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Unread 05-28-2010, 05:56 PM   #1
Dewster196
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Alternator Question

I recently swapped in a 136A alternator from a ZJ and it read right about 14 volts on the dash guage with out any accessory load(haven't tested with a volt meter yet) but my question is about the old 90A with the Jeep running the gauge would read almost 16 volts with out anything on heater, lights? What would be the cause for the high voltage reading with the 90A alt? Was it on its way out. Before I started DD my jeep and it would sit for a while when i would start it the alt would be siezed and I would have to shut down and turn the alt over with a breaker bar to get it feed up again.

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Unread 05-28-2010, 06:41 PM   #2
pasinbuy
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Our 12v system is in reality a 13.2v system.. Your battery has 6 cells that put out 2.2v each = 13.2v.
Just like a 2 x 4 board is in reality 3.5 in x 1.5 in we have a way of rounding off numbers.

Anyway 14v is fine. You need a little more to charge the system.
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Unread 05-28-2010, 11:06 PM   #3
QuantumYJ
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I recently did the same modification and noticed the same thing. I checked mine with a meter and am getting over 14 volts at idle. I am curious if it's a case of the feedback from the PCM is off since the original alternator was 90 amps. The voltage regulator is in the PCM and I assume the gauge is driven from there as well. Just a guess, I'm sure there's somebody who knows more about this.
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Unread 05-29-2010, 04:54 AM   #4
Dewster196
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With mine the higher voltage was when I had the 90 Amp in, I am not concearned about the 14 volt reading but I thought maybe the 90 was on its way out because of the higher voltage reading.
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Unread 06-07-2010, 01:55 PM   #5
tjing
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Im having sorta the same problem, volatge is in the red with the check gauge light on. Dash cuts in and out. I know that the voltage is regulated through the pcm/ecm. Also after jump starting it and turning off again everything is dead, the alternator has to be charging because it will run forever untill I turn it off. My question is do I change the alternator or the pcm???? help please!!
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Unread 06-07-2010, 01:55 PM   #6
tjing
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Quote:
Originally Posted by tjing View Post
Im having sorta the same problem, volatge is in the red with the check gauge light on. Dash cuts in and out. I know that the voltage is regulated through the pcm/ecm. Also after jump starting it and turning off again everything is dead, the alternator has to be charging because it will run forever untill I turn it off. My question is do I change the alternator or the pcm???? help please!!
also its a 97 tj 4.0l
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Unread 06-07-2010, 04:35 PM   #7
Fernando
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You need to move that question to the TJ forum!
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Unread 06-07-2010, 05:26 PM   #8
plym49
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In general, dashboard voltmeters are only useful in relative terms as they tend to be damped and calibration varies unit to unit.

Check with a good quality voltmeter across the terminals of the battery.

12 volt batteries are rated as '12 volts' but their nominal voltage is higher - 13.2 is not unusual. The nominal resting voltage of a battery is affected by temperature, battery age, battery construction and level of charge, so again readings are relative and there is no reason to get your knickers in a knot if yours reads 13.2 and your buddies reads 13.4. (These are, of course, readings with no load on the battery and the engine off.)

In general, you charge a battery at a voltage 15% higher than the voltage the battery is nametagged at. That's where 14.4 comes from, and that is about where your voltage will be with the engine running.

The Jeep's ECU does not know anything about what voltage the alternator is putting out. It also knows nothing about whether you have an alternator rated at 90 amps or something else.

The voltage regulator INSIDE the alternator determines the voltage output. It varies the field current to maintain 14.2 (or thereabouts, depending on the design of a particular regulator). The current rating is the maximum that the alternator can put out. If you have a 150 amp alternator and the car requires 5 1/2 amps to run because you only have the motor running and everything else is off, your fancy alternator is putting out 5 1/2 amps. Switch on all of your DAylighters, and things change.

Voltage and current are two different things. Voltage appears across two terminals. Current (amps) flows through something. Together they make power (watts). Elegantly, in a DC circuit, watts = volts times amps.

