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Discussion Starter · #1 ·
I know this and similar issues have been addressed in other posts. I’ve combed through many of those and am still confounded. And sorry for the long post, but it’s been a long a bumbuzzeling problem.

My son bought a 1994 Isuzu Rodeo (6 cyl, 2wd, 122k miles), he drove it for about 6 months with no issues. Then on November 25th he was out and the car started having what appeared to be transmission issues. He parked the car and called me to come look at it. When I got there the battery was dead so we jump started the car using my vehicle. When the car was running the panel warning lights (transmission, charge, all of them) were flashing on and off at random. I tried to put the car in drive and it shifted REALLY hard. Not wanting to drive it that way, we had the vehicle towed home.

At home, the battery was still dead so we jumped the car using one of those portable battery/jumper units. The car started but as soon as I disconnected the jumper unit the car died. “Huh” says I. Tried it again, same thing. We decided to swap in an extra (charged) battery to get the car going. Car starts, no warning lights flashing, shifted the car through the gears with no issues, took it for a test drive and the vehicle drove as smoothly as the day we bought it, no issues whatsoever.

We got the voltmeter and tested the charge at the battery with the car running, there were only 12.5 volts coming into the battery from the alternator (we should read 13-14 volts at the batter when the car is running). I figured it was the alternator and put in a brand new alternator. There was no change, same issue, 12.5 volts at the battery. We have chased this issue and can’t seem to figure out why we can’t get more than 12.5 volts at the battery.
To keep a long post shorter – here is what we have done (all with no results):
  • New alternator; tested the old alternator and it was putting off 13-14 at the local Auto Zone. Also, with the alternator in the vehicle and vehicle running, if I test the voltage coming off the alternator at the charge bolt with a voltmeter it’s 14 volts (positive lead on charge bolt, common lead ground to frame).
  • Swap out and test the battery.
  • Replaced the charge relay.
  • Checked all wires coming off the alternator for short/continuity issues.
  • Checked between the alternator and fuse/relay box in engine bay.
  • Checked between fuse/relay box and battery.
  • Let the car sit un-driven for a week, the battery does not drain, which also doesn’t indicate a short
  • Removed all the fuses that we possible could, one at a time, with the car running and testing voltage at each interval to try and see if there’s a short in one of the other circuits.
  • Disconnected the wire harness at the transmission to see if there was a short or other issue at the TCM.
None of these steps have made any difference and we’re still only getting 12.5 volts at the battery (the dash voltmeter also only reads 12 volts). The charge light on the dash works as it should (comes on at start up then goes out) so that circuit is good and there are no other warning lights.

At this point were down to replacing the engine control module, but before we did that we wanted to post and see if anyone had any other ideas.

Thanks in advance!
 

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Well, that's weird as hell! But, that's what modern electronics seems to be all about. Seems like you've done quite a bit of troubleshooting already. Hang on, there will be someone knowledgeable along soon to help you with this issue.

I just run the waiting room!

Welcome to the forum by the way!
 

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Have you checked all your connections including grounds? Sometimes corrosion on these old vehicles will creep into the jacketed battery ground unnoticed for example?
 

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14V alternator output when installed on the test bench doesn't automatically = 14v output when the alternator is installed onto the engine.

14v alternator output when installed on the engine doesn't automatically = 14v at the battery.

There are a 2 reasons why a know good alternator will not function in a vehicle.
One is the control circuit of the alternator is not functioning properly. The other is the charge circuit not functioning properly.

Step #1 is to figure out which circuit isn't functioning properly so you can concentrate your on troubleshooting the correct circuit. Instead of wasting time on other parts that are not broken. The easiest way to do this is to start the engine & grab your DMM. Measure the voltage at the output terminal of the alternator. If you have 14v here your control circuit is functioning & your problem must be in the charge circuit.

