Distributors and Contact points
Posted: 22 Mar 2001 8:44DISTRIBUTORS
First of all, let me say that any electronic distributor is better than points - in the same way that doing the Great North Run is better on two legs then on one!
Magnetic pickups and reluctors are best. The only reasons you may have for running a points type system is for originality on restored concourse cars or cost of replacement.
Electronic ignition conversion kits are not that expensive (but hard to find these days) costing between £60 - £150 and it is sometimes possible to modify an existing distributor so that it is hard to tell it has been converted. The ignition amplifier can be mounted anywhere out of sight as long as it is kept cool.
Checking a distributor for wear is quite easy. Side play on the distributor shaft can be measured with a feeler gauge while pushing and pulling on the distributor cam. Shaft or cam wear may also show up as variations in dwell on a meter.
If you slowly increase engine speed from idle up to 1500 rpm and back down again whilst measuring dwell and get a variation of more than 2 degrees, this indicates probable wear in the shaft or cam. Variations at higher rpm can occur because of other factors such as points bounce.
Because the crankshaft rotates twice, on a 4 stroke engine, during the firing order, there is always another cylinder directly opposite on the distributor cap which fires on the same timing mark as No.1 cylinder. If an accurate timing light is put onto this opposite cylinder and the timing found to be more than 2 or 3 degrees out from the timing on No.1, then this also indicates wear or damage.
It is also important to check that there is a good electrical contact between the body of the distributor and the engine block. What can happen is that oil vapour builds up and then hardens between the housing, engine block and hold down clamp. This can act as an insulator. Clean the shaft housing and hold down clamp with a wire brush and solvent. If you paint the hold down clamp be sure to clean off the parts that contact the distributor base and the clamp bolt.
Setting Dwell
If you insist on using the original contact points system it is important that they are kept in proper adjustment. It is impossible to accurately set points by using a feeler gauge. An accurate dwell meter is the best way to set the points gap.
Dwell is the amount of time that the points remain closed and current is building in the coil primary.
The more cylinders your engine has, the more corresponding "bumps" on the distributor cam as it passes through 360 degrees. This means that the amount of dwell possible is less on an 8 cylinder than on a 6, which has less than a 4 and so on.
Dwell can be measured or expressed as DEGREES or PERCENT (%). Dwell degrees refers to the angle of rotation of the distributor through which the points remain closed. Dwell % refers to the percentage of time that the points are closed.
These two ways of expressing dwell are related by the following equation :-
DWELL DEG. = DWELL% X 3.60/No. CYLINDERS
For example, a four cylinder engine has 4 periods of 90 degrees between the bumps on the distributor cam, a six cylinder has six periods of 60 degrees and an eight cylinder eight periods of 45 degrees (total 360 degrees for each).
The points will be opened and closed for a proportion of each period. The closed period is the DWELL ANGLE. Typically this is about 2/3 rds. of the distance between lobes, say 30 degrees of dwell on a V8.
To convert this dwell to percentage;
30 deg. X 100/45 deg = 66.6%
To convert back to degrees;
66.6% X 45 deg/100 = 30 deg.
The dwell can be adjusted by opening or closing the points gap.
INCREASING the points gap REDUCES the dwell.
REDUCING the points gap INCREASES the dwell.
Dwell also has an effect on ignition timing so that when any adjustments are made to the points the timing must be checked.
The points, when closed, supply current to the coil primary winding. The spark is generated when the points open. If the points dwell is too small then the current in the ignition coil primary does not have time to build to a maximum before another spark is demanded. This results in a weak spark at high RPM. The car may start and idle quite well but you may notice flat spots and a lack of power or misfires at high RPM.
If the points dwell is too large then the points are spending very little time open and are also not opening very wide. This may result in sparks arcing across the points, causing pitting of the contacts and a weaker spark. The car may now be difficult to start and may be running "rough" at idle with flat spots at low RPM.
A problem occasionally experienced with very old cars is that when the points are set with a dwell meter the resulting points gap is sometimes much smaller than expected. Often the car will not run very well with the small points gap. This is usually due to wear on the distributor cam or in the shaft bearings. It can also be due to wear on the points pivot pole or bushing. The car may well idle better by increasing the gap but there will be misfiring at high RPM due to the reduced dwell.
Dual Points
Some '60s muscle cars came equipped with dual point distributors. These had two sets of points connected in parallel but out of phase so that if either one was still closed current was still building in the coil. This allows very large dwell angles to be obtained without the points gap being too small, giving good power up to quite high RPM.
