View Full Version : Alternator FYI...

O-Gauge Steamer
May 13th, 2012, 07:29
I am having an issue whereby the volt meter in the cluster drops just below the 14V mark. I confirmed the voltage to be 13.2 so...

I lightly rapped the alternator with a long drift and the voltage promptly went back the the 14.2 it needs to be.


There is, at the least, one of the brushes that is not staying completely in contact with one of the slip rings in the unit.


My replacement alternator will be here on the morrow and that will take care of the issue at hand.


Over time, carbon dust builds in the brush holders that will eventually lead to a brush loosing contact with the slip ring. The carbon interferes with the spring being able to hold the now shorter than optimum brush in contact with the slip ring. The "proof" of the failure is to gently rap the alternator and look for the voltage to return to normal levels.

If I can remember to, I will take the alternator apart and photograph what I find.

May 13th, 2012, 08:25
lectra-motive electrical motor cleaning spray works well for alternators and starters to blow the dirt and grime buildup out of there. It's made by CRC and any parts store should have it (I stock it at my NAPA store). I use it every 6 months or so to clean my alternator out because of it being mounted down low it's exposed to a lot of road dust.

May 13th, 2012, 08:47

Might be as simple as worn out brushes which do not have enough spring tension to maintain tight contact with the slip ring.
A new set of brushes, if you can find them, is a lot cheaper than a new alternator. Make sure the slip rings are clean and smooth.

May 13th, 2012, 12:49
The voltage should really be 14+ volts but a little lower would not hurt. My XJ only got up to 14 volts a few times when I was in Iowa and starting it the first time in the day in early winter. Here in Florida the highest I see is about 13.6 verified at the battery terminals. I have replaced the battery twice in the 9 years I own this XJ, the PCM and the alternator with one from a Dakota and the all do the same thing. I am convinced there is something wrong with either the wires from the PCM to alternator or the wires from the battery temperature sensor to the PCM. What ever the reason I have not had a dead battery while driving and the lights and AC including the fans work very well.

Having said all that, the ideal charging voltage for a 12 volt battery is between 2.35 volt per cell to 2.40 volt per cell multiplied by 6 (the number of cells in a 12 volt battery) give between 14.1 to 14.4 volts. My XJ is consistantly down 1 volt below the ideal charging voltage.

May 13th, 2012, 20:24
A new set of brushes, if you can find them.

That is the problem. I tried to re-build a starter not to long ago and it's not always that easy to find rebuild parts anymore.

O-Gauge Steamer
May 14th, 2012, 06:28
Therein lies the issue. I am not able to readily source a set of brushes.

Trust me on this one, I have rebuilt a ton of both generators and alternators over the last 5 decades...

So, I will be popping in the 136A alternator. What is interesting is that my 90A factory alternator tested to 99A last summer. Who would have suspected that ND underrated the device...

The 136A is installed on the ZJ with the 4.0L. It also came on the 5.2L with a seven rib sheave.

I looked into the "high output" (200A) versions. Every rebuilder I contacted (by phone) stated they were using the 90A/136A case. IMO, you need to use the 160A case as a minimum to build up a 200A unit.

And, as I am rather conservative, I decided to just go with a new unit. Cost wise, the NAPA reman'd 136A unit was going to run me near $200 post taxes. The brand new unit is only $38 more.

Fusing. I replaced the Fusable Link over a year ago with a fuse installed at the front of the PDC.

Sort of looks like this:

As you can see, it is currently a 100A fuse. It will be replaced with a 150A. The feed from the alternator has already been replaced with a suitable gauge wire when I upgraded all of the power cabling.

I found remarkable that the spacing in front of the PDC was perfect for the fuse. Makes one wonder. Did Chryco have us in mind?

As some of you know, I am really a "belt and suspenders" kind of Engineer. That being the case and, and in yet another example of over engineering the vehicle, I run a dedicated ground from the alternator to the battery. The wire is, of course, the same gauge as the rest of the power cabling.

I know, not really needed as you can "trust" the existing grounds. Well, you go ahead and trust them. Me? I don't trust anything... Replacing the alternator has been on my "to-do" list for over a year. Once I replaced both the mechanical fan and the mechanical water pump with electrics, having a more robust alternator was required.

Hopefully, the new unit will be in today so I can start changing it. Figure it should take me three days to get it done. It is a real biatch being disabled.

Three days you ask? Yup.
Pull the battery and the fans, day one. Stop.
Change the alternator, day two. Stop.
Reassemble on day three. Recover.

It took me three weeks to install the BW Flat Flares. Being disabled has not stopped me (much to my Physician's ire) but it sure has slowed me down.

May 14th, 2012, 07:17
Nice. Looking forward to more pics and commentary on your swap.

O-Gauge Steamer
May 15th, 2012, 07:24
Picked up the alternator yesterday afternoon. For those in the Colorado Springs area, I can recommend "Quality Alternator and Turbo" as a good place to go for parts.


Knowing that this alternator was going to put out more than the 99 Amps the factory one did, I asked the staff at Quality if we could put the alternaotr on the testor to see what fuse I needed to buy.