Step #2a
Next, check for the same voltage at the battery terminals. If the voltage at battery terminals do not match (with in .5V) then you have a bad wire connection or bad cable in the charge circuit. To find it, use your DMM to measure voltages along the charge circuit until you come across a voltage drop. This is where you will discover that fault. TIP#1 to test your ground connection (which is probably not at fault). Grab some jumper cables. Connect one end to the engine block & the other two the negative battery terminal. Measure the voltage at the battery. Did the voltage increase to 14V? If so your ground cable connection is faulty. If not, you have verified that connection is sound.

Step #b
If you have no 14v voltage output at the alternator from step #1, your issue must be in the control circuit. The plug on the back of the alternator, a couple fuses, ignition switch plus all the connectors and wiring between is where you will find the issue. The troubleshooting prosudure is the same for the change circuit. Find the bad wire or connector that is stoping the alternator from turning on.
 

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I ran across this same problem on my 98 trooper. Everything was fine for years, then one day out of town the battery was dead, or nearly dead. Got a jump start from someone and I checked the voltage and was only getting about 12.2 volts from the alternator.
Got I home and put in a new alternator and still only getting 12.2 volts. After much testing and prodding, I determined that the gain control in the PCM was not working. Unable to determine why it was not working, I ended up disconnecting the wire coming from the gain control circuit. I now get 14.2 volt charge from my alternator and everything is fine.
Not the proper way to fix the problem but it works for me.
Mike
 

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Discussion Starter · #6 ·
Thanks for the responses, guys. I will do some additional testing as Shawn suggested but I'm leaning more toward the PCM/ECM like Guitarbuilder said. A new development came up in just the past day. While I was getting 12.4v at the battery before, something happened and now I'm only getting 10.3v off the alternator while the car is running! The battery will read 12.5v before start up, then when I start the car it drops to 10.3v and will run for about 7 minutes before it stalls out. HOWEVER - if I keep the DMM on the battery the voltage climbs back up to 12.5v within about 90 seconds!

My only guess is that something (the PCM/ECM) is telling the alternator to drop voltage to 10.3v. I've found rebuilt ECMs online for a reasonable price so I'm going to tray that.
 

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Discussion Starter · #7 ·
Hey, now that Guitarbuilder brought it up - is the PCM the same as the ECM??? Or are they different modules? The stuff that I just looked at says they could be the same or they could be two different modules. I know where the ECM is - under the dash on the right hand side of the driver seat. Where is the PCM (assuming it's different from the ECM)?
 

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In a 94 Rodeo/Trooper there is an Engine Control Module (ECM) and a separate Transmission Control Module (TCM) for the automatic transmission. Starting in 96 the ECM and TCM are combined into a single module called the Powertrain Control Module (PCM).
A 94 alternator did not have a gain control built into the voltage regulator because the ECM did not support it.
 

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Discussion Starter · #9 ·
In a 94 Rodeo/Trooper there is an Engine Control Module (ECM) and a separate Transmission Control Module (TCM) for the automatic transmission. Starting in 96 the ECM and TCM are combined into a single module called the Powertrain Control Module (PCM).
A 94 alternator did not have a gain control built into the voltage regulator because the ECM did not support it.
Sorry if this is a stupid question, but I'm feeling over my head when it comes to the electrical/electronic systems. Buster - does this mean that the ECM does not control the voltage coming out of the alternator?
 

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The ECM in your truck does not control the alternator like a 98 Trooper PCM does.
In your truck the voltage regulator is supplied with positive battery voltage when the Ignition Switch is placed in the Run position. The voltage regulator is connected in series with the alternator rotor winding. By controlling the current flowing through the rotor winding the output current in the stator windings can be controlled. The voltage regulator limits the voltage output to 14.45 volts DC maximum. The voltage regulator is located inside the alternator case.
The attached image shows simplified schematic.
Product Schematic Rectangle Slope Font
 