Setting dwell on these distributors is simply a matter of wedging an insulator into one set of points whilst setting the other to specification and then swapping over. If , as in the sums above, the dwell for each set of points is set at 30 degrees (2/3rds of the available 45 on a 8 cylinder) then the total amount of dwell achieved will be 40 degrees as the points are out of phase by 10 degrees.
Looking For Wear
In normal wear, after several thousand miles, the contact area on the points will be a grey colour and the roughness between the contacts should match so that a large contact area is maintained.
Contact burning or pitting will result from;
1/ High primary voltage due to an improperly adjusted or inoperative voltage regulator.
2/ Presence of foreign material or oil from crankcase vapours due to things like a blocked crankcase breather pipe.
3/ Points gap being to small allowing arcing between contacts.
4/ High resistance in the condenser circuit due to faulty condenser or loose lead connection.
Tests For Contact Points Systems
1) PIVOT POLE AND BUSHING
Put a small screwdriver blade into the centre of the pivot pole. Try to move it in all directions. It should not move.
If pole is firm, use the screwdriver to hold the pivot pole in place and try to lift up and rock the movable point frame. You should immediately feel spring tension resisting all movement. If there is any free play, then the pivot bushing is worn and the points must be replaced.
2) CONTACT CONDUCTIVITY
Bump the starter until the points are closed. Leave the key on. Connect the -ve of a D.C. voltmeter to a good engine ground. Select a scale of around 0 - 3 volts. Put the +ve lead on the movable contact terminal of the points. The voltmeter should show less than 0.25 volt. Anything higher than 1 volt either means the point contact is weak, the distributor plate is not grounded, or the distributor housing is not grounded.
To tell which, move the +ve voltmeter probe to the points plate inside the distributor. If the voltage drops then the problem is the point contact. Replace the points. If the voltage stays above 0.5 volt then check the grounding on the inside of the distributor. If this is OK, put the +ve probe against a good contact on the outside of the distributor housing. This should be no more than 0.125 to 0.25 volt max.
If you have over 0.25 volt between the distributor housing and the engine block you must remove the distributor and clean the shaft housing and hold down clamp.
3) POINT SPRING TENSION
This should really be measured with a small spring balance pulling on the contact spring until the points are just opening. The specification can be found in the factory manual but is usually 17-21oz.
In real life though, you can feel if a spring is weak by comparing it with a new set of points.
4) RUBBING BLOCK
Sometimes called the heel, this can also be compared to a new set of points for evidence of wear. They can also soften with age so probe it with a sharp screwdriver for evidence of this. A small amount of silicon grease on the cam will prolong the life of the rubbing block.
By David Robson (Dave) on Friday, March 02, 2001 - 02:48 pm:
5)VOLTAGE AT POINTS WITH POINTS OPEN
There should be almost 12 volts all the way from the battery to the moveable contact point.
By Pete (Pete) on Tuesday, March 06, 2001 - 06:52 pm:
Dave, Oh Font of all knowledge - Do you have any info on setting up the Electronic Distributor correctly - ie the gap on the reluctor ?
I did mine by eye (& some common sense) but do not know if there is a correct way of performing this procedure.
By MoparKev (Kev) on Tuesday, March 06, 2001 - 08:43 pm:
The gap should be set to 0.008" max. As you have probably realised, you can't use a normal steel feeler gauge cos of the magnet. Mopar suggest a brass feeler gauge(?). I used a steel one and my eye (common sense lacking!). Always check the play on the advance plate and check the gap at full advance etc. (That's my own tip!). Snot only Mr Robson with all the knowledge................Just most of it!
By David Robson (Dave) on Tuesday, March 06, 2001 - 10:10 pm:
Listen and learn young Grasshopper.
The air gap must indeed be set at 0.008in. (1977 and later) with a non-magnetic feeler guage. I 'picked up' my set years ago but have not seen any since.
BUT to set the gap use a 0.006in guage held in place while you tighten the adjusting screw. You will find the resulting gap to be a nice snug 8 thou. Check each reluctor blade to make sure nothing is catching and it is all free from dirt or iron filings.
Time to leave Grasshopper.
By Alex (Alex) on Tuesday, March 06, 2001 - 10:51 pm:
Snap On do the feelers Pete, If ya cant find any, I will lend you mine if ya promise to look after them and whizz them back asap, cos I use them a lot at work.
Take it easy
Alex
By Pete (Pete) on Wednesday, March 07, 2001 - 08:13 am:
Alex - cheers - i can wait 'till a Meet & we can try & do it at about 3 in the morning after lots of falling down water.
Thanks to all who replied..Kev, I just used my experience picked up from the "Basic Auto" course (BT Joke) - you are probably too young to remeber that stuff - I did say I was a candidate old fart.
