Topic: multimeter to test points

[jabs83]

How do I use a multimeter to test the voltage coming from my points on a 73 cb750? A step by step would be greatly appreciated.

Thanks in advance.

[KingCustomCycles.com]

Points are basically switches that are controlled on and off by the cam opening and closing their contacts.  A test lead to ground, and a test lead to the points will indicate battery voltage, and then nothing, as the engine is turned. I like to use a contuniuity light when working with points.  Once system voltage is determined to be adequate, I am mostly interested in the exact moment of the points opening for timing purposes.

[jabs83]

Thanks for that response. So a lead to ground, and lead to either the yellow or blue wires coming from points.

What setting should the multimeter be on? I should see cycles of 12v and zero, correct?

[mystic_1]

Correct, set to measure 12V, connect negative (black) lead to ground, positive (red) lead to points (yellow or blue) at coils.

When points are closed, current flows from battery through coil primaries to ground, meter will read 0.

When points are open, no current flows through coils, meter will read 12V.



When using a test light, connections are the same, light will be OFF when points are CLOSED, and ON when points are OPEN.

mystic_1

[bollingball]

mystic_1 , set to measure 12V, connect negative (black) lead to ground, positive (red) lead to ground.When points are closed, current flows from battery through coil primaries to ground, meter will read 0.

When points are open, no current flows through coils, meter will read 12V.



When using a test light, connections are the same, light will be OFF when points are CLOSED, and ON when points are OPEN.

mystic are you sure the part in red is correct? Look like you are putting both meter leads to grd. or am I misreading?

[mystic_1]

Ah, you're correct, I was incorrect, I've corrected that. 

mystic_1

[jabs83]

Thanks guys for the replies. Do the coils output dc voltage or ac volts?

[TwoTired]

Thanks guys for the replies. Do the coils output dc voltage or ac volts?

The coil output is pulsed.  A positive pulse on one lead and a negative pulse on the other lead.
Even the coil input is pulsed DC.
The coil characteristic behavior is closer to AC than DC unless you look a very small time periods.  In fact, the coils behave more like a transformer, which ONLY function with AC coupling or at least, only when the voltage levels change.  When observing voltage levels that are changing, the time periods of examination become important.

If you are going to try and measure coil output, better get a device capable of accepting 12-15000V.  Instruments with lower capabilities are likely to go poof.  Sometimes they send goodbye smoke signals, other times they vacate the world of function with no further comment.

Cheers,

[jabs83]

Okay so I tried hooking up my multimeter as described in this thread, and I'm getting no voltage reading. I'm trying to diagnose where my ignition system is failing, as im only getting spark on the 2 and 3 coil. I have no experience with electrical stuff, but I figured I could start from the points and work my way through the ignition system by eliminating items as they pass.

But I can't figure out if I'm testing the points for voltage correctly. I set the multimeter to read dc voltage. Then I hooked the blue lead from points to positive test lead on multimeter. The black test lead was grounded to a frame bolt. I cranked the starter and all I got was a zero reading. I did the same to the yellow wire from points, and same result.

Am I going about this all wrong, or am I really getting zero volts? Should I be using a different method to test my points or ignition system for where its failing?

BTW I have no wiring harness so, I just have a jumper cable going from negative on battery to a frame bolt, then I touch the starter motor cable to positive to crank engine. That's it for my wiring if that makes a difference in the reading I'm getting from the points.

[TwoTired]

The coils are supplied voltage via the Blk/Wht leads.  Voltage goes through the coil to the yellow and blue lead and their respective points.
When a point set is open, you should see a voltage on the blue or yellow for that point set.

If you don't power the coils, you won't get voltage on the blue or Yellow wires.

Cheers,

[KingCustomCycles.com]

This schematic shows the points and power system for ignition fairly clearly although, it is an older (smaller) copy of the file and the 2-3, 1-4 leads are shown backwards:

[jabs83]

Thanks for the replies guys, and king, thanks for the schematic. So in order to test the points, and see if voltage is coming across the point wires at their respective times while cranking the engine, then the coils need to also be powered via the black wires to the positive battery terminal?

[OldSchool_IsCool]

Typically, you should have 12v on the black/white wire at the coils when you have a good battery in, key on and kill switch set to run.  Do you indeed have 12v there?

The blue and yellow wires run down to the points.  the points provide the path to ground for the coils.  the coil with the blue wire connect to plugs 1&4, the yellow to 2&3.  With your volt meter on the 20v setting, touch the black lead to the engine case and the red lead to the blue wire.  If you should have either 0v or 12v.  By rotating the engine slowly by hand, the meter should flip flop between 12v and 0v.  Same for the yellow wire.

Now, if you have 12v all the time, you points are not closing or they are dirty.  If you have 0v all the time, then your points are not opening or you have a broken or disconnected yellow or blue wire.

[jabs83]

Okay, so thanks to everyone who posted, I now have a greater understanding of the electrical side of the ignition system. I was able to power the ignition, and test the points with a multimeter.

It seems that on the 2-3 point wire, I am getting voltages of 0 and 12 as the points open and close. However, on the 1-4 point wire, I am getting a constant 12v no matter is the point contact faces are closed or open. Could this be the reason why Im only getting spark on the 2-3 plugs?

If so, what does the constant 12v on the 1-4 point wire indicate, and how do I resolve this issue? Is this a timing issue, or maybe the contact points aren't closing flush? Could this mean a bad condensor?

[TwoTired]

Either your 1-4 points aren't closing fully, or they have an insulative coating on them.
File the points contact to get a clean contact.

If you bridge the points with a screwdriver blade does the voltage go to zero?

[jabs83]

Either your 1-4 points aren't closing fully, or they have an insulative coating on them.
File the points contact to get a clean contact.

If you bridge the points with a screwdriver blade does the voltage go to zero?

I'll see what happens when I bridge the gap with a screwdriver later when I get home from work. I'll also pick up a point file on my home, and i'll file the contact surfaces.

Suppose the voltage doesn't go to zero with a screwdriver in between - what does that indicate?

[OldSchool_IsCool]

After you file the points faces, polish them with a clean, white, non-glossy business card soaked with brake cleaner.  You'll be amazed how much crud will be left on the card!

[TwoTired]

Suppose the voltage doesn't go to zero with a screwdriver in between - what does that indicate?

It means the universe you live in doesn't obey the laws of physics.

[OldSchool_IsCool]

Suppose the voltage doesn't go to zero with a screwdriver in between - what does that indicate?

It means the universe you live in doesn't obey the laws of physics.

I hate it when that happens!