Replaced bad alternator, now not charging

[RvRev2]

My alternator bit the dust. I took it to Autozone to be tested, and it was bad. Got an exact replacement and installed it. However, the alternator is not charging the battery.

TEST 1: I pulled that new alternator and had it checked at Autozone, and it passed.

TEST 2: I reinstalled the new alternator. After starting the engine, I removed the positive lead from the battery, and the engine died.

TEST 3: With the engine running (off the battery), I checked the output at the alternator, and it was producing plenty of power. I must have a break somewhere between the alternator and the battery.

My rig is mostly still all original. I can handle most mechanical things, but that voodoo magical electrical stuff confuses me.

Can anyone guide me in what to check?

My alternator has the large wire going to BATT, and two smaller wires that clip on and go to the harness, so not a single wire alternator.
I still have the blue Isolator (at least that is what I think it is called) under the passenger side hood.
Could it be a blown fusible link? If so, can you help me find it? There are a lot of wires in this general area.

Thanks in advance.

 

[RvRev2]

 

[RF_Burns]

View attachment 12717

Check the voltage on the center terminal of the Isolator (finned blue device). It should read about 14.5-15.5 volts with the engine running. Then check the voltage on the upper and lower terminals of the Isolator. They should read approximately 0.7 volts lower than the center terminal.

If the voltage is more than about 0.9V lower, then the is faulty.

 

[Tybalt39]

(Bruce beat me to it...)

You may want to have the battery/batteries tested as well.

You may also want to check if the battery cables are firmly affixed to the battery, with no corrosion present.

 

[RvRev2]

(Bruce beat me to it...)

I always assumed the alternator ran the electronics and also charged the battery; that the battery was just to start the engine. Apparently I was wrong and the battery does everything electrical on the engine; the alternator just charges the battery. If that is the case, you may want to have the battery/batteries tested as well.

You may also want to check if the battery cables are firmly affixed to the battery, with no corrosion present.

Howdy Luther. I'll have the battery checked, but it is fairly new. My assumption was the same as you in regards to the alternator running all the engine electronics as well as charging the battery. In fact, back in the 1980's I was able to remove a battery from a running car, and it still ran, operating off the alternator. As I said earlier, voodoo magic electrical stuff.

 

[RvRev2]

Check the voltage on the center terminal of the Isolator (finned blue device). It should read about 14.5-15.5 volts with the engine running. Then check the voltage on the upper and lower terminals of the Isolator. They should read approximately 0.7 volts lower than the center terminal.

If the voltage is more than about 0.9V lower, then the is faulty.

Thank you Bruce. I'll check that tomorrow when there is day light.

I see that Applied GMC has direct replacement Isolators in stock for about $100.

 

[RF_Burns]

Thank you Bruce. I'll check that tomorrow when there is day light.

I see that Applied GMC has direct replacement Isolators in stock for about $100.

Isolators are a common item available from many places. avaoid the cheapest ones as they will likely fail in short order.
Also, Removing either engine battery lead while the engine is running is not a good idea. It may lead to the alternator output voltage rising out of control and damaging devices.

 

[pvfjr]

battery does everything electrical on the engine; the alternator just charges the battery. If that is the case, you may want to have the battery/batteries tested as well.

That's 99.5% true. The one weird exception on our coach is the high-speed of the HVAC blower. It taps into the alternator output, and gets an extra 0.7V because of it. I guess they figured it would help with the lackluster performance.

In any case, the engine normally stay running if you disconnect the battery. But, it's not great from a voltage regulation standpoint. It's a good way to make noisy power.

The engine should also stay running if you disconnect the alternator output. Really, the battery and alternator can BOTH feed your power distribution system. If the alternator lacks (like when you're cranking) the battery helps out. The alternator keeps the battery charged, and keeps it from getting discharged. Which one is "powering things"? Well, that may almost be a philosophical argument.

