Points, points, points

[Adam]

Hey guys, I've been a Sundial member since around 2004 but haven't been active in a long time and my bikes have sat for some time. Glad to see the board is still here and still some familiar faces!

I have a '72 GT550 that is still in nice shape and recently I started doing the necessary tune-ups and adjustements to get it going again. I have set timing before but have forgotten some of the details and am getting some worrying numbers for the points. I'm hoping the experts can tell me if I have a problem or not.

I started by checking when the point opens by using a resistance setting on my multi-meter, expecing to see zero resistance when point is closed and infinite resistance when open. Instead I am seeing about 75 Ohms when open on all three points which doesn't seem right. Then I remembered my multi-meter has a "continuity" setting where a buzzer goes off when continuity is lost so I tried that next and it seems to suggest everything is fine. The buzzer goes off as soon as point opens. So I guess my main question is: is it OK that I'm seeing 75 Ohms across an open point or should it be closer to infinite?

[Craig380]

Welcome back

Is the ignition ON or OFF when you're taking these measurements? For what it's worth, when I used my digital multimeter to set the timing (ignition switched OFF), the display would jump from 0.03 ohms (points closed) to about 8 ohms (points open).

The actual opening point was easy to spot as the displayed resistance suddenly jumped.

My meter never showed infinite resistance with points open. The bike started and ran great at all rpm so I just figured it was a quirk of the meter.

[Zunspec4]

Hello Adam,

When the points are closed the primary windings in the coil have 12v applied, which draws current, which produces a magnetic field. When the points open, the current stops and the magnetic field collapses inducing the HT spark voltage in the secondary coil. So when you measure resistance across an open set of points you are measuring the resistance of the primary coil. 75 ohms seems rather high to me, more in the range that craig380 states. You may have a dicy connection somewhere but others more knowledgeable can provide a typical resistance reading for a 550 primary coil.

When I am timing a points ignition (with ignition on) I set my meter to Volts. When the points are closed it will be 0 volts. When the points are open you will read 12 volts (or whatever your battery is giving out). If there is a big discrepancy between a reading taken across the battery terminals and what you see across an open set of points this is also an indication of a dodgy (high resistance) connection.

Cheers Geoff

[jabcb]

Its a good idea to disconnect the battery first if you aren’t too sure what you are doing.
Otherwise you might accidentally try to take a resistance reading of a live circuit.


A multimeter has an internal resistance.
When you touch the two meter leads together you will get something like 0.2 to 0.4 ohms, which is the internal resistance.
When you measure the resistance of something, you get a reading that is the sum of the meter internal resistance + the resistance you are measuring.

When you measure the resistance for closed points, you get a reading of the meter internal resistance + ZERO (for good points). Or something like 0.2 to 0.4 ohms


What you get when the points are open can vary.
On a T500 with the ignition off for example, measuring the left points when they are open you get:
1) resistance across the left points: infinite
2) resistance from open left points to left coil, left coil primary, to right coil, right coil primary, to right point, and across right points. Or something like 0 + 4 + 0 + 4 + 0 + 0 + 0.4 (for meter) = 8.4 ohms
(This assumes you didn’t do something to keep the right points open.)


In your case for the GT550 with all 3 points open, you are measuring a different electrical path.

If you had the bike in neutral, then you are measuring the resistance via the neutral light circuit. The stock neutral indicator bulb is something like 50 ohms. So this may be your case.

Otherwise, there is an electrical path through the rectifier.
In one direction its relatively low and in the other direction its really high.
If you reverse the multimeter leads & the reading goes from 75 ohms to something big like 10,000 ohms, then you are measuring the rectifier resistance.

[Coyote]

Disconnect the battery leads. Time using continuity (buzzer or lamp). Attach fully charged battery and turn on ignition. Attach the plugs to the high tension leads and make sure the bodies are grounded. Open the closed points with a small stick. You should have healthy spark at each corresponding plug..DONE!
Don't worry about resistance, ohms or anything else unless you have no spark. It's a simple process. Don't complicate matters.

[Adam]

Thanks for all the info, guys. I tried with several different configurations and the resistance changed each time. I'll go with the continuity setting for now!