No, I did say digital on purpose because most people just cant find an older analogue test set to use. Sometimes a good quality multimeter will work on old cars but not always. Digital multimeters are affected by RF or radio frequency transmitted by wire spark plug leads and old ignition systems. Also effected by voltage spikes given off from generator commutators and brushes on older cars. Analog meters don't pick up the RF and work better. Regards, Kevin.
Resistor voltage drop.
- Thread starter Phil Gillespie
- Start date
Phil Gillespie
Well-known member
Yes Think He did mean analogue.
I did shift the leads to Com & Amps. Meter supposed to take 10 amps.
But will swap put the condenser for a modern type which I have. Have one that has been checked and was 27mfds. Will also get hold of another meter an keep at it.
Always appreciate all assistance given.
Phil NZ
I did shift the leads to Com & Amps. Meter supposed to take 10 amps.
But will swap put the condenser for a modern type which I have. Have one that has been checked and was 27mfds. Will also get hold of another meter an keep at it.
Always appreciate all assistance given.
Phil NZ
B_man_Al
Senior Member
As I mentioned in post 5 there will always be an error in the measured voltages when making them on a running engine . This applies to measurements made with an ANALOG meter where the meter movement "averages " the voltages and current being measured. This is exactly what happens when you make a dwell angle measurement.
When a digital meter is used all bets are off. They respond to all the glitches and spikes, particularly across the points where there is a significant voltage spike due to inductive kick form the coil as they open.
If you make the measurements with the engine off (and the points closed) you can use either type of meter and get accurate results. To correct for the differences in battery voltage between running and non running just multiply all your non running measurements by 1.2 ( 7.4/6) and you will be pretty close.
Al
When a digital meter is used all bets are off. They respond to all the glitches and spikes, particularly across the points where there is a significant voltage spike due to inductive kick form the coil as they open.
If you make the measurements with the engine off (and the points closed) you can use either type of meter and get accurate results. To correct for the differences in battery voltage between running and non running just multiply all your non running measurements by 1.2 ( 7.4/6) and you will be pretty close.
Al
supereal
Well-known member
The voltage drop across the ignition resistor will vary as the system level rises and falls. It was designed to be a sort of "voltage regulator" to protect the coil and points. From overheating and/or burning as the generator output increases with engine speed. The element is "nichrome" wire, an was used in early toasters. Many though that bypassing the resistor would add power. It doesn't.
B_man_Al So you say that a volt test meter or an ammeter in series reads the average voltage or current at the coil when the engine is running because of the rapid opening and closing of the (distributer contacts) circuit. Its like a voltage and current regulator in effect. The meter is reading in error because of the above effect. I don't think so ! The meter is reading exactly what is happening in the circuit. What is happening in the circuit is different between engine running and not running. The ignition coil and its resistor have a lower current draw with engine running. With the engine not running the current draw of the coil is higher and the resistor is getting hotter and so increases its resistance more so than with the engine running. So whats the point in testing with the engine stopped because that is not the ignition systems operating conditions. Yes this is a little hard to get your head around but I agree to disagree with you and do my tests with the engine running and the ignition primary circuit under actual operating conditions. Regards, Kevin.
Last edited:
B_man_Al
Senior Member
Hi Kevin ...
Your right that the apparent current is lower when the engine is running, but not because the peak current is less (as defined by the battery voltage divided by the total circuit resistance (coil + ballast resistor), but because the AVERAGE current is less. This is because when the points are open there is no current flowing at all. So if what you want is "average current" then you are correct .
This however can be very misleading because so many times I see posts here that talk about how much coil and ballast resistance there is and doing calculations of current based on voltages measured across them. This will be incorrect if the engine is running.
For example if a coil is 1 ohm say, and the ballast resistor is .5 ohms, then you would expect the current to be 6v/1.5 ohms = 4.0A . If the engine is running and you make a voltage measurement across the ballast resistor you will not read 2v, (which is 4A x .5 ohms), you will read 1v which is average on an analog meter (assuming the points are open for half the time)
What I see happen so often is that after someone has made the ballast voltage measurement and done the calculation ( 1v measured/.5 ohms measured =2A) they conclude that the coil current is too low. As far as the coil is concerned it has all the current it needs as it is actually 4A during the time the points are closed. So as long as the dwell angle is adequate the coil and ignition are happy.
Your right that the apparent current is lower when the engine is running, but not because the peak current is less (as defined by the battery voltage divided by the total circuit resistance (coil + ballast resistor), but because the AVERAGE current is less. This is because when the points are open there is no current flowing at all. So if what you want is "average current" then you are correct .
This however can be very misleading because so many times I see posts here that talk about how much coil and ballast resistance there is and doing calculations of current based on voltages measured across them. This will be incorrect if the engine is running.
For example if a coil is 1 ohm say, and the ballast resistor is .5 ohms, then you would expect the current to be 6v/1.5 ohms = 4.0A . If the engine is running and you make a voltage measurement across the ballast resistor you will not read 2v, (which is 4A x .5 ohms), you will read 1v which is average on an analog meter (assuming the points are open for half the time)
What I see happen so often is that after someone has made the ballast voltage measurement and done the calculation ( 1v measured/.5 ohms measured =2A) they conclude that the coil current is too low. As far as the coil is concerned it has all the current it needs as it is actually 4A during the time the points are closed. So as long as the dwell angle is adequate the coil and ignition are happy.
