Fastboat

On my 98 Vette the battery voltage will drop from 13.7 volts to 11.2 and then go back to 13.7 volts at idle speed only. This voltage drop will occurr about every 10 seconds and it appears to be worse with lights on at night. (sitting at a red light) All the lights will dim accordlingly during the drop and return to normal brightness when voltage returns to 13.7v. Anything above idle speed the voltage remains at constant 13.7v. The alternator has been rebuilt but the problem remains. Also, there is a engine whine noticable during this voltage fluctuation... I was told by a local mechanic that there was a service bulletin from GM on this issue but I have not inquired about it to the local Dealership as of yet. Just wondering if any of you guys have encountered this problem or there was indeed a service issue on this from GM........ Thanks for all you comments on this issue.....

Bill Curlee

When this problems happens,,,,what is the voltage reading directly on the battery terminals???? Once you get that reading, take a voltage reading on the terminal. bolt on the BACK of the alternator to chassis ground. The alternator and battery terminal readings should be he SAME!

The PCM controls alternator output. If your having fluctuations, its probably time to do some ground maintenance!!

BC

ajg1915

I have a similar problem however my battery reads 14.4 V and the back of my alternator reads 14.5V, however my IPC gauge reads 12.4V. and so does the DIC.

Also my temperture gauge is reads 200 and if I read the gauge using the DIC it's 186.

All of the other gauges appear to be correct. I checked my grounds, what the heck could my problem be.

I am also getting Service Engine Soon accross DIC however I have no active codes.

I've done alot of work to the car, but I'm not sure about this problem.

CODE4LS1

I too, suffer from this problem. On my '98 it will show just over 14 volts at the back of the alternator and on the battery terminals, but the DIC shows lower. Would love to find out more if anything that can be done to remedy it.

Fastboat

Thanks guys............Ill check my voltage and grounds liked advised.. Strange problem for sure......

ericdwong

Sounds like the Air pump is operating. Its a motor and takes some juice. Its normal, mine does that.

ajg1915

Would love to see this service bulletin, as this problem is driving me crazy.

Anyone ever check to see if their temp gauge matched that of DIC readout ? Mine is off like 15 degrees ?

Thanks

3 vettes

check your voltage at the battery if it differs from the voltmeter in the dash you need to have the voltmeter recalibrated.find someone with a tech2 or go to the dealer.my old 97 was reading 13.8 at the battery and 11.4 on the dash.i work for a cadillac dealer so i have access to a tech2.you need to go into ipc and reset the voltmeter so it matches alternator output.many people have changed alternators, batteries and ripped into their harnesses trying to fix this problem with no results.check this out first before you waste alot of time and money.

Fastboat

Thanks again guys for this valuable info on this issue..........the only thing that bothers me is.....why does the voltage fluctuate? (The volt meter in the dash reflects the actual drop/fluctuation in voltage) If the voltage at the battery and volt meter in the dash differ, why would this cause the fluctuation between 13.7v and the 11.4v. continously at idle?? I feel the voltage is actually dropping due to the lights dimming accordingly. I just cant figure out why its dropping. I could probably live with the volt meter being alittle off calibration with the battery but the drop in voltage continuously drive me nuts too. Ill keep you guys posted on what I find..............Thanks again for all the info for I know where to start looking now.........
Andy

Bill Curlee

Andy

The alternator output is PCM controlled. The PCM controlls the PCM output and senses the voltage level of the B+ , When it dips low, it increases the putput. Sometimes you see that milli second drop between draw and PCM response to increase the alternator output. It should NOT drop below 13 VDC at the battery.

BC

BC

Fastboat

An update to this issue: The voltage drop does show at all locations, the indash voltmeter, the alternator, and at the battery. It does appear to only occur when the motor is at normal operating temperature. This voltage drop does not occur when the car is first started when cold........... Again, thanks for all your help....

tripod5

i agree with eric dewong, its sounds like the air pump, because that makes a whining sound when it operates, and it does drain the voltage when its operating. Something may be wrong why the air pump keeps on operating. I should only go on for a couple of seconds when your car starts for the catalytic converters.

Fastboat

Thanks Tripod5......what does the air pump do? Do you think this one is bad or is there something wrong elsewhere causing the pump to cycle so much. Thanks again...

Bill Curlee

The A.I R. Pump (Air Injection Reaction) Pump is an electric motor operated pump that supplies fresh air to the CATS during COLD start-up to aid the CAT light off. The AIR pump ONLY runs at cold start up for a pre programmed time (1-2 minutes) and during a TEST MODE as directed by the PCM when a whole list of parameters are met. It runs for a few seconds in test mode while the O2 sensors detect the fresh air report to the PCM and then the pump shuts down.

The pump should NOT run constantly or often.

Bill C

65GGvert

This doesn't sound like it would work, but it has fixed two for me with the same symptoms you describe. It gets worst with time. Follow the red wire from the battery down to the starter post. Shake it and see if you have any play. If the nut is loose, tighten it carefully. Or if the post is burned and the bakelite damaged, you will have to replace the starter. If the post is damaged from heat internally, it will pull out as you tighten the nut holding the wire in place. That has been the problem on two Z06's and each caused the voltage to fluctuate under changing engine speed conditions. Doesn't cost anything to take a look at it. Post what you find.

Bill Curlee

Any "LOAD" on the alternator will cause a current draw that is compensated by the PCM increasing the output of the alternator. The PCM does this by increasing the DUTY CYCLE of the alternator. If you apply the brakes, AIR Pump turns on, heated rear window is energized or the big one (window motor is used), you may see a slight dimming or a flicker of the headlights at night.

Its the current draw that causes the issues that many people are seeing. Current is inversely proportional to voltage. As current draw increases, voltage in the system will have a tendency to decrease. Your battery acts as a reservoir for voltage and helps dampen dip in the voltage.

If your battery is old, or very discharged when you crank the engine the starter will draw 300-400 amps on initial engagement and if you closely monitor the voltage at the battery, you can actually see it drop down to 8-9 VDC for a very brief period of time. Thats what causes a lot of DTCs ( Especially the U series DTCs) and causes some of the memory functions to reset to the preset setting and loss of FOB sync.

BC

0TheRadioFlyer97

Check your alternator's primary power wire connection. The nut may be loose. You can use a spring locking washer to secure a good connection.

you may also want to look into the "big-3" wiring upgrade.

Fastboat

Thanks again all you guys...........Ive learned so much in a short period of time.
It was also suggested by a local technician to pull the AIR pump relay out when this condition is occurring to see if the voltage drop does indeed go away...(Possibly a bad AIR Pump) I will do this plus check all the cables and connects between the battery, starter and alternator as recommended.
I cant say enough about how your all advice has made Vette owning more enjoyable and easier for us inexperienced owners. Ill check these items and reply my findings............

Andy

tripod5

I think that is a good choice for you to take the relay for the air pump out first and see if the voltage fluctuates. You will get a service engine light on your dash when you take out the relay, but you can clear it by resetting it. It can also be the pump solonoid relay. My relay was bad because they build up carbon between the contact points and dont work properly