Voltage Drop

[Pat]

It is my experience that my alternator pushes about 13-14 Volts for about 20 minutes and then drops to 10-11. Any idea what may cause the drop? Steve has indicated it may be not reaching a rev limit threshold.

Thanks,
Veek

 

[Tom_Hughes]

Bad diode in the alternator? Bad voltage regulator? Should be easy enough to pull it & test. My .02 cents says it is heat related, be it engine heat or current build up.

 

[Bill Bayard]

Veek,

I agree with Tom. Considering the lack of "normal" air flow

to the front of the engine, heat is a likely cause of your

problem. Does your alternator have an external fan? How

close is it to your exhaust? Do you get higher voltage(13+)

at higher RPMs? Check the low RPM diode in particular.

If that goes, you won't get any charge until above 2000RPMs

or so.

My $0.03 worth.


Bill

 

That sounds like your altrenator wiring might be too small. If it it, it will become a resistor and make a voltage divider, giving you a stable voltage when hot because the regulator regulates the voltage at the end of the wire.

 

[Howard Jones]

Where is your battery located? You might be losing a cell when it gets hot. Measure the battery voltage with the engine off, key off when your problem is present. Get youself a cheep voltohmmeter at radio shack or somewhere. If its not the battery then its the alternator. More than likely the voltage regulator portion. I think Kragen will check out a alternator for you for free. The last thing might be a cable getting very hot and increasing resistance. This is usually a connection at either end.

Good luck.

 

[Pat]

Thanks all.

The battery is in the right rear in the engine bay behind the fuel pump (standard CAV kit). It tests very healthy. Looks like I need to check the alternator and voltage regulator.

 

[Mark Clapp]

I had a similar situation. The flucuating volt meter was fixed when we fully relayed all the fans and added extra grounds. So the voltage was OK, just getting funky readings due to the faulty grounds. Also check the ground to the instrument cluster itself.

 

[Ian Clark]

Hi,

You want to run a ground strap directly from the battery to the engine block as the standard CAV setup grounds to the motor mount.

Also check your idle RPM, with all loads on we have no problems at 1050>1100rpm. The harness and gauges are very well grounded so this should not be an issue.

Nothing wrong with a bigger alternator either

 

All of the above, plus check that the case of the alternator is bonded to engine/chassis with a heavy braided wire, capable of passing 100 amps. The mounting bolts are not efficient enough at passing alternator current.
Tony.

 

[Bill Bayard]

For those guys considering paying big bucks ($200+) for a high amp alternator, you can get your existing one rebuilt at a higher amperage, somewhat cheaper. I took mine to my local alternator shop, where it was bumped from 100amps to 160amps(new stator, etc.), for $165,parts and labor. Fully warranted for 1 year. Only if I turn EVERYTHING on, will it drop below 12 volts at idle. It is MUCH better than before, but I am still going to add some better grounds, to see if that helps the idle situation.

Bill

 

[Frank Catt]

I'm with Ian, the earth strap on the factory CAV is inadequate, follow Ians guidance and put in additional engine earths, Frank

 

[Bill Bayard]

Frank,

That's next on the "list".
Just out of curiousity, in your experience, does the list ever end? /ubbthreads/images/graemlins/smirk.gif

Bill

 

[Pat]

To close the loop on this, I tried several of the suggestions and after the alternator was bench tested (it was fine) the fellow testing it suggested I get a higher amp battery. For reasons unknown, it worked. I also redid the terminal connections.

 

[Frank Catt]

Bill/Veek, with the CAV I found that the sill mounted batterys were just too small, so I move them to the front of the car and fit a much larger unit ( see previous posts ) as this gives much higher CCA but also helps the weight balance of the car. Do not listen to tails of voltage drop by moving the battery to the front, with a decent size cable the drop is minimal and unnoticable. If you want pics of the CAV front battery install just ask, Frank

 

[Frank Catt]

BILL, THE LIST NEVER ENDS, if it do's i'm dead !! Frank

 

[Howard Jones]

My guess is you had a battery that had a cell that was going dead under load(chemicals low in that cell or over deluted), shorting under higher than average load (heat), or shorting because of shock(vibration). Car batterys are really six 2 volt batterys in series, if one cell goes to 0 volts or shorts between + and - plates you are left with 10 volts. Anyway a new battery did it.