RPM drops while driving?

[cerberusiam]

Okay, and why is that? I have to say that it sure feels like it is doing that, because the truck jerks with that happens.

Run the engine up to 2000 rpms and shut the key. The engine has a lot of spinning mass to decelerate, it does not happen that fast. What you are seeing with the tach dropping like that is an electrical interruption that is dropping the signals to the cluster, either at the cluster, the controller, the data bus, or maybe the ECU shutting down.



That could be a spike in the power that cause the ECU to shutdown, a loss of ground, or a loss of power. The jerk is the injectors shutting off.



With a light flashing on the dash like that it is electrical. Have you run the cluster diagnostics to see if anything is popping in it?



As I said before the big connector on the firewall carries bus info and if it looses connection the ECU will shut down. The ECU is programmed to shut down if it can't communicate with the clister controller becuase any security systems are integrated into it. An intermittent short condition in the fan circuit can also cause the same results, or even another circuit.



The FCM can have somewhat the same effect if it has corroded and\or has a broken connection on the board.



Along those lines, how are the batteries, battery connections, and the power connections off the cables? Also, the infamous AC clutch wire will throw a dead short back at the system and cause it to do weird things.

 

[AEdelheit]

Run the engine up to 2000 rpms and shut the key. The engine has a lot of spinning mass to decelerate, it does not happen that fast. .

I agree an unloaded engine won't drop that fast and Im not saying the OP engine is but if you turn an engine off with a load on it it can drop RPMs extremely fast, I don't honest know what the torque converters doing but I'd bet it's possible it could rapidly unlock so it doesn't have the drivetrain keeping it going and cause it to drop RPMs far faster then unloaded. It could recover that fast too if the tourque converter relocks and it's the drivetrain

Spinning the engine, not saying this is the case but I'd deffenitly say it's possible

 

[cerberusiam]

As soon as power is lost the TC unlocks, OD drops, and the sprag in the OD clutch will overrun. The engine will essentially be in free wheel mode just like neutral or park until the OD direct clutches engage. Only then will the drive train apply drag to the engine.



The caveat is this whole sequence can take up 3-5 seconds, or more, to initiate.



A short power loss simply will not unlock things fast enough to make much difference so momentum will keep the rpm's about the same. Thats the jerk that is felt.



Remember, this a 7-8k truck and simple physics states an object in motion tends to stay in motion until acted upon by an outside force. Rotational mass in the engine and wheel speed dictate it ain't slowing that much. If it did the driver and any loose obects would be launched thru the windshield, seat belts or not.

 

[AEdelheit]

I'm not saying the truck would slow down just if you take an engine under load and shut off the fuel it can drop RPM extremely fast if the load stays on it, I've seen it in person. makes sense the OD clutch should disengage , as I said don't think it's actually dropping rpm that fast but to say a loaded engine can't I feel is inaccurate

 

[cerberusiam]

if you take an engine under load and shut off the fuel it can drop RPM extremely fast if the load stays on it

Yes, that I agree with. Given an engine and a load on it dropping the fuel will cause it to slow much faster than it would without load. A static load that is.



What I am trying to say is the load on an engine in a vehilce traveling at speed is not static. The engine is hooked to the wheels, the wheels will translate the kinetic energy of the moving truck into a rotational force that will keep the engine rpms from dropping much if any rpm and a lot slower than even an unloaded engine. Until the transmission completely disconnects energy gets transferred, both from the chassis and the transmission itself, to the engine.



Thinking in simple terms of a mass\energy equation, the amount of energy released to slow the engine from say 1800 rpms to 750 rpms in the time described will be destructive to the engine itself and the chassis also.



The rpm bounce has to be electric malfuntion or there would be parts scattered across the road.

 

[AEdelheit]

Yes, that I agree with. Given an engine and a load on it dropping the fuel will cause it to slow much faster than it would without load. A static load that is.



What I am trying to say is the load on an engine in a vehilce traveling at speed is not static. The engine is hooked to the wheels, the wheels will translate the kinetic energy of the moving truck into a rotational force that will keep the engine rpms from dropping much if any rpm and a lot slower than even an unloaded engine. Until the transmission completely disconnects energy gets transferred, both from the chassis and the transmission itself, to the engine.



Thinking in simple terms of a mass\energy equation, the amount of energy released to slow the engine from say 1800 rpms to 750 rpms in the time described will be destructive to the engine itself and the chassis also.



The rpm bounce has to be electric malfuntion or there would be parts scattered across the road.

All agree with that, I guess there really is no way the transmission could create that sort of load without transfering the kinetic energy

 

[ACerf]

Run the engine up to 2000 rpms and shut the key. The engine has a lot of spinning mass to decelerate, it does not happen that fast. What you are seeing with the tach dropping like that is an electrical interruption that is dropping the signals to the cluster, either at the cluster, the controller, the data bus, or maybe the ECU shutting down.



That could be a spike in the power that cause the ECU to shutdown, a loss of ground, or a loss of power. The jerk is the injectors shutting off.



With a light flashing on the dash like that it is electrical. Have you run the cluster diagnostics to see if anything is popping in it?



