Can a bad battery cause low voltage?

[Ken Cunningham]

Please excuse the obvious ignorance of my question, but...

I have been experiencing voltage problems. Upon most starts, I will get very hard starting. Oftentimes my anti-theft doesn't allow a start, which I attribute to low voltage. I see voltage as low as 9 volts through the ALDL on the scan tool, and this is not during cranking, but key-on voltage. As soon as the car does start, and the alternator is operating, the voltage usually goes up to 12 or so. BUT, I have seen the volts light come on a couple of times while driving. I know I should expect the easy things first, but after cleaning up and relocating the ground wires on the p/s head, I decided to test the battery with the "turkey baster". The specific gravity was real low in all cells. Can the battery act like a sump, pulling power out of the system? Or should I still suspect that something else (like the regulator) killed the battery?

 

[WarpSpeed]

I'd suspect the altenator, first. Might have it tested at Pep Boys or whereever. Voltage should be 13.5 at idle, or more.

 

[Ken Cunningham]

Guess I should drive it in to Kragen and have them roll out the battery and alternator testors. Prolly one or t'other.

 

[Steve Wood]

I would suspect a bad cell in the battery....have it load tested and the alternator checked for proper output.

 

[Ken Cunningham]

OK, so I took the car into a Kragen and they brought out a hand-held meter which attached only to the battery. The results of the meter were all OK: cranking volts, charging volts, everything. So here's the situation: all cells measure low on spec. gravity. Volts light has come on twice while driving. I have seen very low volts at 1st start up (like 9-10 volts), and hard starts are rule now, not the exception. The mgr. of the Kragen suggested that the brushes may be going in the alternator, giving intermittant no charge. Sound reasonable?

 

[Steve Wood]

If all cells are down on sg, then it would sound like the battery is not being charged. Have you put a meter on the alternator to see what it reads when the engine is running? Should be close to 14 v in that state of battery charge....may be a bad diode in the regulator, etc.

 

[Outpost]

Where should the voltage numbers be when the engine is running? On the Scanmaster, we see numbers ranging from 13.2 to 14.2, but mostly 13.4 - 13.7. Is this normal for a TR? We have replaced the alternator and both battery cables, and tried a different battery with no change. I expected the numbers to be closer to 14.2.

 

[Steve Wood]

Scanmaster is not always meaningful as it may be seeing losses down the line rather than true battery voltage.

13.5-14.2 is pretty normal depending upon state of battery, alternator, rpm, etc.

 

[G McCall]

ken,
Assuming you have checked the obvious; all grounds and connections, try a good (borrowed) battery, and recheck the voltage. Good battery vs bad will give decent starts for a short time even if charging is a little down. Install borrowed battery, just don't try jumper cables. Also, these cars develop battery drains thru the years. Various sensors and even theft devices can drain (and ruin) a good battery in short time. With accessories and high battery drains its hard for a car to sit for 4 or 5 days and start easily. Run a search here for battery drains. I've read a few posts on the subject.