LMTV A1 fast idle ghosting

[MatthewWBailey]

I've notice a continuing issue when cold, this fast idle switch does not always trigger a fast idle. Half the time the engine bumps up a very small amount of rpm's instead of the usual 1200ish
Goes away if I restart after warming up.

 

[Ronmar]

There may be some condition not met for it to enable/allow fast idle, like throttle position not low enough, or neutral or some such. Not specifically familiar with the specific requirements for fast idle, but it may be something like this causing your issue…

 

[MatthewWBailey]

There may be some condition not met for it to enable/allow fast idle, like throttle position not low enough, or neutral or some such. Not specifically familiar with the specific requirements for fast idle, but it may be something like this causing your issue…

What's weird is that it only bumps up a small amount of rpm's, barely noticeable but clearly as a result of the switch activation.

 

[Suprman]

Maybe the computer wants to see a certain engine temp before doing full high idle. There may be a setting in the ecm.

 

[Ronmar]

Conditions for Engaging the Fast Idle System
When all of the following conditions are present the idle speed will increase to the selected value and the FAST IDLE lamp will illuminate.

The engine is running
The transmission is in Park (P) or Neutral (N)
Vehicle speed is not detected
The FAST IDLE switch is depressed and released
Conditions for Disengaging the Fast Idle System
When any of the following conditions are present the engine speed will return to the normal engine idle speed and the FAST IDLE lamp will turn OFF:

The engine is not running
The transmission is not in Park (P) or Neutral (N)
Vehicle speed is detected
The FAST IDLE switch is depressed

 

[MatthewWBailey]

Conditions for Engaging the Fast Idle System
When all of the following conditions are present the idle speed will increase to the selected value and the FAST IDLE lamp will illuminate.

The engine is running
The transmission is in Park (P) or Neutral (N)
Vehicle speed is not detected
The FAST IDLE switch is depressed and released
Conditions for Disengaging the Fast Idle System
When any of the following conditions are present the engine speed will return to the normal engine idle speed and the FAST IDLE lamp will turn OFF:

The engine is not running
The transmission is not in Park (P) or Neutral (N)
Vehicle speed is detected
The FAST IDLE switch is depressed

It's definitely going into fast idle mode but the rpm change Varies greatly. Normally to about 1200 rpm but half the time it only goes up by 50-100.

unless... there's 2 setpoints?

this threads info is showing that there's 2

Sergeant York had a CAT scan...

My neighbor's grandson is a mobile mechanic with the local CAT dealer. An exchange for a nice brass CAT belt buckle, he plugged into my truck and gave it a full scan and analysis. Everything looked good!

www.steelsoldiers.com

if so, how is setpoint #2 triggered?

 

[aw113sgte]

Anecdotal, but my A1R has always gone to normal high idle rpm and I've tried it with cold starts well below freezing.

 

[GeneralDisorder]

The switch in my truck (A1R, C7) acts strangely in that sometimes it works and sometimes it doesn't. Acts like a software issue where it's not monitoring the pin that's being activated often enough. Sometimes it will activate high idle and sometimes it just ignores the switch. And when taking it out of high idle sometimes when you press the button it will idle down for a split second and then go back to high idle again. Its acts like a poorly functioning video game controller where the polling is off and it's dropping inputs or getting double presses, etc. I don't think it's my switch based on the behavior. It's obviously seeing the button when it idles down and then back up again. It always works just sometimes takes a couple tries to get it to do what you want so I chalked it up a software glitch. Doesn't bother me - usually takes no more than two or three attempts and I don't use it that often. I have a block heater that comes on an hour before I leave every day.

 

[MatthewWBailey]

That's Helpful feedback. Glad it's not a rabbit hole. Once I setup the cc stuff it'll be an unused button anyway.

 

[Ronmar]

here is an alternate theory for you all to crunch on…

I have come across a few people who have had intermittent issues with the transmission throwing speed sensor codes, myself among them when I select mode. What I am theorizing might be happening is that there is a weakness in the pass-thru connector on the transfer case. That connector carries the transfer case lockup solenoid signal and the output speed sensor signal… The Allison uses a 7Khz PWM signal to control the solenoid valves. It sends it all the time. At a low PW, it uses it to test the circuit and higher PW to actually activate the solenoid. What is happening in my case when I select mode, my speedo needle twitches and my TCU throws a fault as it is sensing output speed with no transmission turbine speed as the vehicle is not actually moving… I havn’t gotten a chance to scope it yet and see exactly where the crosstalk is occuring…

So what I am theorizing, since what I have read indicates vehicle speed detection inhibits fast idle, if there were some crosstalk in that connector on yours, it may be sending back enough signal on the output speed line to tell the ECU that there is vehicle speed, which might be inhibiting the fast idle?

so here is a test or two for you:
While watching your speedometer very closely, does the needle move when you turn on the ign, or after you start the truck?
while watching your speedometer needle very carefully, after it is running, select mode. Does the needle move?

