I've got a 305 in a blazer and in the past 2 weeks it has toasted 3 alternators. When I install them they read 14 volts or over at the battery and .2 volts higher at the alternator. I move the ground lead to a bunch of different spots and get the same readings.

I tested for resistance with a lead on a ground and then used the other lead on each of the 3 wires coming from the alt. Only the brown wire got resistance which means it is grounded right? Is it supposed to be grounded? I traced it back and it goes into my firewall.

Oh yea and its got a new battery, right after it toasted the 1st alt.

Any help?

 

14 or just over is what they should read when they are charging a battery. 15 would be over charging.
What year Blazer?

 

Yea when I first put them in they charge good right around that amount but then in like 2 days they junk out, battery dies WHILE i'm driving. Last one I tested 9 volts when I took it out.

Its an 85.

I'm not sure if theres anything that would kill an alt so quick? The wiring is kind of hacked from the previous owner but I havn't had any problems until now.

Gettin real old real fast too, everythings goin wrong with this thing, had to pull the hood release so hard the handle broke off this morning. Fun fun...

 

When it rains it pours, right!

When you get another alterantor do they test the old one at the store?
Check your fusable links. Don't just look at them, pull a bit. The wire could get corroded on the inside. One fusable link will be a voltage sensing wire. They normally come off the starters battery terminal. You may have a juntion block though that will have the link there. But do check the wires at the starter as well.

Also, just because its new does not mean its good. I just recently had a fairly new AC Delco battery turn out to be bad. actually it was new but in my car for a while. I kept noticing the volt gauge would fluctate a small amount all the time and the battery would never read more than 12.3 volts even after sitting on the charger and driving to the shop where my brother works. (he has a nice digital battery/charging system tester while I still have the "smoker")

 

They tested one new one which passed, then tested the one I took out that was junk which "passed". But that one was already tested while it was in the vehicle and failed (voltage regulator). So I can't believe what their indoor machine says.

You think my luck is that bad to get 2 bad ones in a row?? All my wires seem ok, no corrosion, I didn't see a fusible link anywhere though. What do you think about the grounded brown wire?

 

check your wireing. could be a broken wire loosing voltage going to the battery from the alternator. thats what i found on my firebird. replaced 1 wire with new and voltage problems went away.

 

You stated that brown wire has resistance. Is it resistance or continuity to ground?
If memory serves me that brown wire goes to a charge indicator light and Ts' off to a charging system relay found in the convenience center. This relay is for the choke. Does your choke work?
Also check you gauge fuse and you should have a choke or CH (or maybe its charging) fuse as well.
If that wire has continuty to ground then somwhere along the line that could very well be your problem. Maybe the relay has gone bad.
Sorry I spaced earlier about what you stated on the brown wire.

 

If it is a typical Chevrolet 3 wire alternator you should have 3 wires going into a 4 prong plug. The two heavy red wires provide the main power and field voltage, and come thru fusible links. The smaller brown wire comes from a fuse and is your dash voltage indicator sensor wire. It should NOT be grounded, and should read infinite resistance. I would start with this wire to see if you have a short to ground somewhere, that could be what's frying your alternators.

 

well I tested resistance between ground and that wire and it had resistance, meaning its a closed curcuit correct (has continuety)? I traced this wire around the back of the engine and probed in it in many spots all of which had resistance, so it must be grounded in the firewall somewhere? I will have to check my fuses tomorrow.

 

Sounds like it. How much resistance is it showing? the smaller the number, the more direct path to ground the wire has. The higher the number, the less likely it's an actual short so much as just "crosstalk" across circuits, which can happen.

 

Again does your choke work?
Your choke may be what is causing this. Unplug it and see if you still have ground. Then when you follow the wire unlpug the relay and see it that works.

 

If it's not the culprit, start checking the wire from the alternator back. Mine at the plug was starting to get corroded on my old harness, to the point where the wires were directly exposed for the first 1/8" or so. It's pretty common after years of heat and exposure for the insulation to become brittle or wear away. Also, if you have anything within the circuit you can disconnect, do so, re-adding items one at a time until you get the same low resistance reading. This will help isolate the problem.

 

Oh sorry my bad, the choke actually isn't hooked up, no wires were ever ran to it when the carb was installed.

 

I will have to check the amount of ohms tomorrow. The wire looked ok, but the plastic plug that goes into the alt has chunks broken off of it and the wires are showing, but not touching.

 

Found this diagram which shows the brown going to a fuse right?

 

What happened to the light blue wire for it? Is it grounding somewhere or been used for something else?
Also since its not used then you should see if the relay is plugged in and un plug it.