They were more than happy to help and we discovered yet another case of Denso underrating an alternator. The part number I purchased was the 136A from the ZJ. On the tester, the alternator generated 120A at idle and a very solid 180A at 2,000 engine rpm.

So.... I snagged a 200A fuse on the way home. I am using an AMG so will do some online shopping to see if I can find a fuse closer to the performance. The 100A fuse was a perfect choice for an alternator that put out 99A.

Who would have thought that Denso so underrated their products. I do realize that two data points do not a trend make. But, still all in all, it is two for two.

May 15th, 2012, 07:43
Good stuff - I have a 150A ANL fuse mounted to the firewall connecting the alt to the PDC. I will be switching to the AMG fuse in the PDC like you have, very clean.

I'm running the same 136A ZJ alt, interesting that yours tested at almost 180A, I haven't blown any 150A fuses yet... Mine was pulled from the junkyard has never put out 14v, gonna pick up some of that product Grimm mentioned, hopefully its just dirty.

O-Gauge Steamer
May 15th, 2012, 12:56
OK, I got it out and the bracket modified. This alternator, strictly speaking, is not one from a ZJ. It is a Denso replacement part so...

Here are a couple of shots of the alternators together. I did need to massage the bracketry a bit to get it to fit. Will install tomarrow.


As can be seen here, the replacement part is both longer and slightly greater in diameter. It was the diameter that was the issue. I also needed to swap the B+ lead insulator from the original to the replacemet as the replacement had the cable routed 180* out of alignment. Numerous threads on doing the trim so I did not bother to document it.

It is also interesting to note that the replacement part has two internal fans. One at both ends. The stock alternator only has a fan at the rear.

I should expect to have it wrapped up maybe on the morrow. We will see just how much damage I just did to myself... I may wind up burning a day in recovery.

Lurch. An alternator, in good nick, should be able to drive 20VDc or better. So, yah, yours has an issue. Could be the Battery Temperature Sensor as the PCM uses that Sensor to determine the best System Voltage.

My Heep operated at 14.2VDC all day, every day which is why I reacted as fast as I did to this issue. Will disassemble the stock alternator after the dust settles on this installation.

Another FYI, at a Room Temperature of 25C (75F-80F) the Battery Temperature Sensor must read 10K Ohms +/- 1kOhm. If the Sensor is out of spec at that temperature, it requires replacement.

And yes, I did look up the spec on it... This Sensor is what determines the overall System Votage as per the FSM.

Will update this thread once the replacement is in.


May 15th, 2012, 15:06
That alternator looks exactly like the Durango altrnator I have in my XJ. While I do not know the overall output, I like the fact that the voltage do not drop when the two cooling fans are running, the AC on high, the high beams on and the 4.0L at idle.

I agree fully that the battery temperature sensor play a large part in regulating the system voltage. As said before, My XJ normally reads no higher than 13.8 volts and as low as 13.2 on hot Miami summer days but a whopping 14.3 during the Iowa winter. I have never experienced a battery problem so I let alone. I did see CD based shop manual a way to test the sensor.

I know you will be happy with that alternator especially the low RPM out put.

O-Gauge Steamer
May 16th, 2012, 06:36
There is no doubt about the output at idle. A was presented earlier, this unit is capable of providing 120A at idle.

THe "high" voltage you saw in the cold is what the system is designed to do. By running the voltage up, the curent load goes down.

After all, "Watts are what get the work done". Which is why, in Europe and pretty much the rest of the world, wall voltage in homes is 220VAc. By essentially doubling the voltage, the curent is dropped nearly by half. All of which means smaller conductors in the walls thus reducing copper consumption. Less current means less heat to disapate as well.

Cars went to 12 Volts precisely to reduce the current load. At the time all of the world was running 6VDc systems, the Military was running 24VDc systems. For reliability and ease of maintenance.

O-Gauge Steamer
May 17th, 2012, 06:32
The alternator is in but due to the diameter, I had to remove the AC compressor and it's bracket in order to fish it into place.

Will finish up today and start the disassembly of the failed unit.

O-Gauge Steamer
May 17th, 2012, 08:48
Pictures and analysis:

As it turns out, the slip rings have worn which is the root cause of the brushes lifting.

Photobucket appears to be having issues with uploads as I keep getting a "resize error". Whatever that is... Haven't had this issue before with them.

Basically, the slip rings are worn to a depth of 0.040" on the outboard ring and 0.061" on the inboard ring. The shop here in town says that they can make new slip rings so... I will call and get a price. Repaired, the old unit would make a nice spare.

May 17th, 2012, 11:36
These guys have alternator and starter parts. http://www.aspwholesale.com/

O-Gauge Steamer
May 18th, 2012, 07:09
So I dropped off the old alternator to the rebuilder and it looks like for under $30 I will have a solid spare. They are going to correct the slip rings (turn them back to flat again) and supply new brushes for me to solder into the brush holder. Less thatn $3 for the brushes plus some bench time for me to rework them into place.

Replacing the stock unit with one that produces better is always a good thing. The old unit could, and did, produce 99A at idle, this one lumps out 120A at idle. Should come in handy if I ever do something with the sound system. Like replace it, with aftermarket.

PaulJ, thanks for the link. I added it to the list!