First of all, let me say that any electronic distributor is better than points - in the same way that doing the Great North Run is better on two legs then on one!
Magnetic pickups and reluctors are best. The only reasons you may have for running a points type system is for originality on restored concourse cars or cost of replacement.
Electronic ignition conversion kits are not that expensive (but hard to find these days) costing between £60 - £150 and it is sometimes possible to modify an existing distributor so that it is hard to tell it has been converted. The ignition amplifier can be mounted anywhere out of sight as long as it is kept cool.
Checking a distributor for wear is quite easy. Side play on the distributor shaft can be measured with a feeler gauge while pushing and pulling on the distributor cam. Shaft or cam wear may also show up as variations in dwell on a meter.
If you slowly increase engine speed from idle up to 1500 rpm and back down again whilst measuring dwell and get a variation of more than 2 degrees, this indicates probable wear in the shaft or cam. Variations at higher rpm can occur because of other factors such as points bounce.
Because the crankshaft rotates twice, on a 4 stroke engine, during the firing order, there is always another cylinder directly opposite on the distributor cap which fires on the same timing mark as No.1 cylinder. If an accurate timing light is put onto this opposite cylinder and the timing found to be more than 2 or 3 degrees out from the timing on No.1, then this also indicates wear or damage.
It is also important to check that there is a good electrical contact between the body of the distributor and the engine block. What can happen is that oil vapour builds up and then hardens between the housing, engine block and hold down clamp. This can act as an insulator. Clean the shaft housing and hold down clamp with a wire brush and solvent. If you paint the hold down clamp be sure to clean off the parts that contact the distributor base and the clamp bolt.
Setting Dwell
If you insist on using the original contact points system it is important that they are kept in proper adjustment. It is impossible to accurately set points by using a feeler gauge. An accurate dwell meter is the best way to set the points gap.
Dwell is the amount of time that the points remain closed and current is building in the coil primary.
The more cylinders your engine has, the more corresponding "bumps" on the distributor cam as it passes through 360 degrees. This means that the amount of dwell possible is less on an 8 cylinder than on a 6, which has less than a 4 and so on.
Dwell can be measured or expressed as DEGREES or PERCENT (%). Dwell degrees refers to the angle of rotation of the distributor through which the points remain closed. Dwell % refers to the percentage of time that the points are closed.
These two ways of expressing dwell are related by the following equation :-
DWELL DEG. = DWELL% X 3.60/No. CYLINDERS
For example, a four cylinder engine has 4 periods of 90 degrees between the bumps on the distributor cam, a six cylinder has six periods of 60 degrees and an eight cylinder eight periods of 45 degrees (total 360 degrees for each).
The points will be opened and closed for a proportion of each period. The closed period is the DWELL ANGLE. Typically this is about 2/3 rds. of the distance between lobes, say 30 degrees of dwell on a V8.
To convert this dwell to percentage;
30 deg. X 100/45 deg = 66.6%
To convert back to degrees;
66.6% X 45 deg/100 = 30 deg.
The dwell can be adjusted by opening or closing the points gap.
INCREASING the points gap REDUCES the dwell.
REDUCING the points gap INCREASES the dwell.
Dwell also has an effect on ignition timing so that when any adjustments are made to the points the timing must be checked.
The points, when closed, supply current to the coil primary winding. The spark is generated when the points open. If the points dwell is too small then the current in the ignition coil primary does not have time to build to a maximum before another spark is demanded. This results in a weak spark at high RPM. The car may start and idle quite well but you may notice flat spots and a lack of power or misfires at high RPM.
If the points dwell is too large then the points are spending very little time open and are also not opening very wide. This may result in sparks arcing across the points, causing pitting of the contacts and a weaker spark. The car may now be difficult to start and may be running "rough" at idle with flat spots at low RPM.
A problem occasionally experienced with very old cars is that when the points are set with a dwell meter the resulting points gap is sometimes much smaller than expected. Often the car will not run very well with the small points gap. This is usually due to wear on the distributor cam or in the shaft bearings. It can also be due to wear on the points pivot pole or bushing. The car may well idle better by increasing the gap but there will be misfiring at high RPM due to the reduced dwell.
Dual Points
Some '60s muscle cars came equipped with dual point distributors. These had two sets of points connected in parallel but out of phase so that if either one was still closed current was still building in the coil. This allows very large dwell angles to be obtained without the points gap being too small, giving good power up to quite high RPM.