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If Buster offers advice. Take it to the bank. He knows his stuff. I can't be sure on a 1994 Rodeo, but some Isuzus had a circuit in the alternator that powered up the fuel pump relay. if the alternator isn't charging (10.3 volts), this circuit isn't powering up the fuel pump. The pump is temporarily powered up when you crank the engine. AGAIN, I'm not totally sure on your application, but this was the story on the 2.6 models.
Another thing, unlikely because you are talking idle speeds, but I had a 94 Trooper (same engine) and mine wasn't charging under load and cruising speed. Load being A/C on and headlights on. The load against the alternator was causing the outer ring of the harmonic balancer to slip. The outer ring had the pulley grooves in it. Harmonic balancers for this vintage Isuzu 3.2s were notorious for failing.
I would find and check all fusible links as well. Dennis
 

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Thanks for the responses, guys. I will do some additional testing as Shawn suggested but I'm leaning more toward the PCM/ECM
The PCM/ECM does not control the alternator like other models. The alternator control system for this vehicle is 100% passive.

With this type of system, all the alternator requires to function correctly is it to spin, be connected to a battery, have full ignition power at the IG connection & a working light on the L connection.

If any of these are missing it will not function. Find what is missing by fallowing my troubleshooting prosudure. If electrical is not you strongest talent. I will PM you my number and ill try my best to walk you through it.
 

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Discussion Starter · #13 ·
The PCM/ECM does not control the alternator like other models. The alternator control system for this vehicle is 100% passive.

With this type of system, all the alternator requires to function correctly is it to spin, be connected to a battery, have full ignition power at the IG connection & a working light on the L connection.

If any of these are missing it will not function. Find what is missing by fallowing my troubleshooting prosudure. If electrical is not you strongest talent. I will PM you my number and ill try my best to walk you through it.
Thanks, Shawn (and Buster!). I'm going to put in a new alternator and attempt to work through the trouble shooting procedures you laid out. Though I may take you up on the PM - let me know how to find that on the forum.
 

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What's the latest? Hope you figured it out.

One more thing to look at if still not working.... many of the alternators of this vintage required a field or sense wire that read ignition voltage rather than from the battery peg. If there was no voltage on this wire, there was no output. So, your 12.5 v would drop after starting the engine (due to starter load), and volts would not come up even though alternator was spinning (bat was depleted from start and now powering car load). This was a typical problem when owners would think the alternator was bad and have it rebuilt, and it would check good on a test stand, but still bad on actual car. If there is no voltage on field or sense wire/terminal, jump +12v to it and check for output to be sure that's the issue. I don't have a wire diagram for your car but someone here does that can tell you wire colors.

If there is higher voltage coming from alternator but not getting to rest of car circuits, Would suggest borrowing a DC clamp amp multimeter (make sure it has DC load option) and check amp load on the alternator. I would expect to see about 24-35 amps after start then settle about 12 amps in a few minutes with engine running. If way higher, this could explain the low volts and some voltage regulators have protection circuits that lower output with high draw or high temperatures. Good luck.
 

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Discussion Starter · #17 ·
Update - Still no resolution yet. I tested the battery and it failed one test so I replaced it, I'm now back to the 12.5 volts I was working with originally. I have worked through the charge circuit testing voltage along the way (I've attached the wiring diagram from my Hanes manual for reference). Starting from the ignition plug wire at the alternator (12.2 volts) I measured to the fuse box in the cabin (fuse #8 on the diagram) and got about 12.2 volts (battery was 12.5), so about a 1 - 1.5% drop from the battery reading. I measured at fuse #7 which runs to the charge warning light and again got 12.2. From that point it runs through the charge warning light and back to the relay/fuse box in the engine compartment. At the relay/fuse box I measured 11.75 volts at fuse #5, so about a 4% drop.

I talked to Shawn A. and he believes 3/4v drop may be significant enough to prevent toe alternator from starting the charge process. He said there are a couple of parallel circuits that run off fuse #5 in the relay/fuse box, but I'm not sure what they are (ideas?). I'm going to backtrack and see what the voltage is coming off the charge warning light in the dash, just to be sure.

Any other ideas are welcome!
Handwriting Schematic Font Rectangle Parallel
 
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