Phil Gillespie
Well-known member
Yes I did try this. Just bypassed to resistor and there was no change to running conditions. ie low vacuum and poor idle.
Phil NZ
Phil, Over the last couple of weeks my 34 Ford has been running really rough, wont rev at all, cant drive it. So I have been through everything twice. Serviced the distributer, Replace coil with a rewound one, checked voltages, checked fuel system, changed over carby, put in fresh fuel etc,etc. One of the very first things I did initially was remove condenser and do all my tests on it and all perfect and put it back on. So I did pull the condenser again later a did my tests on my very good tester. While measuring the microfarad capacity which was .30 mfd I happened to move the wire where it goes into the end of the condenser and lost the reading. Turned out there was a intermittent bad connection inside the condenser. Its a Mallory performance condenser not an original ford type one. So I fitted a new condenser and away the car ran like a Swiss watch. Condensers can be a real problem so that's why I say its an easy quick item to replace. New old stock condensers and even new reproduction Ford style condensers have given me a fair amount of trouble on early Ford V8s I have worked on over the years so that's why I started using the NEW Mallory modern ones which have been good until this one packed it in. Just for your info. Regards, Kevin.
B_man_Al, Yes Al I am hearing what you say and its a trickey system for sure to arrive at what is a correct reading both in volts and current draw. The harder you rev the engine the lower the coil current draw and voltage reading. I usually check at idle speed and voltage can be about 3 volts and 2.5 amps current draw ballpark figures. Anyway when checking this I usually do tests with the engine stopped, points closed and then do the engine running test. With engine stopped test the longer the ignition is left on ,then the hotter the resistor becomes and higher resistance which alters all readings. Anyway enough of that and thanks for your comments. Regards, Kevin.
Phil Gillespie
Well-known member
Kevin this information is all good and appreciated for sure. There sometimes is a tendency to overlook the likes of the condenser on the thought thats its new just been replaced and should be good. i believe in a methodical item by item check etc and arrive at the problem as opposed to changing out serveral items at once and crossing fingers and hope for the best.
I will advise as I progress.
Phil NZ
I will advise as I progress.
Phil NZ
B_man_Al
Senior Member
Hi Kevin ….
Me again. The point you make about excessive current when the engine is off and the points are closed is a good one for ignition systems that have ballast resistors with a positive temperature coefficient (the higher the current and hence power dissipation, the higher the resistance). Its doing exactly the right thing by responding to the average current/power dissipation (not the peak) and limiting the coil current accordingly.
For an ignition system that has this type of resistor, the engine off measurements must be made quickly (a few seconds) before the ballast resistor has had a chance to get hot.
If you have an ignition system with a ceramic power resistor, such as used when converting to 12v with the original 6v coil, (typically 1-1.5 ohms) the engine off measurements will be OK as the resistor value is stable and does not increase with temperature. Again the measurements should be done quickly to not overheat the coil.
In the running condition the average power in the resistor and the coil will be approximately half of the engine off value due to the point dwell.
As far as condensors are concerned I had a very similar problem with my model B a few years ago. I was convinced I had a fuel system problem after checking the timing and points and coil. After a few miles my car would start sputtering and quit or only idle but not run at higher RPM. Turned out to be the condenser failing at high temperature!
Hope some of this helps anyone out there.
Al
Me again. The point you make about excessive current when the engine is off and the points are closed is a good one for ignition systems that have ballast resistors with a positive temperature coefficient (the higher the current and hence power dissipation, the higher the resistance). Its doing exactly the right thing by responding to the average current/power dissipation (not the peak) and limiting the coil current accordingly.
For an ignition system that has this type of resistor, the engine off measurements must be made quickly (a few seconds) before the ballast resistor has had a chance to get hot.
If you have an ignition system with a ceramic power resistor, such as used when converting to 12v with the original 6v coil, (typically 1-1.5 ohms) the engine off measurements will be OK as the resistor value is stable and does not increase with temperature. Again the measurements should be done quickly to not overheat the coil.
In the running condition the average power in the resistor and the coil will be approximately half of the engine off value due to the point dwell.
As far as condensors are concerned I had a very similar problem with my model B a few years ago. I was convinced I had a fuel system problem after checking the timing and points and coil. After a few miles my car would start sputtering and quit or only idle but not run at higher RPM. Turned out to be the condenser failing at high temperature!
Hope some of this helps anyone out there.
Al
Phil Gillespie
Well-known member
Just an update on my problems some of it caused by myself!! Sometimes the focus is not clear as it should be.
One of the probles unseen by myself, when choke was pushed back it the cable and kinked upwards giving some choke at idle and the low idle vacuum.
Had tried another coil with adaptor plate on distributor good idle but =
not so good on road test.
Today I fitted a distributor, condenser and original coil with =
resistor. This distributor was recently rebuilt.
Original coil I think was done by Skip Haney.
