Engine/Transmission (1998.5 - 2002) High Idle

[garylmoore]

How about that Idle validation switch. Is that near the Apps?

 

[Ozymandias]

Stop throwing parts at it - have it diagnosed.
You need to know what the PCM is calling for and what he sees.
You already wasted a ton of money.

 

[Ozymandias]

How about that Idle validation switch. Is that near the Apps?

It is function of the APPS and not a separate part.

 

[garylmoore]

Ok.

 

[garylmoore]

I read an old post on Moparman that the ECM Idle is set by a coolant temperature switch, does anyone know anything about this switch?

 

[BigPapa]

Tell us again why you feel the alternator isn’t operating as it should. What’s battery voltage engine off and what’s the voltage running after warm enough for no heater grid?

 

[garylmoore]

Thank you for replying Big Papa. The problem is a high idle. The truck runs 1200 rpm cold and about 1500 rpm when warmed up. It was suggested that this was most likely caused by low battery voltage. This made sense because the truck had been sitting, while I was in the middle of an out of state relocation and UHauling everything out of state.
I checked voltage at one point coming out of the alternator and had 14+ volts, but never had a reading after that above 13.1 at the alternator post or the batteries. The meter that obtained that result, failed due to a depleted battery the next day, so that14+ volts reading is suspect.
I had the 10 month old Interstate batteries checked and Interstate replaced both due to failing a load test. This made sense with the low battery scenario, but still under 13.5 volts when running. I ordered a new Nations Alternator and installed it today. So. I'm pretty sure there is no problem with the alternator, but I have no way to tell if the ECM is commanding the alternator to charge or not.
I do know that after doing the things above, the idle is still running 1200 to 1500 rpm, which was the original issue with the truck and makes it unsafe to drive. I am at a loss as to where to go now. Unless there is another good reason for high idle, ECM failure is all I can see.

 

[garylmoore]

There is a second connector on the alternator.
If this line carries a signal to or from the ECM, it is possible the green and blue wire at that connector is at fault. I have a manual on cd, but it is at the moving destination along with my computer and most of my other belongings.
I am down to one trailer full of stuff, a 3000 lb toolbox, and a Dodge Truck and the move is done.

 

[BigPapa]

It just seems to me you’ve spent a lot of time and money on a charging system issue that you may not even have, while not addressing the original fuel system issue. I’m wondering why the concentration on the charging issue? Why did you start down that road? Low voltage isn’t as big a problem on the 2nd or even 3rd Gen trucks that it is on the newer trucks.

Do you have a tuner, chip, or other fuel system modifications?
When and how did the idle issue start?
Has there been ANY other work done before the idle issue?

As I said, I’m not too good on the 24-valves. Just trying to figure out what you may have going on.

 

[garylmoore]

Well, the battery call was suggested and at the time, it made sense.
No mods on the truck at all, no tuner, bone stock as far as I know. A Fass Titanium is the only fuel modification.
About a week before the truck was parked, both fuel filters were changed. The truck was driven that week with no problems.
The truck sat for at least 3 weeks without being driven.
It ran fine when parked, went to high idle weeks later when driven and never came down.

 

[GAmes]

..........I have no way to tell if the ECM is commanding the alternator to charge or not.

I think the PCM contains the voltage regulator, but you do have a way to see if it is working. Measure the battery voltage after it has rested a few hours and again with the engine running. If there is an increase in the voltage the alternator and PCM are functioning. Voltage output varies due to the demand on the battery and battery temp so if it only goes up to 13.5 (or whatever) there is no need to suspect the alternator isn't working.

The ECM is what controls the idle. Like Scott, I know little about 24 valve trucks, so I don't know if there is a self test procedure or if you need a scanner.

 

[garylmoore]

I have not detected readings above 13.5 either with the new alternator or old. The batteries being new, I thought it was not charging because the ECM saw no need. The guy that is helping felt the same way. We are expecting 14+ voltage in the batteries and at the post of the alternator.
I am going to jump on the Interstate and make a Wisconsin Beer run this morning so I know the batteries are getting depleted.
I will take your suggestion and check resting voltages before leaving.

 

[Topzide]

@garylmoore, Here's a diagram that might help, it's from a Haynes manual.

 

[BigPapa]

I have not detected readings above 13.5 either with the new alternator or old.

I think you're missing what Gary and I have said. Measure battery voltage without the truck running, after it's sat overnight or at least a long while. Then, drive the truck enough to get it up to temp where you know the grid heaters aren't cycling, leave the truck running, and measure battery voltage again. If it's ANY higher than it was when the truck wasn't running, your alternator is charging.

 

[petersonj]

I have not detected readings above 13.5 either with the new alternator or old. The batteries being new, I thought it was not charging because the ECM saw no need.

The alternator ALWAYS charges when the engine is running. It is the PCM (not ECM) that controls alternator output by regulating the field current from the alternator. The battery sensor under the driver side battery provides an input to the voltage regulator part of the PCM. The PCM will set an approximate maximum voltage of around 13.5 volts on a 100°F day and an approximate maximum voltage of around 14.2 on a 40°F day.

I believe you live in Illinois. At this time of year you should be seeing an engine running voltage of around 14 volts at each battery.

Note in the photo below that the dark blue field wire is the source voltage (12 volts) from the PCM to the alternator. The dark green field wire is the field ground from the alternator that is controlled by the PCM. You can check the dark blue wire for 12 volts while the engine is running. If it is not present, there will be no output from the alternator.

Make sure the 140 amp fuse in power distribution compartment is not blown.

Also, use a different multimeter (or double check the accuracy of your meter on another vehicle) to verify your readings are good.

- John

 

[BigPapa]

I found this, mentions idle speed. It tells you what conditions cause the idle to increase from 800rpm to 1200rpm.

 

[GAmes]

Spotted cow. If Wisconsin were closer I'd be up for a beer run too.

 

[garylmoore]

Great information! I will check the voltage now, run around the Interstate, then stop and check again.
I know exactly which fuse you are talking about. It sits between the lead from the alternator and battery positive at the driver's side battery.
Thanks for the great information!

 

[garylmoore]

Ok. You've proved it to me.
Voltage cold 12.29 and 12.28 v
Voltage at full temp
14.38 and 14.37 v

 

[garylmoore]

So. That is a relief. If I have to drive with the high idle I will.

Thank you all for that.

I am sitting in a parking lot at this moment with a warm engine idling 1459 rpm.