battery voltage dropping

I am having an issue with my battery voltage dropping. The battery holds a charge and shows about 12.04 when running at 1500 idle. This initially alarmed me as it should be 13.4 so I thought that there might be charging issues. I suspected alternator & voltage regulator and tested them. They seemed ok, I checked & cleaned the fuses & sanded down large fuse on engine firewall. I checked & cleaned engine,battery grounds & cables...no issues. However with the key turned on (engine not running) the voltage drops from 12.04 down and keeps going lower-like 11.8, 11.6 and keeps going down (without the engine running). I disconnected the alt and had the same issue. I suspected that the alternator was not charging the battery, however that seems to be a secondary issue (if even the case). The battery is just dropping quickly without the engine even being on. I ran this engine on battery alone for hours when I was rebuilding it (it was on the floor) and it ran for a long time without the alternator even being attached to the car. I was thinking of disconnecting the fuel pump & seeing if I still had the voltage dropping so fast. I think that even with the key on the battery initially powers the fuel pump and I was thinking maybe there is a short between the fuel pump & the battery. I guess it must be a short somewhere but not sure where else to look? Any insight much appreciated!

 

ok...battery charged up & showing 12.7 volts...I turn lights on only (low beams) it drops to 12.05 volts & stays there...turn lights off & voltage goes to 12.63 volts(again the motor is off) . At 12.63 volts I turn key on until fuel pump kicks in..voltage drops to 12.30 and stays there. I turn off key & voltage rises to 12.62. I turn key back on to start fuel pump & turn lights on & voltage drops to 11.99 and stays there. I turn lights off (with fuel pump on) and voltage rises to 12.29 and stays there. I turn car off and go to bed (while neighbors suspect odd goings on in my garage at 9 at night!) I have to recheck the field voltage etc tomorrow (but grounds cleaned & fuses checked and/or replaced). Another wise sage suggested the voltage reulator as the culprit. However, I want to check everything before I replacce that. As usual all advice welcomed! Thanks

 

This has been a long process for me. I am not quite finished with it yet but when I am I hope to save many people a lot of time , money & aggravation (by example). There is a shot I will have a bunch done by tomorrow or weds and I will give you installment #1 of this issue. Hint- it was a huge pain in the rear!!

 

OK...finally just about done with my alternator troubles on my '74 us GTS. I was having difficulty getting a full charge to the battery and extensively tested the voltage regulator & alternator, fuses , grounds and battery. I probably ran 25 tests to see what was wrong. I tried the voltage regulator & installed a new voltage regulator ( they are all the same mine came from a VW), I tested the field wire, The output from the alternator. I did everything as I wanted to be 100% sure that it was not the alternator, The bottom line was i couldnt get higher than 11.5 on the field wire nor higher than 11.25-12 on the alternator or battery no matter what I did. I made the decision to pull the alternator which was a VERY tough job. It would have taken me probably less time to pull the engine. Maybe it will be much faster the second time I do it but it was a real pain. It is important to loosen the gas tank strap and shove the tank as far over as you can the extra 1/4- 3/8" is really needed to get the alternator out through the bottom under the oil cooler hoses and along side of the gas tank. The hoses coming from the aluminum radiator tubes have to come out too. I also pulled the alternator waffle housing and bracket that holds it to the engine. I was able to get this out down through the area by the shift linkage & radiator hoses (where they go thru the tunnel in the center of the car). My alternator had been rebuilt before I put the engine back into my 74 246 GTS. However after it was not working & I pulled it I sent the alternator back to the rebuilder and this time he said that the brushes had become soaked in oil & weren't charging. I cant figure this out as the engine was bone dry and has no oil leaks. In any case I decided to try to retrofit a regular GM alternator on the car just to see if i could do it and if it would work. I bought one from Jegs for about 110 bucks and it was an externally regulated unit. I also bought a euro alternator bracket from Dinoparts.de. I had tried to get the bracket(was told that they werent available)when I had the engine out to make the alteration at that time and to get rid of the waffle aluminum cannister housing that is on the Us versions. If your engine is out you should consider doing this...MUCH easier to work on & see. If you look at OMGJON's rebuild he has the euro bracket and thats what alerted me to the fact that they are available. It was pretty expensive 300 bucks or so but well made and I wanted to change mine. So I put the GM alternator in which has a smaller diameter pulley and a slightly wider groove which is giving me some slipping on the belt (as a reminder the US version cars have a narrow "v" belt which is smaller diameter than the euro belts). However, I am waiting for another pulley which I may fit to the alternator & see if it fits better. In any case the new GM alternator was giving me 15.25 at the battery which was too high. I was using my original red Dino Voltage regulator. I switched to a VW voltage regulator( I paid about 35 bucks) and the voltage dropped to high the low 14's . I dont know why this is except maybe the smaller pulley on the GM alternator spins faster than the original(larger) pulley on the magnetti marelli unit and in combo with the original voltage regulator was too much.( I intend to take apart the red Dino voltage regulator & look at it). I am not 100% sure about why this is happening but the replacement VW voltage regulator has given me the right voltage. I also replaced the 60 amp fuse as mine had melted ( and although it was one of the first things that I checked & it was ok -not sure when but at some point along the way it went) after I checked it initially. I bought two of them (about $50 bucks each which is crazy) and I now keep a spare in the glove box. However, here is an easy cheap fix (temporary) get a modern 60 amp fuse and if your original melts you can gently push the new style one into the slot and place it along one side of the original. The thickness of the original will keep it in place & the circuit will be continued through this fuse. It certainly will get you home until you can spend $50 bucks for a new one! If you repalce your alternator with a GM unit or any other unit with a smaller diameter pulley you will have to make a longer adjusting arm for the alternator. The geometry of the original Magnetti Marelli unit had it closer to the block due to the size of the pulley & the original shorter adjusting arm will fit. The new adjusting arm that I made is about 4" x 6" and is bent into a right angle. I may put a different pulley on this unit just to try it but I took the car out for a spint tonight and it all worked great. I really enjoy working on my car and if it wasnt for the helpful people on fchat (and a HUGE thanks to JS) I wouldn't be able to do any of it. I actually enjoy fixing it more than driving it (kinda sick huh?) Thanks and if I can help any one please feel free to ask.