Clarification on rotor resistance.

When I check resistance at room temperature across the two copper strips on the rotor face, I am getting .4-.7ohms. I am assuming this is an internal short, and that the rotor requires replacement?

Thanks!

 

what rotor are you talking about. don't know about others but the only think i know called a rotor is the brakes.

 

I don't have the specs on that rotor but your reading close I believe. Your reading through the windings of it though so they look intact to me. Check from each copper ring to ground, you shouldn't read anything.

Same thing with the field coil. You should read between each leg to leg. But not any one leg to ground.

 

How about the rotor that spins within the stator that makes up the alternator?


#2 there.

 

The reason I ask is because in the shop manual, it states that the resistance on the green and brown is supposed to be 3.6-4.4 ohms as read from the disconnected molex (so reading through the brushes and the wiring). Are the brushes and wiring going to add 3-4 ohms?

 

I looked at the Manual. You have the right info.

Take-off the Alternator Cover and look at the Circular Race paths where the Brushes make contact.

If the ROTOR looks like the two Circular Paths are darkened and scorched from the Brushes Arcing on the Paths ...
Use an ERASER with PUMICE to scrub the Paths free of scorch marks.

Once you remove the scorches ... polish to remove the scratches of any abrasive used on the ROTOR.
The slightest scratch will begin the arcing process all-over.

 

The brushes won't add that much. I don't know the exact specs but most rotors should read around 3.5 ohms. Anything lower means it is shorting out. The stator will read almost 0 ohms. On either one it should be open to ground. Hopefully one of our XJ wiz knows the exact reading.

 

my bad.

spec calls for 4 ohms field resistance. +/- 10%
stator calls for .46 ohms +/- 10%.

you should have low reading between any connection. high reading means it's broken. you also check between any one connection and ground of the rotor. should have NO reading. if any connection has a reading it's grounding on the case.

you can also check for A/C volts coming out of the plug. the regulator/rectifier is what converts it do D/C volts.

EDIT: YOU should be able to check for A/C

 

The resistance of the wire can increase due to there age, corrosion and a build up of crud and oxidation of the copper wires just plain old deterioration. I believe the the wires need to be changed out also. The resistance on the brown and green leads of the rotor is 3.6 to 4.4 ohms. The resistance on the three white wires of the stator is 0.41 to 0.51 ohms.
And these values are taken at 68 degrees Fahrenheit or 20 degrees Celsius and do not allow for humidity and higher temps which will make for higher readings.

 

Very carefully examine the Quick-Connections for signs of overheating and melting.

Sometimes the Connector can be subject to enough heat that the "Barb" on the Captive Blade connector is let free.

When this happens, the Connectors don't get a marriage of the two terminal ends.
One will PUSH against the other and the resulting connection is either OPEN or made with resistance.

If there is tension on the Connectors; the problem is made worse.
The wire of which the Locking Barb has melted or otherwise escaped will get pulled-out and the connection fails to pass current because those terminals never meet.

 

Hey, Alaska, have you still got electrical gremlins? Follow Ricks advice, but when measuring very low resistances, touch the red and black meter lead together and deduct the "Lead resistance" from the value you get when measuring your components.

 

I thought you just held the probes together & turned the dial to zero, each time.

 

I replaced the entire wire harness (without removing the airbox or engine or carbs) ((amazingly it's in the stock location and didn't get cut-up at all)) and the rotor and the brushes. Charging system works as advertised. A comparison of rotor resistance values off bike were: old-.2ohm, new (to me)-4.5ohm

 

Yep. Using a Fluke 87V digital multimeter (i work avionics on military aircraft) with lead resistance of .01Ω

 

My meter is much less luxurious than your Fluke so the leads/connectors are a pita when trying to measure low resistances.

Wizard: You must have an old Avo?

 

It's fairly new, Mark, but it is analogue. lol

 

analog certainly has it's place. our micro ohmmeters at work are still analog units.