Voltage drop

I recently installed a New York AC compressor and clutch (for my AC resotration / conversion) and as we filled the system everything worked well, except the clutch was exhibiting a weird ‘slip and catch’ thing. My mechanic discerned we were not getting a good 12 volts at the wire to the clutch; rather we had between 10.5 and 11.5V. At that time with the car running we had roughly 13.7 at the alternator B+, and high 12’s / low 13’s at the battery. Similarly we only had 10.5-11.5V at the snowflake switch and blower control knob.

I initially thought it might be the alternator…but now I’m not sure….I’ve read a few things on the forum including ‘beefing up’ the alternators, and I know voltage drops on our cars is not entirely rare. This weekend I was able to check the alternator / diodes and suspected voltage drop, and got some confusing results:

- with the alternator disconnected and using 2 different multimeters (and with red lead on the alternator casing and black lead on the B+ post) I got 1380 and 911 respectively. Too high. Not sure why 2 different readings, and the second one is with a ‘newer’ (and probably more trustworthy) multimeter.
- then I cleaned the connection and reattached the wire to the B+ post, and rechecked the diode test. Now I get a reading (with the better multimeter) of 690, and I checked it twice - once with the black lead on the B+ post, and then with the black lead on the battery, and got the same reading. BUT I read the test is only valid when the alternator is disconnected. Is there any truth to that?
- then I turned the car on and checked the alternator (although not a full ‘load test’)- it was putting out 14.1V at the B+ post, and 13.6 at the battery. I also checked to make sure it wasn’t putting out and AC volts and it was not.
- I did recheck the AC clutch wire and found 11.85V with the car running and the battery at 13.5V.
- Lastly I checked voltage drop on the negative from alternator case to negative battery terminal and it was only 120mV, and then on the positive side from alternator to positive battery terminal and it was 0.46V, which seems like ‘borderline’ a lot. NOW, I know I need to do the voltage drop test on the positive side further and check at which connection most of the drop is getting lost; the ammeter shunt, relays, fuse boards etc….Although, I did check one relay (to the right radiator fan motor) and it was only 10.72V (car off, but ignition on)

I may try the diode test again with a ‘really good expensive’ multimeter. I’m also going the change the voltage regulator with brushes just to see if it makes a difference in voltage output. And I may call Centerline Alfa in Colorado that ‘improves’ our alternators - maybe a better alternator does the trick?

I suspect, and as I’ve read, that this is all fairly common with our cars and actually not so bad - although I’m not sure how to rectify the voltage issue at the AC clutch. Maybe install a separate AC clutch relay? It’s not like we can rewire the entire car, and honestly I feel my car’s wiring is…fairly pristine. Super clean and not much dirt, grime or cobwebs anywhere…

Thank you and any opinions are welcomed..

 

guys - thanks for the input.
Yes - I absolutely need a trustworthy multimeter. I may have fallen prey to the Amazon allure of the 'most often bought' item. I will correct this and buy a very good one. YGWYPF (you get what you pay for.....)
I will indeed check the fuses and relays. All my wiring looks great, but I will double check as I certainly do not want this known issue get worse...
When redoing the AC removed both the high and low pressure switches, and change them out to a single high/low pressure switch. I can easily check voltage drop across that. Since the power to the clutch comes from the snowflake switch I was secretly thinking I may have a bad (or going bad) snowflake switch. again I can check the voltage drop across that switch alone to check its status.
visually my ammeter shunt and all associated wiring in and around the relay panel looks pristine. But again I will double check and take some picture documentation.
thanks again.

 

This is a '78 carb'd manual series 1.
Yes I agree that changing the Alt likely will not remedy the issue.
I absolutely need a better Multimeter, and I absolutely must do a better voltage drop test to isolate the circuit causing the drop. Using the jumper is a great idea, (although one of the first things we did the day we were changing the AC system and realized the low voltage was jumper battery to AC clutch and voila - worked fine)
I do have a useable wiring diagram from my car! the original book! In fact, I went to Staples and had it enlarged and laminated! So now I can draw on it - then erase it!