As I said before the big connector on the firewall carries bus info and if it looses connection the ECU will shut down. The ECU is programmed to shut down if it can't communicate with the clister controller becuase any security systems are integrated into it. An intermittent short condition in the fan circuit can also cause the same results, or even another circuit.



The FCM can have somewhat the same effect if it has corroded and\or has a broken connection on the board.



Along those lines, how are the batteries, battery connections, and the power connections off the cables? Also, the infamous AC clutch wire will throw a dead short back at the system and cause it to do weird things.

How do you do a Cluster Diagnostics? Batteries and all connections are fine. I have not checked the connector at firewall or on ECM or is that ECU?

 

[ACerf]

I disconnected the Edge EZ, since some seem to think that might be the problem. I did drive it today on the Freeway and nothing happened. But of course that doesn't mean anything since sometimes it takes a week or so for the problem to show up.

 

[ACerf]

Yes, that I agree with. Given an engine and a load on it dropping the fuel will cause it to slow much faster than it would without load. A static load that is.



What I am trying to say is the load on an engine in a vehilce traveling at speed is not static. The engine is hooked to the wheels, the wheels will translate the kinetic energy of the moving truck into a rotational force that will keep the engine rpms from dropping much if any rpm and a lot slower than even an unloaded engine. Until the transmission completely disconnects energy gets transferred, both from the chassis and the transmission itself, to the engine.



Thinking in simple terms of a mass\energy equation, the amount of energy released to slow the engine from say 1800 rpms to 750 rpms in the time described will be destructive to the engine itself and the chassis also.



The rpm bounce has to be electric malfuntion or there would be parts scattered across the road.

All I can say is, What?

 

[JJPage]

I disconnected the Edge EZ, since some seem to think that might be the problem. I did drive it today on the Freeway and nothing happened. But of course that doesn't mean anything since sometimes it takes a week or so for the problem to show up.

Thank God! I was about to say "for God's sake, disconnect the EZ".



If the EZ proves not to be the culprit, all "FLAME ON"!:-laf:-laf:-laf

Jess

 

[ACerf]

I happen to remember one thing about this problem. When this problem has shown itself I had the Cruise Control on! I had a TSB about PCM connections and I noticed it said to clean and grease with dielectric grease. 3 connections C2, C2 and C3. Sometimes when I have the cruise on it feels like it is surging, but it is hard to tell if it is the truck or the road, it is barely noticeable. Well, I have the EZ disconnected and I can try cleaning the PCM connections.

 

[cerberusiam]

How do you do a Cluster Diagnostics?

Witht he key off, hold down the trip meter reset button and turn the key to the on position. Once the dash lights up you can let off the button. The controller will run diagnostics on the cluster and report any code sin the odo window.

 

[ACerf]

Witht he key off, hold down the trip meter reset button and turn the key to the on position. Once the dash lights up you can let off the button. The controller will run diagnostics on the cluster and report any code sin the odo window.

thanks for the info. Did this test and everything working but I got a "C - Code and 00 00" what does that mean? All the gauges seem to be working okay, but I have to say that it is a funny thing to watch.

 

[ACerf]

How do you do a Cluster Diagnostics? Batteries and all connections are fine. I have not checked the connector at firewall or on ECM or is that ECU?

I did the Cluster Diagnostic, and I got to two things on the ODO screen; 1st a "C - Code" second a "00 00". So what does this mean, I can see the 00 mean nothing but what about the C - Code?

 

[ACerf]

Because the 03' were having problem with the PCM connections being corroded. I checked them today, they were good. I even checked the main connection thru the Firewall, no problem there. So now I have ruled out the PCM connections, disconnected the EZ. Checked the Gauge Cluster for errors of which I am not sure if there is problem or not because of the "C - Code" and the 00 00. At least I am getting it narrowed down... . I think!

 

[JJPage]

Because the 03' were having problem with the PCM connections being corroded. I checked them today, they were good. I even checked the main connection thru the Firewall, no problem there. So now I have ruled out the PCM connections, disconnected the EZ. Checked the Gauge Cluster for errors of which I am not sure if there is problem or not because of the "C - Code" and the 00 00. At least I am getting it narrowed down... . I think!

I did the test for the gauge cluster, and got the same result you got, C CODE and 00 00, so that must be normal? The process of elimination can be frustrating, but sometimes it's all you got.

 

[cerberusiam]

Yes, that means there are no issues with the cluster and the tests all passed.

 

[ACerf]

Okay, whats left to check? There has to be something else that you all can think of... . I have not driven it much without the Edge disconnected so I might see how that goes before doing anything else. I am going to call Edge and see what they think about the EZ causing a problem.

 

[ACerf]

I called Edge and told them about the problem I was having. The tech did not say it was or wasn't the EZ causing the problem. He did say that when they get old they do have a tendency to cause problems... ummm. He offered me a new one at discount of $250. I told him I will run the truck for awhile to see if the EZ is cause the problem before I invest in a new one. You guys just might be right about the EZ! Thanks!

 

[BLoging]

not steal this thread but i did the the cluster test and got a the "C_code of 81 00 then the 00 00 this is on a 2004 and this is to cerberusiam what is that code mean?