Reman squareback alternator internal short?

[propilot74]

My 67 Monaco is in need of a new alternator, the bearings are shot and it's growling something awful.
I purchased a reman unit from Napa Auto that is rated at 60amps (squareback), so unlike my current alternator, it has 2 field terminals. I understand I just need to ground one of these terminals and connect the field wire to the other. I figured I'd do some basic checks to the new unit before installing it. I got my ohm meter and checked continuity to the case (ground) from each of the 3 terminals. I found both field terminals to have continuity (no resistance) to ground, and the Bat+ terminal showed continuity with resistance to ground. It's this indicative of an internally shorted alternator? Should I return it? This is my first Mopar, so I'm not too familiar with these. Thanks in advance!

 

[Vaanth]

My 67 Monaco is in need of a new alternator, the bearings are shot and it's growling something awful.
I purchased a reman unit from Napa Auto that is rated at 60amps (squareback), so unlike my current alternator, it has 2 field terminals. I understand I just need to ground one of these terminals and connect the field wire to the other. I figured I'd do some basic checks to the new unit before installing it. I got my ohm meter and checked continuity to the case (ground) from each of the 3 terminals. I found both field terminals to have continuity (no resistance) to ground, and the Bat+ terminal showed continuity with resistance to ground. It's this indicative of an internally shorted alternator? Should I return it? This is my first Mopar, so I'm not too familiar with these. Thanks in advance!

View attachment 743107

With the listed measurement results, the rotor wiring is shorted to the core, and subsequently to the alternator housing. Or one, or both, of the brush mounts are shorted. Resistance from either terminal to the housing should be an open circuit in an isolated field alternator.

Resistance between the two field terminals in a non shorted good rotor is typically about 4-6 ohms depending on the winding size.

Note that one terminal has been cut off. This brush connection might have been intentionally shorted to make the alternator a "single" field connection for a pre-1970 setup. If the cut terminal measures ~0 ohm to the housing, and the other terminal measures 4-6 ohms to the housing, this might be the intent.

---

 

[propilot74]

With the listed measurement results, the rotor wiring is shorted to the core, and subsequently to the alternator housing. Or one, or both, of the brush mounts are shorted. Resistance from either terminal to the housing should be an open circuit in an isolated field alternator.

Resistance between the two field terminals in a non shorted good rotor is typically about 4-6 ohms depending on the winding size.

Note that one terminal has been cut off. This brush connection might have been intentionally shorted to make the alternator a "single" field connection for a pre-1970 setup. If the cut terminal measures ~0 ohm to the housing, and the other terminal measures 4-6 ohms to the housing, this might be the intent.

---

I just rechecked, and both field terminals read 0.4 ohms to the case, and the Bat+ is showing approximately 1000 ohms to the case. Resistance between the two field terminals is 0.3 ohms.
So I guess I have an internal short.
I suppose it was a good thing I checked!

 

[Vaanth]

I just rechecked, and both field terminals read 0.4 ohms to the case, and the Bat+ is showing approximately 1000 ohms to the case. Resistance between the two field terminals is 0.3 ohms.
So I guess I have an internal short.
I suppose it was a good thing I checked!

With the measured numbers, that alternator is bad. It may have the shorted brush for the application use, but the rotor is definitely shorted. It should never have made it out of the rebuilder shop in that condition.


FYI:
Measuring from the charging post (BAT+) to the housing is measuring the resistance of the internal capacitor. Some resistance is normal, but none is another short. Depending on the meter, while keeping the leads connected, you should see the resistance number climb or drop, then reversing the leads, you should see the reverse, drop or climb, in the numbers. This is the capacitor charging/discharging. It's not a full test of the capacitor, but it is a basic check that the capacitor itself is operational.

 

[MoPar~Man]

I'm looking at a '73 service manual, and the test procedure shows one of the FLD contacts jumpered to ground, but it doesn't go into detail as to which one. In the photo of the alternator, the field contacts are identified as "field brush - horizontal" and "field brush, vertical". The horizontal brush is the one closest to the battery output stud. The test procedure doesn't identify exactly which field brush is to be grounded. And they give no static resistance measurements.

I would think there should be a very high resistance from the field terminals to ground, and a low resistance between the terminals.

 

[propilot74]

Thanks, I will be returning it tomorrow.