The way I like to see it: The battery powers things, and the alternator compensates for the loads being placed on the battery.

 

[Tybalt39]

That's 99.5% true. The one weird exception on our coach is the high-speed of the HVAC blower. It taps into the alternator output, and gets an extra 0.7V because of it. I guess they figured it would help with the lackluster performance.

In any case, your point is right on. The engine should stay running without the alternator output connected.

So, is that another way to diagnose an alternator problem; if you don't have high-speed on the HVAC fan, your alternator may have problems? Not definitive, but another data point?

When I first received my coach, it would only run off of fully-charged batteries; they were not being charged by the electrical system and the vehicle would die if the batteries ran low. Based on previous vehicle experience, I suspected a bad alternator (I had already purchased two new type 97 batteries to test the coach). While other pieces of the charging system were mentioned by others, my gut went with replacing the alternator. Problem solved! Some day I will have the original rebuilt.

 

[pvfjr]

So, is that another way to diagnose an alternator problem; if you don't have high-speed on the HVAC fan, your alternator may have problems? Not definitive, but another data point?

Sure is! That's if your coach still has the factory wiring. I've seen some folks reroute the power sources for the blowers when they can't figure out how the blower relay and resistor work in tandem. Circled in red, there's the splice where a 12 gauge red wire taps into the 10 gauge alternator-to-isolator wire. This then goes to a fusible link, then the blower relay (via the 'X' terminal on the schematic).





You can see how the "BAT" output on the "Generator" above goes to the center terminal of the isolator (labeled as "GEN OUTPUT DIODES", because that's all it really is). So the blower relay, alternator, and isolator are really the only things on the output wire. It's easily accessible at the isolator, so you may as well test there. But as you surmised, if your blower works on HIGH, then there's a really good chance your alternator is working.

However, if your blower motor does NOT work on HIGH, there could be any number of things causing the fault.

 

[RF_Burns]

(Bruce beat me to it...)

I always assumed the alternator ran the electronics and also charged the battery; that the battery was just to start the engine. Apparently I was wrong and the battery does everything electrical on the engine; the alternator just charges the battery. If that is the case, you may want to have the battery/batteries tested as well.

You may also want to check if the battery cables are firmly affixed to the battery, with no corrosion present.

I'm not sure how you came to your conclusion from what i wrote.

When the engine is running, a properly operating alternator should be supplying all the loads and charging the battery (if it needs it). The alternator should be outputting a voltage higher than the battery's resting voltage, so current will flow into the battery. The alternator will also be supplying the current to operate all the other chassis loads.
The only time the chassis battery will supply power to the chassis loads is when the alternator output voltage is lower than the battery voltage. This might happen when the engine is running too slow, or the fan belt is slipping , or the chassis loads exceed the alternator's capacity. At that point the alternator voltage will drop and the battery will pick up some of the load current.

 

[Ken B]

I suggest you go buy a cheap volt / ohm multimeter from Harbor Freight for under $20. Then start taking some voltage readings to tell us what is happening. Thee statement "plenty of power with the fed wire disconnected and the engine running" tells us nothing. When taking readings use the aluminum plate that the isolator is mounted on for your minus meter lead connection to the meter.

I would check the voltages at the 3 terminals of the isolator WITH THE ENGINE RUNNING and all wires connected. If there is no or low voltage at the isolator c center terminal it could be many things including a slipping belt.

Also you could check the voltages on both sides of the fusible link while you are there.

Let us know what readings you have to 1/10th of a volt and we will help you more from there.

Ken B.

 

[Tybalt39]

I'm not sure how you came to your conclusion from what i wrote.

When the engine is running, a properly operating alternator should be supplying all the loads and charging the battery (if it needs it). The alternator should be outputting a voltage higher than the battery's resting voltage, so current will flow into the battery. The alternator will also be supplying the current to operate all the other chassis loads.
The only time the chassis battery will supply power to the chassis loads is when the alternator output voltage is lower than the battery voltage. This might happen when the engine is running too slow, or the fan belt is slipping , or the chassis loads exceed the alternator's capacity. At that point the alternator voltage will drop and the battery will pick up some of the load current.

