Volts?

[ncgirl05]

Hi All!

I have what I believe is a fairly simple question - what is the "normal" reading for Volts on the gage screen? Mine is always around 12.9 to 13.2, my boyfriend's Reatta generally has 14.1 so my numbers seem low. Both cars are '89, BTW.

Thanks in advance for your help!

[Edselguy]

Search for "voltage antics" on this forum in the subject box, some info posted there a few weeks ago. My 89 voltage jumps around from 13.5 to 14.5 I don't know if that is a problem or not. When I got my 89 a couple months ago I thought it was the cause of the touch screen changing sizes (slightly) while driving, however, it is now stable all the time and the voltage still jumps around. These cars are fun.

John

[Guest EDBS0]

Mine is closer to the 14.1. If the car starts and funs regularly then I wouldn't think anything about it. Have you ever tried to compare oil pressure with the engines turned off? You might be surprised.

[ncgirl05]

It does start and run fine, but I wondered if there was an alternator problem lurking in my future, or some other item... I couldn't figure out why the difference in readings in the 2 cars.

[Guest kennyw]

The only way to tell for sure is to use a digital volt meter across the battery termanals. Do this after the car has run for a while. Mr Reatta settles down to around 13.3 on the screen but the meter tells a different story. Did you know that the altanator can be checked in service mode? You can pull up all the reading on windings, etc............ken

[luftweg]

A fluctuation in voltage is perfectly normal, as long as it's between about 13.5 and 14.5 v...

This is caused by the voltage regulator cutting in and out.

Why does it do this?:

The only way to get energy to go from the alternator to the battery is if the voltage from the alternator is higher than what the 'sitting' voltage of the battery would be (that is, if it were not connected with the electrical system).

Contrary to some common belief, '12 v' cars actually run on voltages in the 13 to 15 volt range.

When the voltage falls below a certain point (in this case somwhere in the mid-13 volt area), the regulator engages the field in the rotor of the alternator and causes current to be induced in the stator of the alternator (which also puts a torque load on it); this current both maintains the battery voltage and is used to run the rest of the electrical system (a heavy burden in a Reatta).

When the voltage rises too high (the mid to upper 14's), the regulator stops the field current in the rotor (this releases the load from the engine and stops stator current).

This 'cycling' continues all the time while the car is running, and is dependent on what is drawing on the electrical system and what rpm the engine is at (as well as what state of charge the battery is in).

It's more obvious in the Reatta because of the digital gauge.

When the car is off, the battery should be reading at least 2.1 volts per cell, or 12.6 volts total.

If it's less, than either the battery has a 'bad' cell, or is otherwise on it's way out, or the electrical system (alternator) is not charging enough for some reason (which could be one of many).

I have worked rebuilding alternators, starters, AC and DC motors, and have diagnosed charging/starting systems as a job before (did it for a few years, mostly part time and some full time).

It comes in handy...

Recently I had an interesting problem:

My Reatta's voltage was starting to drop slowly (over a day or two), then the 'brain' (computer) told me there was an electrical system fault, when the voltage read about 13.1v...

The voltage continued to drop over the next day or so, even though the car would start and drive...

I swapped a known good battery, and the voltage was still getting lower...

At that point I knew almost immediately the problem was either with the regulator or the bridge rectifier or stator in the alternator.

I pulled the alternator off, and apart, and tested reverse current on all the diodes in the bridge rectifier; sure enough, one was passing current both ways...

I went down to my old workplace and bought a new bridge rectifier (also, I replaced the front and rear bearings and brushes in the alternator); tested the voltage regulator too -- while it was out (it was okay).

I think the total cost for parts was $25.

Put it back together and on the car, and bingo, the codes cleared (of course, the battery had been disconnected while working on the car), and the voltage range quickly became 13.6 v to 14.5 v.

K

[Guest kennyw]

well this is new news. So what you are saying is that even if you alternator is charging well it still could be bad and be the cause of some of our battery drain problems? This is very interesting and something that has never come up on this board as far as I know. Is there a simpler way to make this test other than taking the alt. apart and doing tests? I do not have the equipment or know how or what part to test.........ken

[harry yarnell]

A leaky or shorted alternator diode (rectifier bridge) will run the battery down in a day or so. That's the layman's test.

As for charging voltages, if it's over 12.5, and less than 16 (short term), then I'm happy. Too many variables in there to have a 'correct' voltage. Load, battery condition, temperature are some that come to mind.

[luftweg]

Sure. The alternator stator or bridge rectifier could be sources of a 'short' (as I wrote before, a diode can end up passing current both ways (some 'newer' design bridges actually intentionally pass current both ways --this is not the case with Reatta), and a stator winding can end up shorting to the stator frame (this happens when the insulation burns off of the stator windings))... remember (and I'm 99% sure this is the case with the Reatta too), the alternator is always connected to the positive terminal of the battery whether the car is running or not running with the key off.

To check for a slight short elsewhere in the car, one should really disconnect the terminals on the alternator to rule it out.