Roundback and Squareback refer to the style of housing. Roundback through model year 1969. Roundback for model years 1970-71 usually has a slightly different rear housing Squareback Revised Squareback Is most easily differentiated by the thin middle between the housings. Overall these are slightly bigger but can usually be made to fit in the same location as the earlier alternators. THE STYLE OF CASTING CAN NOT BE RELIED ON TO MATCH THE WIRING WITH THE VOLTAGE REGULATOR First lets look at identifying the two standard alternator connection types. That will be followed by a link to a post by @67Dart273 illustrating some of the ways rebuilders chop up and change them. Terms: Brush is a spring loaded electrical contact made of carbon. This is a brush assembly. Slip ring is the spinning contact surface. Field refers to the rotor's slip rings and electrical windings. Field is short for the electromagnetic field created when electricity flows through those windings. Functional Identification Grounded-brush Alternator One brush assembly is grounded to the housing. The other brush is insulated from ground and has a terminal. This terminal gets connected to the output wire from the voltage regulator. from The 1970 Alternator & Regulator (Session 269) from the Master Technician's Service Conference Isolated-Field Alternator Both brush assemblies are insulated from the housing and have connecting terminals. Both terminals get connected to the voltage regulator. from https://www.hamtramck-historical.com/images/serviceHighlights/70 sh 71.jpg
Reality when dealing with replacements, especially emergencies. An isolated-field alternator can be used with a pre-70 voltage regulator circuit by grounding one field terminal. This is generally OK to do. Normally this is fine. Here's an example. Picked this up at a junkyard when the alt in my 67 siezed up. Added the wire to ground and it was good to go. With later revised squareback and 'high performance' alternators its possible the field current demands will be in excess of the regulator's capacity. The other thing to watch out for with higher capacity alternators is high rates of battery recharging. Alternators are often replaced when they stop working. This generally means the battery is low on charge. A battery with low charge and a high output alternator can be hard on the wiring and the battery. Even a standard alternator can cause damage. That's discussed in detail in this thead and others. Finally, as mentioned previously, unscrupulous rebuilders hack up these alternators in various ways. @67Dart273 shows examples in this post why it is critical to check the terminal connections for how they are, or are not, insulated from the housing. charging system blues
What would also be helpful is a listing of alternator part #'s and their AMP ratings. That way we can determine the AMP rating of our alternator w/o major research.
How this works. Line voltage is a power source. Controlled current is how much, if any, electricity is allowed to flow through the rotor. The more frequently the electricity is allowed to flow through, the stronger the electromagnetic field is. There is only one wire to the field because the regulator controls the current flowing into the rotor. The wire to the regulator is usually dark blue or blue with a white stripe. The output wire from the regulator was usually dark green. The rotor works the same way. The difference is the regulator controls the ground connection. When current flows through the rotor, it flows like this.
I agree. It is very difficult even when we do know what the original rating was. Then that's complicated by Chrysler's alternator part numbering. Service part numbers were not always the same as the original, and the same alternator would be given a different number depending on which pulley it got. Then for those of us with 'driver' cars there is the whole issue of 'ratings'. There seems to be no standard on how alternators are rated. On top of that rebuilders frequently substitute or mix up parts in a batch. Not just the pulleys but also the rotors and stators.
