Odd VOM reading on VAC

old · Oct 21, 2021

So jumped the VAC to get it to the shop. Checked voltage when running and got 8-10 volts across the battery running. Checked battery not running after I shut it down and battery had 5-6 volts. So can a battery be so bad that an alternator cannot put out the 13-14 volts it should when running??

Geo-TH,In · Oct 21, 2021

Old,
A bad battery can damage the alternator.

old · Oct 21, 2021

Well the way it seem to be working good chance both have problems. Just odd that it runs just fine but I get those readings. My just take the alternator to town and have it checked. I'm pretty sure it is one that is under warranty since I only buy them with life time warranty.

wore out · Oct 21, 2021

(quoted from post at 11:39:58 10/21/21) So jumped the VAC to get it to the shop. Checked voltage when running and got 8-10 volts across the battery running. Checked battery not running after I shut it down and battery had 5-6 volts. So can a battery be so bad that an alternator cannot put out the 13-14 volts it should when running??

LIKELY it has a "shorted cell". NOT uncommon.

Geo-TH,In · Oct 21, 2021

Lifetime alternators is the only thing I'll buy.
I'll bet your battery damaged it.
My rule is never jump start anything that way a bad battery won't damage the alternator.

old · Oct 21, 2021

Things are getting more confusing on this. I put the battery on the charger and after about 30 minutes it reads 11.76 volts. Put a different battery in the VAC and it fired up just fine as it should but wasn't charging from the alternator. Messed with the excite wire and seems it may have a bad diode in line. Put a different diode in line and it now seems to charge as it should but does seem a bit high on voltage as in 15 plus volts so may still have an alternator problem

old · Oct 21, 2021

I use a jump box not jump from another car or tractor.
I put a different battery in and it started as it should and run as it should. But reading across the battery said it was not charging I.E. battery read 12.56 volts. Checked the excite wire and took out the diode and it then started to charge. But maybe a bit high as in 15-16 volts.

Battery I took out has been on the battery charger for about 30 minutes and it now reads 11.76 volts so maybe a couple of problems. Also with no diode in line it would not shut off when turned off so maybe a bad diode. Thinking about rewiring it when I put in the new ignition switch

John T · Oct 21, 2021

Sounds like the battery may be totally shot with one or more shorted cells and hope it didn't damage the alternator????

John T

old · Oct 21, 2021

Since I posted this I have done a few more thing to it to try to figure it out and it keeps getting more confusing. Put battery on a charger and it now reads around 11.76 volts. Put different battery in and started it up and took a reading and only battery volts. Messed with the diode in the excite wire and took it out and placed the excite wire where the diode had been and checked voltage at the alternator out put and it read 16 or so volts which seems a bit high. Tried to turn it off and the alternator back feed as it should. Put in a different diode and now it shut down as it should but still seem to be charging a bit high. Wiring isn't very good so likely to rewire it when I change the ignition switch

ArleninOr · Oct 21, 2021

George, jump boxes can and will damage alternators too if not careful.

Janicholson · Oct 21, 2021

The original battery may be good. As low as it was, it will take a 4 or 5 hour charge, or an automatic charge for a day. hten check it for voltage after it is off the charge for an hour. 12.6 and good. Does your meter read 14.2 to 14.6 on another operating vehicle when running above idle? If high there as well I would test the meter against a standard, known good battery to assure reading correctness. Jim

old · Oct 21, 2021

Have the battery I took out on a 2 amp charge right now and will let it charge over night then unhook it and let it sit a while and check it again. Digital VOM so could be off some

JD Dave · Oct 21, 2021

Suspect the battery. Hook up a known-good battery and see if you get 13.5-14.5 volts charging with the engine running at about 1,000.

old · Oct 21, 2021

I change the battery and running at the alternator charge stud I get around 16 volts

SVcummins · Oct 21, 2021

rlp in Co. · Oct 22, 2021

A very accurate way to set a VOM is with a 3 terminal voltage regulator.

Geo-TH,In · Oct 22, 2021

Old,
I don't own a jump box. If my battery is dead, I'll remove it, put a different battery , charge the dead battery. It may take 24 hours if completely charge a dead batttery. Only takes a few minutes to replace a battery.

I have damaged alternators in the past with a dead or almost dead battery.
Takes more time replacing alternator, going to town to get another one and installing new alternator. NO thanks.

old · Oct 22, 2021

This tractor was out in the field so taking a battery out was an option but would have been a painful option. Plus the battery I put in will not let me close the hood so I'd have to have moved more then one battery around from tractor to tractor to get one that fit

Odd VOM reading on VAC

old

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