I suppose another check in your case might be to simply disconnect the transfer case cannon plug on the front of the transfer case and then see how your fast idle behaves…

 

[MatthewWBailey]

here is an alternate theory for you all to crunch on…

I have come across a few people who have had intermittent issues with the transmission throwing speed sensor codes, myself among them when I select mode. What I am theorizing might be happening is that there is a weakness in the pass-thru connector on the transfer case. That connector carries the transfer case lockup solenoid signal and the output speed sensor signal… The Allison uses a 7Khz PWM signal to control the solenoid valves. It sends it all the time. At a low PW, it uses it to test the circuit and higher PW to actually activate the solenoid. What is happening in my case when I select mode, my speedo needle twitches and my TCU throws a fault as it is sensing output speed with no transmission turbine speed as the vehicle is not actually moving… I havn’t gotten a chance to scope it yet and see exactly where the crosstalk is occuring…

So what I am theorizing, since what I have read indicates vehicle speed detection inhibits fast idle, if there were some crosstalk in that connector on yours, it may be sending back enough signal on the output speed line to tell the ECU that there is vehicle speed, which might be inhibiting the fast idle?

so here is a test or two for you:
While watching your speedometer very closely, does the needle move when you turn on the ign, or after you start the truck?
while watching your speedometer needle very carefully, after it is running, select mode. Does the needle move?

I suppose another check in your case might be to simply disconnect the transfer case cannon plug on the front of the transfer case and then see how your fast idle behaves…

I tried this today. Needle is off zero to begin with, then goes to zero on ignition switch on. No change if I cycle mode once the truck is running. However, this speedometer does act squirrelly at times when driving. It's gone to zero a few times while on the street and I have to stop and cycle ignition to get it back. More dirty connectors I gather?

 

[hike]

Our 2003 M1078A1 does on its own cycle to high rpm (though I haven't yet checked exactly how fast) when starting in colder weather. Though it does so after running a bit and doesn't stay there long. Clicking the high low switch always works while in neutral (1,350 rpm) and goes back when clicked again (700 rpm). It worked today—

 

[Ronmar]

I tried this today. Needle is off zero to begin with, then goes to zero on ignition switch on. No change if I cycle mode once the truck is running. However, this speedometer does act squirrelly at times when driving. It's gone to zero a few times while on the street and I have to stop and cycle ignition to get it back. More dirty connectors I gather?

Dirty or failed, what I am getting is 7KHZ differential solenoid PWM signal being fed back thru the speedometer signal lines. These 2 signals share a 4 pin passthru connection on the front of the transfer case. If you were getting something similar, the rogue speed signal input might be great enough that the ECU detects it and restricts fast idle… if you unplug the transfer case connector, does your fast idle work consistently?

 

[Wingnut13]

I will give it a try tomorrow morning and record the Speedo. Should also be cold enough for the ECU to prompt a high idle. Or, rather try to high idle.

 

[MatthewWBailey]

Dirty or failed, what I am getting is 7KHZ differential solenoid PWM signal being fed back thru the speedometer signal lines. These 2 signals share a 4 pin passthru connection on the front of the transfer case. If you were getting something similar, the rogue speed signal input might be great enough that the ECU detects it and restricts fast idle… if you unplug the transfer case connector, does your fast idle work consistently?

I haven't tried this specifically yet. The next cold day start will be the best time to test it as that's the only time there's an issue. Never happens above 40F.

 

[aw113sgte]

Our 2003 M1078A1 does on its own cycle to high rpm (though I haven't yet checked exactly how fast) when starting in colder weather. Though it does so after running a bit and doesn't stay there long. Clicking the high low switch always works while in neutral (1,350 rpm) and goes back when clicked again (700 rpm). It worked today—

My C7 will change idle a little in the cold similar to what you describe but no where near the rpm of the fast idle switch. I'd guess a few hundred rpm change. This is including down in the single digits.

 

[Wingnut13]

Well, in the last week I replaced my fuel hand primer, fuel lines, IDP and high pressure sensor. Last night was in the low 30’s, this morning I started the truck. Initially It had a hard time staying running, had to restart three times. However once a good idle was established, it idled right up as it never has before.

I took a video, no movement detected on the Speedo.

 

[GeneralDisorder]

Well, in the last week I replaced my fuel hand primer, fuel lines, IDP and high pressure sensor. Last night was in the low 30’s, this morning I started the truck. Initially It had a hard time staying running, had to restart three times. However once a good idle was established, it idled right up as it never has before.

I took a video, no movement detected on the Speedo.

Hhhhhmmm - you sure your grid heater is working? My experience down to the ether set temp (about 20) has been that if you wait till the grid heater light goes out they will start right up. I've never once had to run the starter more than one time. Even down to 16 degrees where the Ether kicked in.

 

[Wingnut13]

I believe the grid heater is working. But I can check it. I wonder if there is a learning curve with the ECM for the new IDP, the old one certainly was not functioning properly. The ECM I’m sure was trying to compensate for it somehow.

 

[GeneralDisorder]

I believe the grid heater is working. But I can check it. I wonder if there is a learning curve with the ECM for the new IDP, the old one certainly was not functioning properly. The ECM I’m sure was trying to compensate for it somehow.

IDP? I must be having a senior moment. Not recalling what you are referring to.

There shouldn't be any leaning curve to any of the engine sensors. The ECM has no ability to check any of them really - there's no redundancy and no exhaust sensors to use for feedback. The ECM expects all sensors to read within factory specifications at all times and all programming is static. There aren't fuel trims or any sensor calibration settings that I'm aware of.

The grid heater solenoid is a common failure item. The connections rust really easily - at least on the C7. It's right behind the cab where water drips directly on it. I broke mine trying to remove the nuts holding the lugs on. Was a cheap replace though.