Identifying original parts. First. I am not an expert on this. This is just a cheat sheet on some of the basics and some links. Here's a nice example of a standard pre-1970 alternator from Plum Crazy original tech and date code info The thing that will survive when the markings have worn off is the raised rib of the rear bearing. This is an example of a standard 1970-71 roundback from this moparts thread. Here's a pic Scott Smith Harms took of probably an original 70-71 installed. I believe the orange paint indicates the 'rating' and is in addition to the part number. Also interesting that this one has the same rear casting shape as as a '60 to '69. As far as I can tell, all of these alternators had the part number stamped on the side after assembly. Here's an example from a beat up one I own. P/n 2642 547 Later alternators got a colored round tag for identification. There is a list, or a partial list, of these tags in a book by Paul Herd. picture from 1975 Dart Sport - Alternator Issue? The green color of the stator windings I think is another clue. edit: here's some notes from Paul Herd 1967-73 Barracuda and Challenger Tags started in 1974. Color indicates 'rating'
My compliments on an informative post AMP RATINGS...........the problem here is these girls are OLD and likely have been REBUILT. Once a factory OEM item goest through a rebuilder "it's anybody's guess" BRUSHES.......BE CAREFUL I've seen cases that "appeared OEM" than had both a spot for a grounded AND an isolated brush, AKA positions for THREE brush holders. Worse, some rebuilders DRILL a spot for a modified, isolated brush holder to convert a grounded brush unit to an isolated unit. AND THE PROBLEM with all this is that sometimes a grounded brush accidently gets installed along with an isolated brush........hooking one of these up can result in either massive overcharge or a burned up harness The final nail in the "grounded brush" coffin (may have been mentioned) is MISSING OR BROKEN hardware / insulators on isolated brushes which cause them to ground Unknown casting with evidently factory positions for both grounded and isolated brushes
And infamous "rebuilder modified" has been crudely drilled to accept an isolated brush. Interestingly, it STILL HAS a grounded brush
This is all good input. Buyer beware! I still think a list of factory part#'s with their amp rating would be a helpful starting point with the caveat that the only true measure would be to spin it up and see what it will really do. A person would need to glean info from all the parts books to make a master list. And no, I am not volunteering.
If we could find that information I am sure it will get posted. The Galen Govier books are probably the most complete as far as part numbers go and of the 3 books I have, neither have any alternator information.
It seems this has been a problem since the 60s. That's my interpretation of why Chrysler posted a service bulletin about the ratings and part numbers in 1966. The tech working on the car and the parts manager didn't have an easy way to figure out what was supposed to be on the car they were working with. The 1960s service manuals I've seen don't provide a full picture, nor do the parts books. page 1 here: https://www.hamtramck-historical.com/images/TSBs/1966/66-78_page1.jpg Briefly restated. The Production part number stamped on a 1966 alternator, refers to the alternator and pulley assembly. The 'rating' for each are given on page 2. https://www.hamtramck-historical.com/images/TSBs/1966/66-78_page2.jpg When ordering a replacement alternator, one then had to look up in the parts book the replacement part number. Yes it sure would be helpful to have a list of both production and replacement part numbers. The best hope might be an old cross reference book, say from the 70s. I think even Chrysler superceded the lowest rated alternators when replacing them. I forget where I saw that, but thought it was Chysler document like a TSB or parts book. If so the reasoning is obvious. They could save money in production by making lots of cheaper alternators with the minimum copper needed. Inventorying them probably made less sense especially since the consumer was going to pay for it. Individually the savings in copper per unit was probably minimal.
My 1982 Dodge Aries had the square-back alternator in a transverse 4 cyl engine (2.5L, based on slant-six block w/ 2 cyl lobbed off). The alternator was bad when I bought it and failed again several times. Each time, the 3 positive diodes disappeared (post 1, 3rd photo, top 3 diodes). Those are isolated from ground, and conduct heat to the floating rail, plus the air. In the round-back, those diodes also float but are pressed into a thicker aluminum holder. Regardless, the square-back seems to work fine in earlier longitudinal engines, so my guess was it didn't get enough airflow when sideways. Instead of always having to unbolt the AC compressor to work the alternator out, I got good at splitting it apart underneath the compressor and replacing just that diode assembly. By cutting "cooling fins" in the rail, I got them to last 2 years instead of the normal 1 year. A guy at an auto electric shop who sold the innards parts said that was a common failure. The 3 negative diodes (at bottom) are bolted to the case so get adequate cooling. The only cabin monitor was a battery idiot light, that when it came on you were almost done and might barely start and make it 20 miles, since they decided the dash ammeter was too confusing and added expense. Today, you can get a little cigarette-lighter voltmeter (some w/ USB charger) if your ammeter is bypassed. You should read 14.3 V while driving if the alternator and Vreg are working.
