Ok, about three months ago the alternator went out in my 78k 2000 Blazer. I had it replaced at a shop with a supposed Delco reman unit. Ever since the incident occured, my interior backlighting has been flickering like crazy. Specifically the digital displays for PRND321, Odometer, clock on stereo, and overhead console. I figured this might be a result of a failing dimmer switch control. Last week the battery crapped out, and I had a new Interstate battery installed. Two nights ago I drove it (normally my wife drives it, I drive my Caprice most of the time) and noticed the headlights, interior lights, and foglamps were all flickering.

So, what is the root of this problem? I have had it back at the repair shop once and they say they can't find anything wrong with it. Any clues?

 

Sounds to me like one of the diodes in your alternator is bad.

An alternator is actually an AC generator with diodes to convert it the voltage to DC. If you have a volt meter set it on the AC seting and read the output terminal of the alternator, If you have any signifigant reading this is the case. Another tell tale is that the flickering can change speed with the RPM of the motor.

Just because it is a new to you alternator, doesn't mean somebody else didn't blow it up and return it with the good ol' "I don't know whats wrong with it..." routine. Placing a really dead battery or hooking up a battery backwards can cause diode failure.

 

Thanks for the quick help DCFluX. I figured it was one of the two. I have the same situation with my 95 blazer. Battery is 5 years old and alternator is 3. LOL! Time to replace! I also have a bad fuel pump that needs changing tonight. Too cold out. Having a mechanic do it. pump = $75 Strainer = $8 Labor = $225
Is that labor a rip off? that was the cheapest I could find so far.

 

If a battery is 5 years old I just flat out replace it. No sense in beating a dead horse. Also when replacing the alternator you should be certain that the battery has a good charge before hand so 'Alternator Death' does no occur.

Sounds about right for the parts. Supposedly it takes special tools to get the ring off the tank but everyone I have asked used 2 straight screw drivers, including me. I did it once on a 3rd gen Camaro and it took like 4 hours to get the tank out without droping the rear axle and drive shaft. So it sounds about the right price for what my time would be worth plus you are getting a mechanic to do the repairs which will come with a warrenty and little risk of blowing your self up from the fumes and gas still in the tank.

A common thing that can go wrong with this repair is bending the tube sructure that the fuel level sender is on, which will give false gas tank readings. Some people have said that there fuel gauge will no longer function when under a half a tank.

 

Thanks for the insight. Will be usefull when i pick her up after work today. I got the battery and the alternator tested at auto zone yesterday and they were both good. any other ideas on what could be the source of the electrical problem? Or should I just replace the battery and alternator and call it a day?

 

I wouldn't trust the staff or equipment of Autozone when it comes to checking to see if an alternator is ok. Unless you load the alternator and check it with either a AC voltmeter or preferably an oscilloscope there is no way of seeing a bad diode.

Their battery tester might be ok though as the one I have seen locally looks like it puts a 200 amp load onto the battery and monitors the voltage for a couple of minutes. Out here in the desert it is seldom we see a battery last that long so I am used to changing them by default.

There isn't a whole lot in the car that can cause flickering that is running in a pulsed configuration. About the only things that come to mind are the injectors and the ignition.

It could be possible that one of the wires feeding the injectors is shorted to ground but I would think that the fuse would blow that power the injectors and you would get noticablly less gas milleage. A dead short on a spark plug wire may cause the same conditions.

Another way to isolate the problem is the duty cycle of the flicker.
Assuming a V6, If it is 1/2 dim and 1/2 normal I would suspect an injector or injector wire.
1/6 dim 5/6 normal shorted plug wire or bad distributor.
1/3 dim 2/3 normal or 2/3 dim 1/3 normal it would be the alternator.
All problems would change speed (not duty cycle) with the RPM of the engine.

 

Could be a lose ground, like on the battery.

 

Update: I went to the shop where I drive a wrecker part time, and had a brand new Interstate 650CCA battery installed. Now the headlights, taillights, interior lights are "strobing" really bad. It's very noticable, even if you pass my truck going down the road or coming up from behind. It's driving me batty. They volt gauge doesn't really move more than usual. I called the dealer where I bought it. The head mechanic said it sounded like a bad voltage regulator. I took it back to the shop who installed the alternator and they had it all day. I just talked to them and they are telling me it has "a 150 millaamp draw on it. It's not the alternator, not our problem..." I say hogwash. edited for language

So, let's assume for a moment it's not the alternator. What could be causing the problem? This thing is loaded with everything but sunroof. It has cruise, pw, pl, pwr seats, heated seats, lumbar, cd./cass, push button 4X4, automatic headlights, foglamps, drls, rear defog, overhead console, etc, etc, etc. What should I do to test for the draw? Start pulling fuses?

 

Best way to trouble shoot an issue like this is to hook up an volt meter and start pulling fuses one by one. Then you will route it down to a general area. From there you can see what is all hooked up to that cluster. I hope you only find one area and not mutipal areas.

Per say Curt 2 fuse: unplug each item that is connected to that fuse one at a time til you find the item that is pulling the power.
If its multi area then: make list of the fuses that cause the issues take out all but one of the problem fuses fix that are first then hit the next cluster.

Good luck due to these are Mechanics nightmares due to they take sometimes days to find and fix.