View Full Version : Battery Volts

03-10-2008, 06:03 PM
Ok guys, my battery kept dying on me, and I replaced the old Lucas alternator with a 75 amp Bosch unit..

The car runs (idles, with no accessories on) @ 11.7-11.8 volts, and with the accessories, it runs @ about 11.5 volts..Does this sound correct? I noticed with my other cars, they run @ about high 13 volts when idling with no accessories on...

Trevor Jessie
03-10-2008, 06:16 PM
No, that is not correct. Should be about 14 volts. How many wires does the alternator have?

03-10-2008, 06:34 PM
The alternator has the same plug connector as the original Lucas one, so alls I did was plug in the harness to the back of it. I also noticed a spade terminal next to the connector, but im guess that is used for a different car?? This is a rebuilt Bosch alternator that also fits on an older mini and Land Rover..

https://cgi.ebay.com/ebaymotors/MG-MGB-Au...emZ260217912495 (https://cgi.ebay.com/ebaymotors/MG-MGB-Austin-Mini-Land-Rover-Alternator-Bosch-75-Amp_W0QQcmdZViewItemQQ_trksidZp1713Q2em153Q2el1262 QQcategoryZ33573QQihZ016QQitemZ260217912495)

Thats the one I bought..

Trevor Jessie
03-10-2008, 07:08 PM
OK, I'm not familiar with that unit, but it seems as though the connections should be correct. Does your "charge" light on your dash stay lit?

03-10-2008, 07:22 PM
Hmm, theres a charge light on the dash? I didnt know that, lol. Ive never seen this light up on the dash, and everything on the dash works (speedo, switches, rpms, etc..) I wonder if maybe the bulb blew out..

Maybe its a bad battery (or incorrect size)?? Its a 500 CCA, and 625 CA battery, in which the guy I bought it from said he bought it new...

Trevor Jessie
03-10-2008, 07:29 PM
OK, I'm wondering if the alternator is failing to sense a load. Do you have a test light with a probe? If so, then see if it lights (while the car is idling) when you touch it to the small wire on the alternator plug.

03-10-2008, 07:41 PM
Nope, I dont have a tester..Maybe I can get one from Pep Boys? Also, which small wire do I touch it on the alternator? Now Im thininking that my original alternator was ok and I installed this new one for nothing (but it IS supposedly double the amps...)

03-10-2008, 08:06 PM
Many alternators need the charge light to charge. If yours is blown out or not hooked up, that could be the cause of your alternator not charging and the low voltage reading.

Trevor Jessie
03-10-2008, 09:24 PM
Right, the smallest wire goes to the "ignition" or "charge" lamp. When the key is turned on this light comes on and once the alternator is generating power the lamp will not be lit.

03-10-2008, 10:52 PM
yes the small wire needs be activated with 12-v everytime you turn car on and than broken when off. On my race car its on a seperate switch but can be done with a relay as well. Its the wire that tells the alternator to charge for a lack of a better axample. Without that sensing 12-v the alt will not put out.

03-11-2008, 01:02 AM
Wow, so basically if that light bulb is blown out or not connected, the alternator will not charge? Ok, which light is it? I have a yellow light on top, Red light in the middle, and blue light on the bottom. When I turn the key on, the yellow light lights up, and when the car starts, it goes off..

What are each of the lights for?

Thanks guys...

03-11-2008, 01:59 AM
Ok, did some research..the yellow light is for oil pressure and the red one for ignition, SO, since my red light doesnt come on, this means the bulb is probably blown out, thus not allowing the alternator to charge! How do I remove this bulb out (just pop it out the dash??) Thanks guys!!

Trevor Jessie
03-11-2008, 07:26 AM
The bulb holder simply pulls out of the back, then change the bulb and press it back into the light housing. 30 second job.

03-11-2008, 02:33 PM
Thanks guys, it turned out that the bulb was missing! I swapped it with the oil pressure bulb and now it works!! Im seeing 14.5 volts now @ idle...Now, where do I find this replacement bulb (i need it for the oil pressure and high beam indicator)?? Thanks guys!!

03-11-2008, 02:37 PM

03-11-2008, 03:28 PM
Think you will find em at the local auto store as well. I did.

03-11-2008, 05:07 PM
I have the same alt set up. Highly recommended. I can know use the blinkers when the lights, wiper, CD and heater are all on.

off, flicker, smolder Mark

Trevor Jessie
03-11-2008, 06:05 PM
Must be something about those rubber bumpers sapping electrical power because my old Lucas unit has always supplied me with all the juice I need. :smile:

03-12-2008, 09:48 AM
Yes, this new alternator definitely is an improvement over the lucas one and I highly recommend it..My lights are now brighter and the turn signals blink faster when my lights are still on...

03-14-2008, 09:28 AM
Yes, this new alternator definitely is an improvement over the lucas one and I highly recommend it..My lights are now brighter and the turn signals blink faster when my lights are still on...

The Lucas alternators were only 35 amps. This is pretty marginal, especially if you add any electrical equipment. Also, the modern alternators' internal voltage regulators do a fantastic job of regulating the system voltage. This alone will keep the battery in better charge--so you have the advantage of increased output and more efficient charging. Replacing an old OE Lucas alternator with a modern one (even a 55 amp unit) is a very worthwhile improvement.

03-14-2008, 11:04 AM
Here's the Teglerizer homepage, for those who haven't seen it...

Alternator swap (https://www.teglerizer.com/alternator/index.html)


Conversion from Lucas to GM Delco "Chevy" unit, on both a 1275 and a 1500. Navigate via the blue links near the top of the page.

03-14-2008, 11:47 AM
I've the GM version. It went pretty well. The alternator is a bit large but after a while you get used to it.