

Points

This is the single most problematic component as it wears and deteriorates. However, they are very easy to replace and are well stocked in most components factors as well as being cheap

There are variations in types in Amazons and 1800’s. B18A’s have 2 piece points that are a little more difficult to get hold of nowadays. B18B’s and B20A’s have points that face one way and B20B’s have points that face the other. Both types are common parts to many cars with Bosch ignition

The points open and close as they rub against the cam on the distributor shaft by a rubbing block. When the rubbing block is “off-cam”, the points are closed and as they are wired to the live feed of the coil, the current goes through the points, and is earthed via the distributor body into the engine block. As the distributor shaft rotates, the rubbing block goes “on-cam” and the points open, in turn closing the earth. Instead, the current goes via the coil which builds up a very high voltage. This enters the top of the distributor, is distributed inside the cap via the rotor arm whilst jumping the gap between the rotor arm and the pin, exits to the spark plug where it jumps the gap causing a spark

When the points gap opens and closes, there exists a very small arc which eventually causes a pit on one surface and a peak on the other. Eventually, the surfaces go from a shiny, domed one to a flat pitted and rough one

To replace the points:

1. Undo the screw which secures the points to the points base plate and disconnect the lead

http://i869.photobucket.com/albums/a...ervice/015.jpg

2. Take the points out of the distributor and replace with new. Do up the screw to reasonably small tension and insert a flat-headed screwdriver between the two raised notches in the base plate and the inverted notch in the bottom part of the distributor. Turn it one way to open the points gap and the other to reduce it

3. To turn the enigine, use a spanner or socket on the crankshaft pulley so that the rubbing block of the points is at the highest point of one of the 4 cams

4. Now you need to use 2 feeler gauges on a “go/no-go” basis. The recommended points gap is 0.4mm – 0.5mm. In other words, the 0.4mm gap should not fit but the 0.5mm is slack. Once this is achieved, do up the screw. It is wise to do a subsequent check to ensure that you have not accidentally altered the gap whilst doing up the screw. Reconnect the lead

http://i869.photobucket.com/albums/a...ervice/014.jpg

The rubbing block also wears out, so lubricate the cam with a smear of grease. As the rubbing block wears as it rubs against the cam, the ignition timing alters resulting in pinking and poor running

Too large a gap results in insufficient time for the coil to build up energy. Too small a gap can result in misfiring and a weak spark because too much current is going to earth. As the points surfaces deteriorate, the points gap reduces. With deteriorated surfaces, it is impossible to get a proper adjustment. There is no point in trying to renovate the surfaces when replacements are so cheap and filing them takes off some of the coating thus causing the rate of pitting even faster

Condenser/Capacitor

The condenser is a type of energy storage device which is attached in parallel to the points. It prevents the high voltage in the system jumping across the points gap thus reducing the rate of deterioration of the points surfaces
There’s no way of testing the condenser, but if the points are deteriorating very quickly, it’s a good sign that it is not good

Simply detach from the side of the distributor body by undoing the screw and replace. Again, a very commonly used component. Some are 2 piece components which comprise of the condenser and a small bracket

http://i869.photobucket.com/albums/a...ervice/017.jpg

Vacuum Unit

Attached to the side of the distributor body is a vacuum unit. Most B18B distributors do not have a vacuum unit. B18A, B20A and B18B’s (where fitted) have vacuum advance units. B20B’s have vacuum retard units

Inside the sealed canister is a diaphragm which is connected to the points base plate by an arm. The unit is connected to the inlet manifold by a hose. When the vacuum from that port is sufficient, the diaphragm alters the position of the base plate in relation to the shaft

The purpose is alter the timing of the spark in relation to how varying mixture ignites. At weak mixture, such as at vacuum at low rpm and small throttle openings, the spark timing needs to be advanced. As such, the vacuum advance unit advances the spark timing. This results in better response from low rpm and increased fuel economy

To burn strong fuel mixture, ignition retard is advantageous. As such, some cars were fitted with vacuum units that retard the timing under heavy vacuum such as engine braking, changing gear quickly and going downhill. These vacuum units do nothing for performance and power, but improve exhaust emissions. These types are usually seen to have been disconnected and the vacuum port bunged

It might be the case that Volvo did not fit vacuum advance to twin carburettor cars is because they have a stronger mixture at low rpm and small throttle openings

Problematic vacuum units are a concern now as replacement parts are difficult to get hold of. The system can simply be checked by sucking the pipe and checking that it holds good pressure. The points base plate should move. If it does not hold vacuum. It is best to plug the vacuum take off. This will obviously reduce throttle response and increase fuel consumption

http://i869.photobucket.com/albums/a...ervice/018.jpg

Mechanical Advance

A full distributor strip-down is not covered here, but it is possible to carry out a simple check

Inside the distributor body is a pair of weights attached to springs. As the shaft rotates, the weights go outwards with centrifugal force dependant on the rotational speed of the shaft. This alters the position of the rubbing block of the points in relation to the cams and in turn advances the ignition timing as the points close earlier

Advanced ignition timing is required because when the piston speed increases, the reaction in the combustion chamber stays relatively fixed. Therefore, it is necessary to bring forward the point at which the combustion takes place – too late and the piston has gone past its optimum point and pressure in the combustion chamber is too low

There is a point of maximum advance. This is governed by stops which prevent the springs from going any further outwards and is at between 3,200 and 3,900 rpm depending on engines. At that point, the dynamics of the airflow inside the inlet manifold results in sufficient turbulence to overcome the above mentioned problem of the fixed reaction and the combustion takes places at the optimum pressure inside the chamber

The advance curve varies from distributor type to type and was designed with a number of contributory factors including cam shaft profile, combustion chamber design, manifold and carburettor type. Once optimum advance is achieved, it makes no difference of what type of distributor is fitted

It is possible to carry out a basic check on the mechanical advance system by twisting the shaft while the rotor arm is attached. There should be good resistance and it should spring back to its original position when let go

http://i869.photobucket.com/albums/a...ervice/021.jpg

It is also possible to check the advance curve using a strobe timing light. At idle, it will be about 9 to 14 degrees. At above 3,200rpm, it should be above 32 – 36 degrees depending on the engine and distributor type

If not, then the springs could be stretch or broken, the weights worn or the distributor shaft jammed. It could be possible to un-jam things by a good strip down and lubrication. But if worn, it’s bad news as these individual components are no longer available

Not present on the distributor photographed is an oiling canister. These were present on later distributors and allowed the springs and weights to be lubricated through a small hole with engine oil or light oil. On the top of the shaft is a felt pad for lubricating the shaft. This is often missed and severely reduces the longevity of Bosch distributors

http://i869.photobucket.com/albums/a...ervice/006.jpg

In summary, the standard distributors fitted to Volvo B18 and B20 engines are extremely serviceable and perform well when properly serviced. The points do deteriorate faster than normal servicing intervals and to maintain good performance and efficiency, need regular attention. A good regular service is not expensive and can cure many engine running problems