Setting dwell on these distributors is simply a matter of wedging an insulator into one set of points whilst setting the other to specification and then swapping over. If , as in the sums above, the dwell for each set of points is set at 30 degrees (2/3rds of the available 45 on a 8 cylinder) then the total amount of dwell achieved will be 40 degrees as the points are out of phase by 10 degrees.
Looking For Wear
In normal wear, after several thousand miles, the contact area on the points will be a grey colour and the roughness between the contacts should match so that a large contact area is maintained.
Contact burning or pitting will result from;
1/ High primary voltage due to an improperly adjusted or inoperative voltage regulator.
2/ Presence of foreign material or oil from crankcase vapours due to things like a blocked crankcase breather pipe.
3/ Points gap being to small allowing arcing between contacts.
4/ High resistance in the condenser circuit due to faulty condenser or loose lead connection.
Tests For Contact Points Systems
1) PIVOT POLE AND BUSHING
Put a small screwdriver blade into the centre of the pivot pole. Try to move it in all directions. It should not move.
If pole is firm, use the screwdriver to hold the pivot pole in place and try to lift up and rock the movable point frame. You should immediately feel spring tension resisting all movement. If there is any free play, then the pivot bushing is worn and the points must be replaced.
2) CONTACT CONDUCTIVITY
Bump the starter until the points are closed. Leave the key on. Connect the -ve of a D.C. voltmeter to a good engine ground. Select a scale of around 0 - 3 volts. Put the +ve lead on the movable contact terminal of the points. The voltmeter should show less than 0.25 volt. Anything higher than 1 volt either means the point contact is weak, the distributor plate is not grounded, or the distributor housing is not grounded.
To tell which, move the +ve voltmeter probe to the points plate inside the distributor. If the voltage drops then the problem is the point contact. Replace the points. If the voltage stays above 0.5 volt then check the grounding on the inside of the distributor. If this is OK, put the +ve probe against a good contact on the outside of the distributor housing. This should be no more than 0.125 to 0.25 volt max.
If you have over 0.25 volt between the distributor housing and the engine block you must remove the distributor and clean the shaft housing and hold down clamp.
3) POINT SPRING TENSION
This should really be measured with a small spring balance pulling on the contact spring until the points are just opening. The specification can be found in the factory manual but is usually 17-21oz.
In real life though, you can feel if a spring is weak by comparing it with a new set of points.
4) RUBBING BLOCK
Sometimes called the heel, this can also be compared to a new set of points for evidence of wear. They can also soften with age so probe it with a sharp screwdriver for evidence of this. A small amount of silicon grease on the cam will prolong the life of the rubbing block.
By David Robson (Dave) on Friday, March 02, 2001 - 02:48 pm:
5)VOLTAGE AT POINTS WITH POINTS OPEN
There should be almost 12 volts all the way from the battery to the moveable contact point.
By Pete (Pete) on Tuesday, March 06, 2001 - 06:52 pm:
Dave, Oh Font of all knowledge - Do you have any info on setting up the Electronic Distributor correctly - ie the gap on the reluctor ?
I did mine by eye (& some common sense) but do not know if there is a correct way of performing this procedure.
By MoparKev (Kev) on Tuesday, March 06, 2001 - 08:43 pm:
The gap should be set to 0.008" max. As you have probably realised, you can't use a normal steel feeler gauge cos of the magnet. Mopar suggest a brass feeler gauge(?). I used a steel one and my eye (common sense lacking!). Always check the play on the advance plate and check the gap at full advance etc. (That's my own tip!). Snot only Mr Robson with all the knowledge................Just most of it!
By David Robson (Dave) on Tuesday, March 06, 2001 - 10:10 pm:
Listen and learn young Grasshopper.
The air gap must indeed be set at 0.008in. (1977 and later) with a non-magnetic feeler guage. I 'picked up' my set years ago but have not seen any since.
BUT to set the gap use a 0.006in guage held in place while you tighten the adjusting screw. You will find the resulting gap to be a nice snug 8 thou. Check each reluctor blade to make sure nothing is catching and it is all free from dirt or iron filings.
Time to leave Grasshopper.
By Alex (Alex) on Tuesday, March 06, 2001 - 10:51 pm:
Snap On do the feelers Pete, If ya cant find any, I will lend you mine if ya promise to look after them and whizz them back asap, cos I use them a lot at work.
Take it easy
Alex
By Pete (Pete) on Wednesday, March 07, 2001 - 08:13 am:
Alex - cheers - i can wait 'till a Meet & we can try & do it at about 3 in the morning after lots of falling down water.
Thanks to all who replied..Kev, I just used my experience picked up from the "Basic Auto" course (BT Joke) - you are probably too young to remeber that stuff - I did say I was a candidate old fart.