Bad voltmeter readings

I have a Wallymart plug in meter and watching that I see 14.7 at idle, 14.27 with headlights on (H4). 14.10 with the addition of the heater. stopped at a light, the idling at between 700 and 1000 rpm the cars sits fine and holds the volts BUT, at some point waiting for the light I hear the engine kind of lug down a bit (except the rpms dont change, it is just working a tad harder). when the engine sound changes, the voltmeter tanks right into the 9-10 volt range and I have to bring the revs up (sometimes when i do this, I get a belt squeel as well) The belt is new and well adjusted.

Alternator tanking on me?


It is the 73 GT with a Bosch/Fiesta alt

Carl,

Just curious...are you reading voltage at the battery?

Dropping to 9 or 10 volts indicates the battery is low. The alternator pull and belt squeel is the alternator trying to charge it up. I'd expect your starter to be turining a tad slow with that battery voltage. A loose belt would cause every symptom you stated. If you can turn the alternator by it's fan the belt is to loose.

I don't know Carl...maybe the alt. shaft is starting to sieze. Did you look at the pulley nut maybe it's come loose and causeing the pulley to jam up?

JDW, I do not quite get your question;

""If you can turn the alternator by it's fan the belt is to loose"".

Paul, I was wondering about something like that. The car goes really normal except for the time when the engine appears to lug down at the light and that is when the belt makes that problem. At no other time does the belt make that noise. Despite the full darkness and 18f weather I am gonig to go out and check the tightness again.

Belt seems to be right in spec tightness wise and the pulley nut is tight.

The engine grunting sounds like you suddenly have a heavier load on the alternator rather than a belt slipping. You may be right on the ragged edge of of the regulator cutting in and out on the alternator. Normally, the alternator starts at a low output voltage until the engine is reved sufficiently after start up for the regulator to kick in. From that point on, it should hold the voltage steady (withing the parameters of the alternator) even if the rpm drops below the point where the regulator first kicks in. You may have a regulator going south or just the low rpm and the load on the alternator being such that the regulator can no longer hold the optimum voltage. If you can tickle the throttle a bit and get the voltage to come back up, you are probably ok unless it gets worse. Cheers - Dave

I've had the same thing happen on our 71. If the battery has been drained to a low power reserve and the alternator load is heavy the belt would slip because of the load. We had done the conversion to the higher amp alternator. We narrowed the problem down to the new alt pulley was a bit wider than the old one and the h20 pulley as well as engine pulley, Changed out the alt pulley to a narrow one and the problem was solved.

Are your terminals clean and tight? If they have resistance in them everything will dim at low rpm. How old is the battery?

What Dave said! I think the alt. is dropping off the regulator. It "may" be that the belt is slipping slightly because of the load and the alt. slows enough to drop off. Alt. belts do need to be significantly tighter than generator belts - or you might try some "belt dressing", or even a new belt.

What RPM are you idling at? If you are down in the 800 range, you may need to up it to 1000 for winter driving!

Somewhat aside - my GT has the upgraded Lucas alternator and when the car is first started the volt gauge will show about 10. After I "blip" the revs up to about 2,000 the regulator cuts in, volts go to 14 and stay there even when the revs drop to idle. Very strange, but it seems to be O.K.

I run my idle at about 1000 rpm set with an external tach which is more accurate. With AC I set it about 1100 .

Does it coincide with an electric fan operating? If yes, measure current to the fan motor - shouldn't be more than 6-8 amps. Especially if the car has an alternator, it shouldn't dip like that. Suspect the battery. Also check the belt tension and see that there is nothing on the belt causing it to slip.

I vote for a finicky regulator. Good to try all the tests and sugestions given. But, the diodes and regulators are so notorious for failure in the Lukas alt that I'm leaning that way just by the law of averages. OK, I said it, now let me have it.

""Are your terminals clean and tight? If they have resistance in them everything will dim at low rpm. How old is the battery?""

Connections are good, the battery is new. I had 12.7 at the battery and 12.7 at the fuse box (brown lead).

Swamper, not sure about the pulley issue. The alt came 2 years ago with a pulley on it (again, Fiesta)

JackMG It does not have an eletric fan.


Limey, the belt is new (1 month). It only does the slip when it gets into that dropped zone and I rev to bring it out. My RPM does vary a bit. Some times it is 1100 and some times it is 800. The 800 is fine till it gets that load sound and then the charging tanks with no belt noise then or drop in rpms.

Dave Dubois; How can I get an intermittant heavier load when I have changed nothing in the car (it appears out of nowhere)?


NOW, here is another issue;
Every now and then when I turn the key, I get nothing. The next time I turn the key it is fine (new battery) (new starter). That was what made me watch the battery/volt level.
BTW, just went went out and checked the battery after it sat all night and temps got into the low teens and I still have 12.6 so I am confident the battery is good.

Forget specs. If you can grasp the fan behind the alternator pully and turn the alternator the belt is too loose. The friction coeffecient of all brands of belts isn't the same and changes with wear.

That puts it to a bad connection or the starter solenoid is giving up.

cfrench Wrote:

Dave and JDW are probably on the right track. I had to replace the starter on my car recently when both the solenoid and the field coils went south. When that happened, I could not get the starter to kick over but current got by the solenoid's switch as tho the solenoid's switch between the starter motor and the battery cable post was closed. If something like this happened to you, it would show up as a heavy draw down of your battery and no ability of the starter solenoid to pull in.

Your real problem may be a starter problem, Carl, and nothing to do with your alternator. Mercifully, when my starter went, it died suddenly, not intermittently, and it may be that yours is going bad intermittently. As soon as I replaced the starter, my voltmeter showed higher than usual charging for a while as it recharged the battery. FWIW

Me either. No external fan.

"Dave Dubois; How can I get an intermittant heavier load when I have changed nothing in the car (it appears out of nowhere)? "

Carl - That can happen if the regulator is getting flaky or an intermittent internal short comes and goes, Anything that causes a large current flow from the alternator (including anything internal) will cause a larger back EMF, causing drag on the alternator.

"Limey, the belt is new (1 month). It only does the slip when it gets into that dropped zone and I rev to bring it out. My RPM does vary a bit. Some times it is 1100 and some times it is 800. The 800 is fine till it gets that load sound and then the charging tanks with no belt noise then or drop in rpms. "

This makes me even more suspicious of an internal problem in the alternator.
Cheers,