At non running: 5.94v before resistor and 3.12 v after.(Points must have =
been closed). Resistor cold: 0.9 ohms. Coil : primary: 0.9 ohms. =
secondary:5.57 (20k).
Running: To resistor 6v. Amps drawing to resistor: 2.89amps. Vacuum: =
19=E2=80=9D at idle rpms. Still awaiting analogue meter for volts after =
condenser.
Cranked up idle and current to resistor then 2.2amps.
The road test was good and just to do the final with the vacuum gauge to =
slightly adjust timing for best possible vacuum.
Does the secondary on the coil seem a bit low to you?
The new style round type which I tried on the car did not go well but =
primary was 1.2ohms secondary: 7.94 (20K).
So its good with the small fine tuning to complete.
Over last few days have had these distributors off and on many times.
Have a Vertex magneto condenser of 36mfds fitted.
Its good to get a result and when my bits arrive from the USA will have a good distributor along with a good coil as the spare I tried did not go well at all.
Many thanks to all the helpful assiatance from this site.
Still await my analogue meter for checking voltage after the resistor when running
but guess 2.89amps draw to resistor would seem ok?
Phil NZ
One of the probles unseen by myself, when choke was pushed back it the cable and kinked upwards giving some choke at idle and the low idle vacuum.
Had tried another coil with adaptor plate on distributor good idle but =
not so good on road test.
Today I fitted a distributor, condenser and original coil with =
resistor. This distributor was recently rebuilt.
Original coil I think was done by Skip Haney.
At non running: 5.94v before resistor and 3.12 v after.(Points must have =
been closed). Resistor cold: 0.9 ohms. Coil : primary: 0.9 ohms. =
secondary:5.57 (20k).
Running: To resistor 6v. Amps drawing to resistor: 2.89amps. Vacuum: =
19=E2=80=9D at idle rpms. Still awaiting analogue meter for volts after =
condenser.
Cranked up idle and current to resistor then 2.2amps.
The road test was good and just to do the final with the vacuum gauge to =
slightly adjust timing for best possible vacuum.
Does the secondary on the coil seem a bit low to you?
The new style round type which I tried on the car did not go well but =
primary was 1.2ohms secondary: 7.94 (20K).
So its good with the small fine tuning to complete.
Over last few days have had these distributors off and on many times.
Have a Vertex magneto condenser of 36mfds fitted.
Its good to get a result and when my bits arrive from the USA will have a good distributor along with a good coil as the spare I tried did not go well at all.
Many thanks to all the helpful assiatance from this site.
Still await my analogue meter for checking voltage after the resistor when running
but guess 2.89amps draw to resistor would seem ok?
Phil NZ
B_man_Al
Senior Member
Phil ... I looked at your measurements and have a few questions and comments.
1- Engine off …..From your measurements of V bat=5.95 V, coil=3.12 and R resistor=.9 ohms then V resistor= (5.95V-3.12V=2.83V. and I resistor= (5.95-3.12)/.9 = 3.14 A .
2- Engine off …..Similarly from your measurements of V coil=3.12 and R coil=.9 ohms, then I coil= 3.12/.9 = 3.46 A .
3-The currents calculated in 1 & 2 should be the same but there is a 0.32A difference in the two current calculations. This may be due to an excessive voltage drop across the points (Since V coil measured is actually V coil + V points. ). If this is the case the point drop is in the order of 3.14V-2.83V=0.29V which seems high .
4-Or there is an additional resistance in the ground path from the points back to the battery. When you made the measurements did you use the same ground point in all cases. If not that may explain the differences. Also you may also want to make a voltage measurement between the ground post on the battery and the actual ground at the points/engine block ( with the points closed) to see if this is where the extra 0.29V has gone.
Al
1- Engine off …..From your measurements of V bat=5.95 V, coil=3.12 and R resistor=.9 ohms then V resistor= (5.95V-3.12V=2.83V. and I resistor= (5.95-3.12)/.9 = 3.14 A .
2- Engine off …..Similarly from your measurements of V coil=3.12 and R coil=.9 ohms, then I coil= 3.12/.9 = 3.46 A .
3-The currents calculated in 1 & 2 should be the same but there is a 0.32A difference in the two current calculations. This may be due to an excessive voltage drop across the points (Since V coil measured is actually V coil + V points. ). If this is the case the point drop is in the order of 3.14V-2.83V=0.29V which seems high .
4-Or there is an additional resistance in the ground path from the points back to the battery. When you made the measurements did you use the same ground point in all cases. If not that may explain the differences. Also you may also want to make a voltage measurement between the ground post on the battery and the actual ground at the points/engine block ( with the points closed) to see if this is where the extra 0.29V has gone.
Al
Phil Gillespie
Well-known member
I used the same ground point in these measurements. Also as rpm increased up with throttle the amps dropped to 2,2 amps.
My feeling is that its in the ball park.
My query being if the secondary resistance on the original style coil may be low at 5.57 (20k range).
Phil NZ
My feeling is that its in the ball park.
My query being if the secondary resistance on the original style coil may be low at 5.57 (20k range).
Phil NZ