High Voltage Reading

Good morning!

I have a 2000 Z71 LS w/ the 5.3L, Ext Cab w/ 84,500 miles on it. Within the past week, I've noticed that my volt gauge on my dash jumped up to around 16.5-17V. Finally with some time to spare, I took it to Parts America to have 'em test the battery and Alternator. They put it on their tester and it came back as recharge the battery and then retest. The problem is, the truck had been running for 45 minutes so they advised that I had a dead battery and to replace it. They also advised that my alternator was putting out 14.8 V and therefore running fine. I had noticed no real difference in really starting with the lone exception being yesterday, and its been anywhere from 5 to 15 below this past week in Denver and the mountains.

So I Bought the battery, had them install it (I was without tools and its free installation :naughty: ) and was hoping that this would fix the problem.

It didn't. The volt gage still reads 16.5-17 V. It immediately jumps to 14V and then creeps its way up to the 16.5-17V mark within five seconds of starting. So, really searching for a second opinion, I took it to Checkers where they put it on their gage/multi meter/battery and alternator testing apparatus and it also came back as a good battery (640 CCA) and the alternator was putting out 14.3 V, exactly as expected.

So...should I just slap a new alternator on there or wait until it "truly" fails. I'm tempted to believe that the gage in the dash is bad; then again, its about that time when things like alternators and water pumps and other components can start to fail.

Or does anybody else have any suggestions? Bad regulator inside the alternator (thus needing a new alternator), bad wiring, or anything else that would/could cause an overdraw on the alternator?

I'm a recent college grad living in an apartment thus no garage to work on it :banghead: So any suggestions help!

Thanks!

Check all the grounds and connections from the alt. It sounds like a bad internal regualator though. Confirm the gage reading by checking the voltage with a dvom when the dash gage gives the high reading.

I am assuming that by checking the voltage w/ a DVOM you mean a digital voltometer? Or am I losing something in the acronym (I'm a newbie). If that is the case, where would I check that voltage at? :uhoh:

This is a slightly more technical question, but how can the alternator testing apparatus be telling the guys at the parts stores that it is working okay and be putting out the appropriate voltage when my gage is showing high voltage? I am assuming that the regulator has a type of sending unit/connection with the dash gage and thus the alternator can be showing appropriate voltage to the tester but in reality it is sending a high voltage because the regulator is broke? I'm only guessing...

Thanks so much for your help!

Using a voltmeter check the out put at the large wire on the alt., remember the alt is in a different environent in the truck varying loads and temps, so the bench test may not reveal a problem.

I have a 06 silverado with a 5.3 engine that the volt gauge reads high and the fuel gauge is off by a lot now also the problem is exactly as CoClone descibed have tried the suggestions posted with no luck:crying: :pullshairout

The testers they have typically check for voltage value and load test the batt. You need to get into like a sanp-on or midtronics to test regulator opertaion.

I have a 06 silverado with a 5.3 engine that the volt gauge reads high and the fuel gauge is off by a lot now also the problem is exactly as CoClone descibed have tried the suggestions posted with no luck:crying: :pullshairout

the dash gauges are not to be used for troubleshooting...

With a voltmeter measure the voltage at the alternator red wire output , using the alternator frame as the ground...

measure this on a cold start give air temp...

then after vehicle has been operating for about a 30 min period measure the voltage again ..then report your findings..

the colder the air temp the higher the voltage out of the alternator ..a 20 below zero f cold start may indeed give a voltage of almost 16volts for a short time...

on the other extreme 115 deg f this voltage would be 13.25 volts..

most importantly the voltage should never quickly vary with RPM changes ..if so the alternator is most likely n/g..

removal and bench testing will reveal a bad regulator/diode condition..

with special current testing probes this can be measured on the vehicle..normally a silverado with all equiptment / lights on ,will draw about 66amps at 80 deg f battery fully charged..

the older GM vehicles had defective regulators [they got corroded circuit runs] this would cause no output..

I have a 02 Z71 and I have the same problem since the weather has been inthe teens twenties. Is it possible it may fix its self when the weather warms up in a few days.

I have a 02 Z71 and I have the same problem since the weather has been inthe teens twenties. Is it possible it may fix its self when the weather warms up in a few days.

like I said to others use a voltmeter and give temp engine hot, should be around 14.25volts ..when engine cold depends on what the temp is !

dash guages are not accurate most times...measure at alternator stud to alternator body ground.

according to haynes service manual the voltage reading shgould be 14.25 with engine hot and around 2000 rpm..... a increased fluctiaon of 1.5 volts is ok but anymore than that represents a Bad Alternator (internal charging component)..... or possibly a short in the wiring

according to haynes service manual the voltage reading shgould be 14.25 with engine hot and around 2000 rpm..... a increased fluctiaon of 1.5 volts is ok but anymore than that represents a Bad Alternator (internal charging component)..... or possibly a short in the wiring


if the alternator /battery is good / proper charge, the voltage should not change with rpm changes. the manual is incorrect.

only temperature of the alternator regulator will effect voltage output.

if this is changing as rpm changes the alternator is defective.

when in the very old days of dc generators this voltage variation with rpm increases is a fact...that is why we today have the alternator.

I do not disagree that the variation is the reason for going to alternators, I only quoted the service manual.

I do not disagree that the variation is the reason for going to alternators, I only quoted the service manual.


If the manual did say , rpm will change voltage output , then this is incorrect with THIS vehicle ...

just wanted to make clear rpm change has no effect, only the temperature of the alternator.

cold: higher volts/ hot: lower volts...