Also, from Ohm's Law, voltage = amps times ohms (resistance). So now everyone knows why you install a relay when you add driving lights. The lights take a lot of amps, and a long and/or small wire has a higher resistance. Running long lengths of thick wire costs money and adds weight, so isn't it better to run thin light cheap wires to CONTROL the device, and a shorter fat more expensive wire to OPERATE it?

But I digress. Perhaps you should check the voltage accurately before drawing conclusions about what might or might not be happening, because those dashboard voltmeters are not necessarily conclusive.
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Unread 06-07-2010, 07:47 PM   #9
sentinal02
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Quote:
Originally Posted by plym49 View Post
The Jeep's ECU does not know anything about what voltage the alternator is putting out.

The voltage regulator INSIDE the alternator determines the voltage output.
your ECU doesn't, but the OP has a 95. 91+ jeeps are regulated through the ECU similar to ford's.
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Unread 06-07-2010, 09:36 PM   #10
Que89YJ
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TJING, your problem is probably a bad alternator. Pull it and get it tested. To the other people following the thread what Plym49 said for the most part is true except like what was stated the later models were all regulated by the ECU. The normal minimum output is 14.5Vdc. That is +- .5Vdc tolerance.
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Unread 06-08-2010, 04:23 AM   #11
tjing
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thanks! and sorry im new
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Unread 06-08-2010, 10:48 AM   #12
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Originally Posted by tjing View Post
thanks! and sorry im new

It will help if you fill out your jeep info.
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Unread 06-08-2010, 02:56 PM   #13
dodger889
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Two things about a 91+ alternators on jeep is regulated by the ECM , the second only will provide what is needed to recharge the battery and what is needed to power the electrics on the jeep. So here is a good way to tell when you have a problem is when you load the alternator with everything turned on that you can. Then if you see a voltage drop let's say your voltage meter is reading about the same reading then you are good if you see a big deference then you need to do some checking.
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Unread 06-08-2010, 08:35 PM   #14
plym49
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Originally Posted by sentinal02 View Post
your ECU doesn't, but the OP has a 95. 91+ jeeps are regulated through the ECU similar to ford's.
I am not too familiar with the models cited. Can you verify that, on these particular vehicles, that the voltage regulation duties associated with the alternator have been outsourced to the main ECU?

This, design-wise, would seem to be the long way around the block. Is it possible that the native, internal voltage regulation performed by the ECU for itself (input voltages can swing from below 12 v to over 14.5 v, but the chips want 5 vdc) is being confused with the charging voltage regulation?

I would appreciate clarification on this point.

One other item regarding voltmeters. To get an accurate reading, you want to attach the leads from your voltmeter directly across the battery terminals. An ideal voltmeter uses no power (has an infinite internal resistance, so that it does not load down the circuit you are connecting to), so it could be left attached this way forever without adding any drain to the battery. In practice, voltmeters do have a measurable internal resistance, especially the slim shady voltmeters manufacturers install on dashboards, so they would present a small but measurable current drain. Also, it is expensive to run dedicated wires from the dash to the battery just for the voltmeter. That's why the voltmeter on our Jeeps are switched (they go off with the key); they are wired in right at the dashboard. Now we already know that we can have problems with dashboard connections that degrade over time due to corrosion and other factors. There might be a circuit upstream of the voltmeter that for whatever reason s loading the voltmeters feeder line more than that designed. And the voltmeter itself is damped so that the needle does not jump around every time you hit a bump.

No wonder, then, that the dashboard voltmeter does not do much more than inform you of something badly wrong. Other than that it just fills a hole in the dashboard.
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Unread 06-08-2010, 09:18 PM   #15
sentinal02
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Quote:
Originally Posted by plym49 View Post
I am not too familiar with the models cited. Can you verify that, on these particular vehicles, that the voltage regulation duties associated with the alternator have been outsourced to the main ECU?
check out the diagram attached. pin 20 on the PCM drives the field of the alt. the alt still rectifies the output as usual of course, but the regulator has been moved to the computer. what really sucks is when the regulator goes, the whole computer needs replacing. like you said, it's a round about way of doing things and it does make you wonder what the advantage is.
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