 

Correct. I found an 86 Firebird wiring diagram online that shows the circuit, same basic setup used. In the Firebird it goes to a 20 amp fan fuse. You really need the manual for your Blazer to be able to do a thorough troubleshooting of the circuit. You can try Autozone.com, if you create an account (free) it gives you access to a lot of info, sometimes wiring diagrams. If not that, a lot of libraries carry the collected versions of the Haynes / Chiltons manuals, you can try to get the diagrams from them as well. I would definitely start with that brown wire tho, trace it thru the firewall to the fuse block, remove the fuse and check the continuity to ground from the side that goes to the alternator, and the side that goes to the gauges. That should tell you which side of the circuit to focus on.

 

I have nooo idea, there are some wires here and there that arent plugged into anything. I will trace those wires and see if they have relays.

 

Ok thats a good plan, I will def start at that tomorrow. This is a big help.

 

what year is your blazer, and what engine? 4.3L I assume, but to be sure?

 

The blazer is an 85 , and the engine is an 1986 5.0 from a truck I believe, its got the 416 heads though.

 

Harness is the important part. I'll look for an '85 Blazer, give me a few minutes to see what I can find.

 

There isn't much as far as wiring diagrams for an '85 Blazer available online, but Autozone does have some for the alternator circuit. There are two different models used, based on the original engine. If you still have the original harness going to your new engine's alternator, then go by whichever setup the vehicle used to have to troubleshoot it. I would still suggest checking a better source like a Haynes manual, it will give you a much better overview of all the interconnecting circuits, and let you pinpoint the things you need to concentrate on.

 

Got a Haynes manual and the brown goes to the charge indicator. I checked all my fuses and 1 25 amp fuse was blown, however I dont know what its for because the words are worn off.

I tested continuety on all the wires connected to the alt.

Brown: steady beep reading 18 ohms.

2 reds: 1/2 second beep only when I first touch it reading 100 ohms.

Does this mean all 3 wires are grounding somewhere?
Im going to look online now to see if I can find a fuse panel labeling.

 

18 ohms is a fairly low resistance, not direct continuity to ground but enough to check that circuit thoroughly. You might have a wire partially exposed someone and it's making intermittent contact with ground or another wire. Trace the wire as much as you can and check its condition. I would also tape up the damaged plug you mentioned yesterday, each wire wrapped separately and the whole thing wrapped after that.

 

Well found nothing really online of a panel diagram. Can't believe theres not one in the haynes manual. But i snapped a pic of the wiring diagram with my phone. Its hard to see but I wasn't sure how helpful it would be.

I tried moving around and spreading out the wires to see if it would affect my readings but nothing. So that is telling me it is inside the dash somewhere. I layed under the dash trying to find the brown wire going to the battery indicator but had no luck. Might just have an electrical shop look at it because this is my first time troubleshooting anything electrical.

 

It's not that hard, don't get discouraged. You've got the basics down, and you know how to use a multimeter so you're ahead of the curve already. Just check along the wire and do the tests I mentioned and it should help to narrow down the problem. Pull the gauge cluster itself and do a continuity check between the brown wire at the plug going to the gauges and the chassis. If you don't have a path to ground, it may be a problem in the cluster. You can also always go up to your local auto parts store and look at their Haynes manual, a lot of them will let you open them in the store to look something up, esp if it's to troubleshoot something like this.

 

Well, I spent most of my daylight today removing my starter and walking 2 miles to get another, tomorrow I'm going to put it in and hopefully finding that short but we will see. By the way things are going the motor will spin a bearing parked tonight lol.

 

I've had that luck before. I had just finished putting an 8.8" rear in my 94 Mustang and was test driving it when it got t-boned by a Korean lady in an Explorer. This was in '07, and it's been down since. I had just gotten my '83 Camaro on the road 2 weeks before, and hadn't gotten all the kinks worked out of the tuning yet. I have 4 cars now, and make it a point to never have all of them down at once.

 

That sucks about that mustang, must be just about totalled?

Well, somebody told me that the brown wire should be grounded since it goes to a light and the light has to have ground to work. Does that sound right? But I have been driving it since monday and its been doing fine. However, yesterday the needle was jumping around from 13 to 15.5 randomly, now its staying steady an 13, so I have a feeling that this ones about to be bad or is already bad.

Goin at the rear brake line tomorrow morning too.

