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Checking grounds and resistance

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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I have a simple question (I think!)...

What is the proper way of checking my ground connections?

Right now, I am using my multimeter @ 20 Ohms to check all of the ground points and the negative battery terminal.

So far everything is around 0.4 to 0.5 ohms.

The problem I have though, is that when I turn the ignition on, the resistance on the engine ground bolt jumps to 1.2 Ohms. Then when I flip the kill switch (coils on), it goes even higher to ~5 Ohms. Is this all normal? Do I have a short?

Thanks in advance,
 
Check the Stator Papers IV article on this site (garage section). The technique used to check the ground wire on the RR is a good one. Connect the neg. test lead to the RR neg and the pos. test lead to the batt. neg. Sounds wrong I know but you get a reading of the voltage differential between the RR and batt. When i ran a separate ground wire directly from the RR neg to batt. neg the voltage differential went from .35VDC to .17VDC. Meaning there was less resistance to keep voltage from getting into the batt. Do the same for the RR positive only this time it's pos. test lead to RR pos. (red) and neg test lead to batt pos. Again less than 0.2VDC is good. Some guys run a separate hot wire from the RR to the battery too.

I don't know what the resistance is on the ground points, never measured it. But I did go thru and clean every connector on the harness and resoldered some of them. Some guys go so far as to completely untape the harness and check each wire for breaks, corrosion etc. Haven't had any problems with the lights or charging system since I did that and replaced the RR and stator.
 
You have the right idea, sort of. You certainly can measure Ohms across connections to gauge their worthiness, but when you check resistance you must check it on a dead circuit. Generally resistance less that .5 ohm is an acceptable reading. It is perfectly normal, too, to have resistance readings dance all over the place when the circuit is activated. Resistance readings are only accurate when no current is flowing.
Another quick way to check for the presence of a ground connection is to clamp a test light on the positive terminal of the battery, and probe at your ground points. A good ground will be indicated by the test light.
The third way to check for high resistance in a ground circuit is a voltage drop test along the ground side of the circuit. This takes a little more time to explain, though, and my ribs have just finished. Gotta go.
[/quote]
 
Ok... makes sense.

When you are done enjoying those ribs, I'd love to hear that last method, qslim :)

The thing that started me on this trail was my coil voltage supply. I was checking it and found it to only be at 11.5 volts while my battery and everthing else was at 12.6-7 volts.

I tested my kill switch and it reads 12.6 until I flip it, and then I lose a whole volt.

I thought maybe I had a short in my grounds somewhere.
 
Allright. Now that I have a touch of BBQ in me, here goes the voltage drop test - the best way to chase down and pinpoint a high resistance problem.
The deal behind this test is using a meter on DC volts to measure exacly how much voltage a given part of the circuit is using when activated. Now unlike resistance testing, drop testing can only be done when the current is flowing. The theory here is that in any given electrical circuit, all the voltage will be used up by the time is gets over to the battery ground post. It is used up by resistance; the load in the circuit, etc... Now in an ideal circuit, you will have all the voltage (12 in our case) used up by the load (the bulb, coil, etc...). However, there is always a small amount of resistance in the wires, connectors, and switches, so our load usually recieves a bit less than the actual 12v (but not by much).
So, what a drop test does is measure exactly how much V is being used in any part of the circuit. So, take your case. You are trying to figure out why you are losing a volt between your battery and your coil. If you take your leads and place one on the + battery terminal and one on the + coil terminal, your meter will show you just how much voltage is being used up across that connection (as a general rule, anything much more than a couple of tenths of V drop across a wire is too much - it indicates high resistance). A lot of times you will be able to find a volt or more lost across a connection or splice due to corrosion. Sometimes its the wire itself, becoming brittle from years of heat and use.
Another thing to remember is that the high resistance may be on the ground side of your coil circuit, too. If the coil is grounded to the frame and it's not connected well, that resistance may be eating some of the available volts to your coil. The best way to start this test is is by doing a drop test on the hot side and the ground side of the circuit, determining the side that is using that extra volt, and narrowing your search from there. And remember, the circuit has to be on for this test to be valid!
I hope that makes sense. Post if you need a hand.
 
A key point not mentioned is if you are going to measure low resistances you need to 0 out the meter. Connect the leads together and wither adjust the meter to read 0 exactly or press a calibration button if it has one,

If you cannot zero the meter subtract the resistance the leads show from you test so you know the "true" resistance.
 
But now if you are checking your wiring harness, and you want to check the condition of connections you should have 0 resistance or below 0.5 ohms. I believe that 0.5 would indicate that the connections are not very good in the circuit.
 
Yeah, I have a feeling that 0.5 Ohms is equal to zero on my multimeter. It's a cheapie and I can't find a "zero" calibration button anywhere.

When I connect the two probes together I still get 0.5 Ohms.

Thanks for all of tips so far. I'm starting my poking and proding now.
 
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