Sorry for any confusion. I didn't mean to give the impression I was responding to your post. I was speaking from information I had received from other sources. At the time I created my post there were no replies to the first post. Of course, once posted I saw there was a new post; yours. I removed my instructions for testing the isolator in deference to your post after I saw it. I think I'll just remove that section entirely....

 

[RvRev2]

I suggest you go buy a cheap volt / ohm multimeter from Harbor Freight for under $20. Then start taking some voltage readings to tell us what is happening. Thee statement "plenty of power with the fed wire disconnected and the engine running" tells us nothing. When taking readings use the aluminum plate that the isolator is mounted on for your minus meter lead connection to the meter.

I would check the voltages at the 3 terminals of the isolator WITH THE ENGINE RUNNING and all wires connected. If there is no or low voltage at the isolator c center terminal it could be many things including a slipping belt.

Also you could check the voltages on both sides of the fusible link while you are there.

Let us know what readings you have to 1/10th of a volt and we will help you more from there.

Ken B.

Okay Ken and gang, here are the readings:
Center --- 20.53
Top ---- 10.16
Bottom --- 19.49
Pictures attached. And I have ran the engine with the floor off and can see that the belt is not slipping.

 

[RF_Burns]

So your alternator is outputting way too much voltage. It appears that the Isolator is faulty as the Top terminal is only outputting about 10 volts and I believe that is going to the chassis battery.

So the alternator sees the chassis battery as very low at 10 Volts, so it is outputting 20 volts in attempt to charge it. That 19 volts from the bottom terminal is going to the House batteries and will damage them.

Get yourself a new Isolator and install it, then test the terminal voltages again. it should be about 15v at the center terminal and 0.7 volts lower at the upper and lower terminal.

Let us know.

 

[RvRev2]

So your alternator is outputting way too much voltage. It appears that the Isolator is faulty as the Top terminal is only outputting about 10 volts and I believe that is going to the chassis battery.

So the alternator sees the chassis battery as very low at 10 Volts, so it is outputting 20 volts in attempt to charge it. That 19 volts from the bottom terminal is going to the House batteries and will damage them.

Get yourself a new Isolator and install it, then test the terminal voltages again. it should be about 15v at the center terminal and 0.7 volts lower at the upper and lower terminal.

Let us know.

Thanks bunches!!!!!! I'll place it on order with Applied GMC. I'll get back to you all after it is installed.
Thanks again!!!!!!!!

 

[Bhart]

You can get an isolator at any of the local parts stores.
Here is a 120amp at NAPA. It is spendy but you might find cheaper at other part stores.

https://www.napaonline.com/en/p/BK_48120BX

 

[RvRev2]

You can get an isolator at any of the local parts stores.
Here is a 120amp at NAPA. It is spendy but you might find cheaper at other part stores.

https://www.napaonline.com/en/p/BK_48120BX

I already placed the order with Applied GMC. Plus, I wanted to give them my business in order to help support them so that they will be there for when I an other GMC'ers need those unobtainium items.

 

[Ken B]

In the Interim if you want to verify that the alternator is working, or you want to use the coach, switch the wires between the top and bottom terminals on the isolator. This will connect the engine electrical stuff, including the alternator, and vollage sensing stuff together, allowing the alternator to operate normally. It will put the house stuff,, including the battery, on the suspected bad isollator terminal.

 

[RvRev2]

In the Interim if you want to verify that the alternator is working, or you want to use the coach, switch the wires between the top and bottom terminals on the isolator. This will connect the engine electrical stuff, including the alternator, and isollage sensing stuff together, allowing the alternator to operate to operate normally. It will put the house stuff,, including the battery on the suspected bad isollatot terminal.

Excellent! Thank you! Will do!