 

You've got something in that system that is allowing the voltage to either short to ground or spike and fry your alternator. If you keep replacing alternators you're just fighting the symptom. If you can get it to an Autozone have them check the charging system and they can at least point you in the right direction.
The Mustang was totaled by the insurance company value wise, but not mechanically. Since I'm in Texas, I can fix her and she'll still have a clear blue title because it was not mechanically totaled or greater than 100% value (the insurance company uses a 65% value, and their math was WAY off). I already have all the parts to fix it, the damage was mostly cosmetic and limited to the fenders, fiberglass hood (too far gone to fix unfortunately), radiator brace, and subframe ahead of the strut towers. It needs about 12 hrs of frame to to tweak the front subassembly back 1/2", and the new radiator brace and inner fender aprons spot welded in. Other than that it's fine, and will be back on the road as soon as I can spare the cash...not likely soon, since my wife just had our first baby girl last Monday.

 

Have Autozone do a charge check with the alternator on the vehicle with the engine running. Then if it tests bad, pull the alternator and have it tested inside on their machine. If the alternator tests good at that point, then you definitevely know that there is a problem in the wiring. I still think you need to thouroughly check that brown wire. It should NOT be showing continuity to ground.
A test you can try is to do a battery test with the red lead of your multimeter on the positive post of your battery, the black lead on the negative post, and your multimeter set to 20VDC, as someone cranks the vehicle. Voltage should not drop below 10 volts. If it does, you could have a short within the starter solenoid. Chevrolet is notorious for running the alternator output thru the starter solenoid, which if it shorts internally will kill your battery.
Another troubleshooting trick you can try is to run an 8 gauge wire or better directly from the positive terminal of your battery to the charging (output) lug of the alternator (the big threaded connection, not the plug). If this solves your issue, you either have a short on the wire going from the starter to the alternator, or within the starter itself.
Other than those two checks, the only other probable culprit would be a problem on the brown wire behind the dash. The wire should show high (greater than 50 ohms) only when the key is in the on position, and infinite any other time.

 

Congradulations on your first child man! Ha those little rascals are expensive so thats understandable that the mustang will be put on hold.

Well one of the alts I had tested in the truck and it tested bad, voltage regulator failed, according to the hand tester. Then they tested it inside and it said passed but he couldn't tell me the voltage output or anything really.

I had to put a new starter in it last weekend so that solenoid shouldn't be a problem.
How about this... is the only purpose of that brown wire only to light up the charge indicator, and is that the volt gauge? If that is the only purpose of it I can temporarily snip that wire and see if that solves it right?

That really has to be the problem because the other day i go to start it and the gauge read 9 volts, so I think "oh great its not gonna start" but then it turned over nice and fast and fired up. So obviously something is messed up in that circuit.

 

If the alternator is failing on the vehicle but passing on a stand alone test inside, then the problem is in the vehicle's circuit, not the alternator. At least now you know before you keep buying new ones! Trace that brown wire all the way to the gauge cluster, and within the cluster itself. It may be the solid state inside the cluster that has the short. As for cutting it, I'm not sure what effects that might have. But it is the best place to look for the problem, so keep at it and you'll find your gremlin.

 

I have been fighting the same problem for 2 years on my 87 gta with a digital dash when i first crank the car it reads 14 v as it heats up the voltage drops a and once the fans come on that's all she wrote it usually takes me about 3 or 4 months to kill one it's like the combination of the head lights, fans and ac take it out pretty quick I even killed a 200 amp power mate in 2 months all wiring is stock I have trouble shot everything under the sun but just can't pinpoint the issue

 

luckily I made friends with a guy there so hes givin me 2 replacements under warranty. But I was thinking if I cut that wire then it would no longer have juice running through it therefor eliminating it... or is that not how it would work?

Yea these types of electrical problems dont cost much to fix, but man they take time... esp if you are like me and don't have much experience in that area.

 

I really think you should troubleshoot more before making any cuts. You don't want to create more problems for yourself down the line.

 

I think it could be bad wiring or maybe some of your wiring has gotten really hot and maybe bubbled abit? Possibly rust got into it somewhere? Anything is possible.

 

Well its been doing ok, I thought it was gettin bad but I havn't had a problem. Maybe that dash wire is just barely not grounding, but unless it gets bad again I will let be.

 

So what did you do to fix the problem? Did you snip the brown wire or what?

 

I brought it to a professional shop. They couldn't see anything wrong with it because I had just put another alt in it, luckily all under warranty. Then went through another alternator and a ton of other problems. Said screwwww this and traded it for a beater vw golf that is much more reliable. I feel like I would have had to pull the entire harness to find wtf was going on with that thing. Yea that truck got the dishonorable discharge.