| Author | Message | ||
| Chuck Brush (Chuckbrush)
Registered Member Username: Chuckbrush Post Number: 7 Registered: 1-2007 Posted From: 205.188.116.13 Rating: N/A |
Guys, I just found this : The 4104 as delivered from the factory had the positive terminal of the battery connected to ground. While this is how GM did all their vehicles, including cars, in the early 1950's, it is unlike almost all other vehicles. This can be a real problem, as most electrical devices for RV's use the standard connection - that is, the negative terminal is connected to ground. I have a 1951 GMC PGA 3301. It, too, has a positive ground. What is our 1955 4515? I'm sure it doesn't have a battery in it. When I go to try and put a battery in it is it a positive earth? The photo I have of the engine shows an alternator mounted. Would that mean that someone converted it to negative ground? Thanks, Chuck | ||
| Bob greenwood (Bob_greenwood)
Registered Member Username: Bob_greenwood Post Number: 578 Registered: 7-2006 Posted From: 64.136.49.228 Rating: N/A |
look at your amp meter as the bus is running,if it discharges when you rev. it up...change polarity. you three need to take a film crew with you  | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 61 Registered: 8-2006 Posted From: 207.231.80.150 Rating: N/A |
Chuck, DON'T PUT A BATTERY IN IT UNTIL YOU ARE SURE WHAT THE POLARITY IS! Backwards battery through an alternator is catastrophic!! All the diodes will fail from overcurrent, and if you're lucky, you won't be injured from the flash that occurs when you connect the battery(s). I won't even go into the thought of a battery explosion. Now, to the bright side. All battery terminals are different sizes deliberately. You SHOULD be able tell the polarity from the previous battery by putting one cable on at a time. If each terminal fits properly, one on, then off, and then the other one on and off, you're probably ok. If one terminal is real sloppy, and the other one doesn't want to go on, you're backwards. Bob's comment above refers to GENERATORS ONLY! A meter with a diode test position will tell you, if you know which wire out of the meter is positive. Take the alt output cable off, touch one wire to the terminal, the other to the alt frame, then reverse the wires. One way should show continuity, the other should show open. A NEGATIVE GROUND ALT WILL SHOW OPEN WITH THE POSITIVE WIRE FROM THE TESTER ON THE OUTPUT TERMINAL. A 9V transistor radio battery will tell you, pull the output cable off the alt, and touch one term of the battery to the output terminal, with the other connected to the alt frame. If no spark with the POSITIVE terminal connected to the output, bus is neg ground. Make sure you reverse the connections and get a small spark with the negative terminal of the battery connected to the output terminal for verification! HTH, George | ||
| Chuck Brush (Chuckbrush)
Registered Member Username: Chuckbrush Post Number: 9 Registered: 1-2007 Posted From: 152.163.100.138 Rating: N/A |
A NEGATIVE GROUND ALT WILL SHOW OPEN WITH THE POSITIVE WIRE FROM THE TESTER ON THE OUTPUT TERMINAL. Thanks George.... I can do this. Is there such a thing as a positive earth alternator? Chuck | ||
| George M. Todd (George_mc6)
Registered Member Username: George_mc6 Post Number: 63 Registered: 8-2006 Posted From: 207.231.80.150 Rating: N/A |
Chuck, I don't know, in the US, probably not very many. Yes, the 4104's were originally positive ground, as were most cars until the mid 50's. The British cars stayed positive earth until much later. I'm sure you will hear from many more of us in the morning, and many more qualified on early GM's than I am. We don't know what all has been done to this thing in 51 years, so we will just have to guess? A ground reversal requires some additional modifications, as I understand the 4104's had some diodes beside the dash, which had to be changed to run neg ground. If this bus is 12V, and has a radio mounted to the dash without insulators and a second wire coming off the case, its probably been converted... (if it was positive earth to start.) | ||
| Phil Dumpster (Phil_dumpster)
Registered Member Username: Phil_dumpster Post Number: 9 Registered: 5-2005 Posted From: 71.112.104.215 Rating: N/A |
I'm looking at the pictures on Ebay and they aren't very clear. The alternator is belt driven in those pictures. The original generator should be a shaft driven unit, if it is still there. It is possible whoever did the conversion added the belt driven alternator for house power but is still using the original generator for the bus power. If that is the case, it is most likely that the bus is still positive ground. Both bus circuits and house circuits can share the same ground and work fine. Please take a video crew with you - this should be real entertaining. I'm almost tempted to take time off work just to watch, if not to keep you from blowing the thing up. | ||
| Steven D. Braud (Steve77amg)
Registered Member Username: Steve77amg Post Number: 43 Registered: 5-2005 Posted From: 24.144.4.182 Rating: N/A |
What I'm curious about is why are older cars and busses positive ground and why the change to negative ground? I have a '77 AMG and it is positive ground. Can anyone enlighten me on that point? | ||
| Jim Wilke (Pd41044039)
Registered Member Username: Pd41044039 Post Number: 111 Registered: 2-2001 Posted From: 69.77.151.82 Rating: N/A |
I understand that when using the chassis/body of a vehicle for the ground path, that there is way less corrosion with positive ground due to the direction of the electron flow. With an aluminum unit body/chassis this would be a good idea and GM did just about everything they could to make the 4104 live forever. (Plus it would be "jumper cable compatible" with everything else that was on the road at that time.) Jim-Bob | ||
| Tom Caffrey (Pvcces)
Registered Member Username: Pvcces Post Number: 1032 Registered: 5-2001 Posted From: 65.74.69.234 Rating: N/A |
Chuck, it's possible for an alternator to be positive ground, and it's really not very difficult to do. Replacing the diodes with the opposite polarity will pretty well do the trick. In the case of a lot of heavy duty and marine equipment, there is no ground to the case of the equipment. So, you could have a floating ground or choose which polarity you want to be by just hooking the correct lead to the frame. For what it's worth. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 290 Registered: 11-2005 Posted From: 63.97.117.18 Rating: N/A |
As Tom said, there are pos ground heavy duty alternators. I have one on my '59 ALF fire truck. As I remember the US went to neg ground because GM decided to do it that way!! | ||
| Chuck Brush (Chuckbrush)
Registered Member Username: Chuckbrush Post Number: 11 Registered: 1-2007 Posted From: 64.12.116.138 Rating: N/A |
"As I remember the US went to neg ground because GM decided to do it that way!!" Thanks Gus...... Any Idea what year this happened? Seems like a lot of things happened at the time of the switch from 6 to 12 volts. I even have a vague memory of an 8 volt Mercury, could this be true or am I remembering things goofy? | ||
| Bob greenwood (Bob_greenwood)
Registered Member Username: Bob_greenwood Post Number: 583 Registered: 7-2006 Posted From: 64.136.49.228 Rating: N/A |
it was in the early 50's...they all changed to neg. ground..yes,they 'tried' to fix the slow turning 6 volt starters,with an 8 volt battery,was a poor attempt, I had a '50 olds engine in my '39 ford & the main problem was 'patched-up with the 8 volt battery,but,the starter bushings needed to be replaced often,for some reason. |