 

[72RoadRunnerGTX]

As mentioned, it appears one terminal has been clipped, very common practice for rebuilders to do that after grounding the terminal to make a dual isolated field alternator into a single field alternator for use on a pre ’70 application. I would remove that brush assembly, or just the mounting screw, and check continuity to the case from the other brush terminal before condemning that alternator. The clipped brush is likely grounded to case intentionally using a metal washer under the head of the mounting screw in place of the original insulating washer for a dual isolated field brush assembly. Have seen this countless times.

 

[propilot74]

As mentioned, it appears one terminal has been clipped, very common practice for rebuilders to do that after grounding the terminal to make a dual isolated field alternator into a single field altnator for use on a pre ’70 application. I would remove that brush assembly, or just the mounting screw, and check continuity to the case from the other brush terminal before condemning that alternator. The clipped brush is likely grounded to case intentionally using a metal washer under the head of the mounting screw in place the original insulating washer for a dual isolated field brush assembly. Have seen this countless times.

OK. Now I feel like a complete idiot, but I guess this is how we learn. I was measuring using the screws on the brush terminals, thinking they were the wire attachment points.
I retested using the brass terminal and found the following:
The insulated brush shows 5.5 ohms to the case, and the grounded brush (it does have a metal washer) is still 0.3 ohms to the case.
Resistance between the two terminals is 5.8 ohms.
The Bat+ shows 1k ohms with the negative lead on the post and the positive lead on the case, but shows infinity when I reverse the leads. So maybe this alternator is OK? I'm still not 100% sure; maybe one of you guys will be able to tell me. Your help is very much appreciated!

 

[Vaanth]

OK. Now I feel like a complete idiot, but I guess this is how we learn. I was measuring using the screws on the brush terminals, thinking they were the wire attachment points.
I retested using the brass terminal and found the following:
The insulated brush shows 5.5 ohms to the case, and the grounded brush (it does have a metal washer) is still 0.3 ohms to the case.
Resistance between the two terminals is 5.8 ohms.
The Bat+ shows 1k ohms with the negative lead on the post and the positive lead on the case, but shows infinity when I reverse the leads. So maybe this alternator is OK? I'm still not 100% sure; maybe one of you guys will be able to tell me. Your help is very much appreciated!

With the new information, the alternator sounds okay.

 

[Big_John]

My 67 Monaco is in need of a new alternator, the bearings are shot and it's growling something awful.

That's pretty easy to fix. Its always the bearing in the front, as the rear bearing doesn't have much load on it.

 

[413]

Is there a insulating washer under the bolts securing the field terminals? There should be one on each for a dual field.

You can remove one insulating washer to ground it. The clipped off terminal looks like some one was going for that.

 

[propilot74]

Is there a insulating washer under the bolts securing the field terminals? There should be one on each for a dual field.

You can remove one insulating washer to ground it. The clipped off terminal looks like some one was going for that.

There is under one of the terminals (the one that will still accept a spade connector), the other one has a metal washer under it, which grounds it. It appears to be fine. I installed it this morning and it's charging without the bearing racket.

 

[propilot74]

That's pretty easy to fix. Its always the bearing in the front, as the rear bearing doesn't have much load on it.

Since there was no core required for the new alternator, I may try and repair this one and keep it as a spare. I've never changed one of these bearings before. Not even sure i have the right tools to do it.

 

[Big_John]

Since there was no core required for the new alternator, I may try and repair this one and keep it as a spare. I've never changed one of these bearings before. Not even sure i have the right tools to do it.

A puller for the pulley is the only tool that you might not have. You might even be able to rent one.

 

[propilot74]

A puller for the pulley is the only tool that you might not have. You might even be able to rent one.

What do i use to drive the old bearing out and the new one in? Just a couple of appropriately sized sockets and a mallet? (I'm just guessing, I haven't even looked at it yet).

 

[Big_John]

What do i use to drive the old bearing out and the new one in? Just a couple of appropriately sized sockets and a mallet? (I'm just guessing, I haven't even looked at it yet).

While the rear bearing is pressed in, the front bearing is just held in with a spring clip.

Rear bearings rarely fail, but they can be pressed in and out with a vice.

New brushes are a good idea too.

Download a service manual at Service Manuals – MyMopar if you don't have one.

 

[propilot74]

While the rear bearing is pressed in, the front bearing is just held in with a spring clip.

Rear bearings rarely fail, but they can be pressed in and out with a vice.

New brushes are a good idea too.

Download a service manual at Service Manuals – MyMopar if you don't have one.

Thank you!
I actually have the service manual downloaded to my PC. I'll pull it up later and find the relevant section to print out.
Thanks for the advice, much appreciated!