 

Yes exactly my plan to first test alternator to ammeter- then ammeter to fuse block- and the fuse block to snowflake.
Changing all the connectors would be ambitious…but time prohibitive. Mine look ok with no ‘burnt’ connectors. Maybe a future project.
Improving the grounds is always a good idea. Fortunately my alternator to negative battery was only 120mV. But I’ll check them all once I get the car in the air…

 

Is a half volt drop acceptable? Or too much?

Yes good idea. Man! I can not believe how expensive some of the silver elec greases are!

I guess I’ll make sure the snowflake switch is good/bad, but I’ll bypass it as I troubleshoot the system

Relay would be an easy thing to do…but I’d rather figure out why the drop in the first place…

Thanks!

 

So I didn’t use the silver conductive grease (yet) mainly because the one I ordered, despite being advertised as conductive, specifically says on the side of the jar ‘non-electrical conductive’. BUT I did clean all connections in the alternator/ammeter shunt/ battery axis; I used a dremel with wire wheel and shined all the connections and associated washers, including on the alternator B+, both ammeter shunt connections and of course the battery terminals.
At best readings with the car running and roughly 200 rpms I still got:
- alternator B+ and ground - 14.1
- shunt in and ground - 13.64 (drop of 0.46)
- shunt out and ground - 13.59 (drop of 0.05 within the shunt)
- battery - 13.46 (drop 0.13 from shunt out / 0.64 total from alternator)
- AC clutch - 11.9
I didn’t check ammeter shunt to fuse block, or fuse block to snowflake switch mostly because I forgot - but I was also running short on time.
Frankly I think my results show that most of the drop is in the cable from the alternator to the ammeter shunt; but both ends are definitely clean, and I just can’t imagine the mid-section of the cable ‘just went bad’… so it’s still a little confusing. And I’m certainly not going to replace/change the cable in question; that seems like a pretty monumental task considering how tight that engine bay is especially where the wires enter the passenger foot well where the relay board is.

More to follow as I figure stuff out….
Right now I’m leaning towards adding a new relay for the AC clutch…HOWEVER 11.9V seems like it should run the clutch, doesn’t it?…..
Thanks - any help or constructive criticism always welcomed…

 

Good question on current being drawn by the AC clutch. I did not because I did not refill the system with Freon since I was troubleshooting the voltage drop. And I did not want to run the compressor without Freon. In the end, I may have to fill it with Freon just to check the amperage and the worst case scenario is, we may have to suck the Freon back out to change the clutch if needed. If the clutch ends up being the problem

I did not get a new multimeter, but I’m confident in my better one as I tested it in a few different scenarios and it seem to give me pretty accurate results.

does it seem reasonable that 11.9 V at the AC clutch wire is enough to activate the clutch without any issues?

 

Gotcha..i may try that… ty!

 

Yes, when we tested the clutch, it was full of Freon. And I would never run the AC without any Freon in the system.

It’s good to hear that you did exactly what I was thinking. Yes it dawned on me. These old cars have old wiring and I’d love to snap my fingers and have all new wiring but that’s just not something I’m gonna do right now. I’ll do a few more tests, but I’m, not opposed to doing the relay thing.

The B+ post is not a bad idea. But why do you suggest to do that? just to make sure you have full power going to the clutch? Can the clutch handle 14 V? (as opposed to 12.5 -13)

 

Yes single alternator..
For clarity the only B+ post is on the back of the alternator (i’m fairly certain)
But still..could run a new cable with the existing one (yes correct the cable goes up front between the headlights and then emerging in the right front corner to follow the right fender up to the battery and passenger footwell/fuse/relay area) and put a new fuse/relay somewhere up front right

 

I’ll remove the battery again and take a look. I was under the car on Sunday to try and see from the bottom where the cables entered the passenger foot well to go to the fuse panel and relay board. It certainly didn’t seem easy, but I’ll take another look. I have the tools to make my own cables so I’m not concerned about that.

A mechanic friend of mine had an idea to just run an additional cable from the alternator directly to the battery. Leave the existing one in place but just put a second one. It’s an interesting idea and I’ve never done that before. Does anyone have any opinions about that?