Battery Light.....on at idle, off at high speed

[tight as a bull's ass]

on my 1970T,

my battery dashboard light stays on (brightly) at idle (around 1K RPMS) but completely goes off after 2K or so.

Last year, I replaced the alternator with a completely rebuilt one and also installed a brand new voltage regulator of the external type.

Any ideas of what's going on? thanks.

[Jdub]

Check all of your grounds (engine to frame, battery (-), and any others, esp. at the alt.) and be sure that the battery is not weak or is not "hiding" a fractured plate. You might want to bench test the battery.

Did the rebuilder say anything about the alt. being on its last rebuild? Maybe the brushes or commutator are just about worn out?

John

[RoninLB]

Originally posted by tight as a bull's ass

Any ideas of what's going on? thanks.

what does your volt meter say with blower and headlights on and off..........Ron

[tryan]

i have to sound stupid, but is the belt tight. which fuse needs a clean? #9? or is that a 914? pull the voltage regulator and see if the single shunt resistor is burnt on the bottom. i had no start trouble last night. good standing voltage on each side, but a loose connection at the drivers bat. never should have cleaned the fuz off.

[Phoenix-MN]

My '70T does the same thing, any rpm below ~1200 and the light glows. Never did this years ago when it had one battery installed. When I returned it back to stock with the two batteries the alt. light would glow at idle. I was told by my local mechanic that this was some what normal for cars with two batteries. It has always started and I have never had a problem with low battery voltage conditions. I just live with it -

[jpnovak]

My light comes on when the car is first started. It will go out when the car is reved above 2-2500rpm. Then it stays out. There was a post long ago about this and it was determined to be normal.

What is your idle set at? I also remember that low idle cars the light my glow slightly at really low rpms due to the alt. not spinning fast enough.

Sounds like you need to check the output with a DVM. A simple plug can be made through the cig lighter.

All the more reason to not let the car idle.

[targa80]

I had this problem when I first bough my car. With the belt tight, RPM at idle correct and all the grounds /battery are all good. Then it is possible that the alternator exciter diodes are starting to go. The exciter diodes are not the main rectifying diodes. The exciter diodes are small diodes that feed output to the voltage regulator from the alternator. There are three of them in the alternator, mine just fell apart when I probed them with a multimeter. I replaced them with 1N4000 diodes that you can purchase at any radio shack/electronic parts store. I did this repair over 12 years ago and have ahd no problems with it since.

[targa80]

The exciter dodes provide output from the D+ terminal on the alternator to the D+ terminal on the voltage regulator. There is a detailed schematic in the 74 schematics of the shop manuals.

[Early_S_Man]

This issue is a classic illustration of the effect of aging Faston connections [oxidation of the brass connectors] developing additional resistance ... and its' ultimate result in raising the 'Zero-Ampere Speed' of the alternator!!!

Try a serious clean-up with metal polish ... of the alternator warning lamp holder outside body which it gets its' ground connection, and try the parallel 68 Ohm 5 Watt resistor connection instead of the 91 Ohm resistor advised in the alternator upgrade bulletin and documented in the Bosch Automotive Handbook. The pre-excitation current for the field winding comes through the warning lamp circuit, and is directly responsible for what rpm level the 'Zero-Ampere Speed' phenomenon occurs at! Clean the female #61 Blue lead connector at the warning lamp holder with vinegar, followed by rinsing with distilled water and then isopropyl alcohol.

[Superman]

Could you be more specific, Warren?

[pwd72s]

Super? Be nice here...we both are among the many who are personally grateful for Warren's help! (This typed after laughing, BTW. ) Warren, there's a post on Rennlist you may want to visit...a 2nd year electrical engineering student, wondering about career choices, directions to go...title of the thread is something like "any electrical engineers here?"

[Superman]

I know Warren. He and I were among the anchors on this board three years ago and yes, I owe him dearly. If I piss him off, then maybe he'll write.

[cary]

Originally posted by Superman
Could you be more specific, Warren?

Now that cracks me up ................... LOL

[jk911]

Tight, How's your charging system going now ! did you fix that alternator problem you had a while ago.

I have been and still am in the same situation ! i.e rebuilt alternator, new battery, new voltage regulator, resistor on the warning lamp, checked and cleaned all wiring and 14 pin connector.... and still I can't get it to charge at idle especially when the headlights are on ! It charges fine when revved above 2000 rpm's.

I even pulled the alternator out for a second time, had it bench tested + load and I was told it was O.K.

I sick of having to rev the car every time I have to stop in traffic, the sporto gearswitch doesn't like it either any suggestions ? Anyone ?

[tight as a bull's ass]

jk911 ,

I've rebuilt the alternator (new brushes, diodes, new everything), brand new voltage regulator....

I don't know what else to do....when I come to a stop, the car idles momemtarily at just above 1K rpm, then drops below just about 1K rpm and the red light comes on.

then I get going again and the battery light goes off..

I haven't replaced the battery yet b/c it's fairly brand new..

so for now, I'm just gonna drive it like this and pretend the light is defective....(until of course, I get stranded somewhere)

it's a 30+ year old car and it runs well.....not perfect, but well for it age and originality.

[jk911]

I have the exact same problem 100%

I have just ignored it by trickle-charging my battery constantly and avoid driving at night (headlight use).

However my new battery only last 11 months and I just purchased a new one again this weekend.

I am sick of it and am now determined to beat it . Let me know if you have any insights.

[jk911]

Early-S Man . I took your advice and acted on it (68 Ohm resistor) and removed all corrosion. But unfortunately it didn't help.

Targa 80's advice seems logical. Can anyone guide me as to the process of replacing the excitor diodes, which one's of the 1N4000 do I use?

Please let me know as I have booked the car in to the auto electrician tomorrow.

I would however prefer to do it myself as I have spent $100's on auto electricians already and they have not helped.

[targa80]

The 1N4000 is the part number of the diode and they are used in many types of applications. Buy three of them and maybe a spare in case you mess one up in the installation. I have not bought them in a while so the diodes might set you back a buck or two (cheap). You will need to unsolder the old diodes and solder in the new diodes. You will need to take the alternator apart to get to them. The old diodes are small and should have a band/stripe on one end just make sure you install the new diodes with the band/stripe on the same side as the old diode. If you have never taken an alternator apart before may I suggest that you score a mark on the case where ther alternator will seperate that way it will help you to realign the parts when you put it back together.

[jk911]

Thanks for that Pat.

I just came back from the electronics store with a whole bunch of 1N4004 diodes. I was told that these would still work as they were the closest thing thay had to the 1N4000.


I am willing to give them a try but I am not confident on the proceedure. Are there any diagrams or pictures I can refer to ?

Where are they located in the alternator.

Where can I get the rest of the diodes for the alternator from its a Motollola.

[targa80]

The 1N4004 should work just fine for the exciter diode application. I posted a picture in the other post for the diode with a quick theory of operation.