Mystery NO start issue, 2001 Blue Bird All American RE with Cummins 8.3 & Alison 3060

[ewo1]

IF you claim there is "NO physical connection between the two"....you are most clearly incorrect. IF they are both grounds, they share, AT SOME POINT, a physical connection.

I am most clearly CORRECT. I mentioned earlier grounding goes deep.
The way the sometimes overlap could get confusing.

One is SIGNAL GROUND. Signal when transmitted down two wires will have a signal+ and a signal return.
That return is your signal ground and should be identified as such. It is not tied to your VCC source ground!

Power ground is another animal. It allows for electrons to flow, allowing for the powering up of devices.
Power grounds always are tied to the negative terminal of a battery, in a neg grounded system, and in every automobile the ground is tied to the frame of the vehicle.

signal ground and power ground must be separate from each other otherwise you will have a ground feedback loop which can make your signal transmission go crazy.

Factor this in... within signal transmission.
On a DC system you got ...zero hertz. NO frequency!
On a DC signal transmission, that data can be travelling at say a 14k baud rate. It cannot tolerate any feedback loop (noise) injection or the timing of the data movement can be thrown off and corrupted.

this is one major reason they are NOT tied together.

 

[ewo1]

iF you claim there is "NO physical connection between the two"....you are most clearly incorrect. IF they are both grounds, they share, AT SOME POINT, a physical connection. OR, one is not a ground. It is then called "a problem" and it needs to get fixed, by connecting to ground.

I totally agree that signal grounds should not be connected directly to, say, the starter ground wire for reasons listed in your linked article way above.

So you agree but then disagree .. Yeah it took me a while to understand this reality...
Two different grounds at different potentials, not connected to each other.

If you try to "Ohm" the two you just might be reading circuit impedance which has nothing to do with what we are talking about.
That's Another subject matter.

BTW, there could be instances where circuit design might call for the two grounds to be together. I said this earlier, depending on circuit design.

There is room for variables but for measuring "circuit" voltages, many times it is best done within the circuit itself versus using a common reference point such as a common ground. It all depends on the circuit before you and how it is designed to work.

I will agree that common ground is what is out there more than the others so from an automobile troubleshooting perspective...it works just fine.
I will also agree with you, that there are instances where the signal and power grounds could be tied together at a common point but that is all by circuit design. We as technicians should be able to identify both scenarios and perform accordingly.

High frequency machinery, a different set of troubleshooting procedures for sure.

Electronics theory runs wide and deep!

 

[Mr P]

On a school bus, a truck, a car, (excluding all electric models), assuming a 12 vDC, negative ground, ALL grounds are "connected" to the negative battery terminal. By "connected", I mean they will have continuity.

Agree?

 

[ewo1]

On a school bus, a truck, a car, (excluding all electric models), assuming a 12 vDC, negative ground, ALL grounds are "connected" to the negative battery terminal. By "connected", I mean they will have continuity.

Agree?

I will agree with you that all power grounds are tied together.

I have also seen many times where signal grounds are independent of power grounds!

I am not 100% referencing this second statement to only automobiles. I am speaking in wider terms.
I have repaired medical equipment, industrial high power SCR switching equipment, two way radio communications systems, consumer items, and lots of PLC circuit boards. I have seen floating grounds in many circuit designs...it's all out there!

What I share with you is NOT the only rule in the book, just one of them.
It simply important to know it is out there and how do I respond once I have identified it is before me.

 

[Mr P]

I'm telling whoever reads this in the future, that on every 12v DC auto/car/truck I've ever seen, ALL grounds MUST "connect" (electrically) to the ground at the battery. IF NOT, that "ground wire" is NOT grounded and that component will NOT work.

 

[ewo1]

I'm telling whoever reads this in the future, that on every 12v DC auto/car/truck I've ever seen, ALL grounds MUST "connect" (electrically) to the ground at the battery. IF NOT, that "ground wire" is NOT grounded and that component will NOT work.

Your free to do as you wish.

In a 12v power distribution network, your statement holds truth.

When it comes to signal grounds, there are variables one should be aware of!
If your not aware of them, good luck figuring out why the circuit is gone plum loco..

 

[ewo1]

If "circuit ground" and "frame ground" are not connected, one is not a ground. That in itself would be a problem.

Good info otherwise!

I am gonna circle back to this, you made me think Mr. P.

Inside the ECM, yes your signal ground is tied to the ECM ground which then in turn is tied to the chassis ground.
This signal ground IS NOT chassis ground.
It is tied to the ECM ground in an effort to prevent a voltage differential between the two grounds which in turn help squash ground loop feedback, or noise in the circuit.

It is "signal" ground because it job is to "deliver" a signal, not deliver power.

So in this sense, I will agree with you, only because we now understand why it is tied to ECM ground in the first place, to reduce noise.

I said it earlier, it is a circuit design, one that an engineer used because it served his engineering purpose.
I said it earlier, it is important to recognize this event when working on signal circuits versus power circuits.


It is also important to know how to use "terms" correctly.
When using the word "Ground or Grounding" it is important to know if it is power, signal, earth...they all serve differently.

 

[nikitis]

Let me chime in and clear this up. I'm familiar with micro electronics as they work the same as on an automotive vehicle just much smaller.

Grounds all must end up to a negative terminal, yes. No one is disputing that. You can have a separated ground plane though and have two ground planes as long as they don't touch anywhere. You just don't often do so on a vehicle application. Maybe some of the more modern complicated ones do (unsure).

Typically and in almost all cases, the ground is referenced with the word "Plane". This is to signify that It's a much larger field or surface area/material than the flow of power before it is sent down to the ground plane.

Electrons flow in a much tighter existence on a power line, and it's constrained by outside forces (Larger ground plane). This is how we are able to direct electrons for signaling much easier down wires. Electrons like to take the path of least resistance, and an open plane with vastly more atoms for electrons to have to fight against helps back flow electrons down the positive wiring. This is why Ground is always larger. Wires are the path of least resistance. Put your load in the middle of the wires, and it becomes the least resistance when backed up against a ground plane. Amperage pushes it through the load.

In micro electronics ground is very carefully sized for precision signaling. It doesn't often matter too much with something as large as a vehicle.

 

[ewo1]

ok, back to Rock-n-ruth's thread....
Sorry for the sidetrack...

 

[ewo1]

@Rock-N-Ruth,
I just found this "colored" wiring diagram for the Allison 3000 WTEC3 tid2 tranny.
It might be easier on your eyes if you use this to compare the other diagrams I posted so far.

The Regular Push Button Selector is powered up by the Transmission Electronic Control Unit (ECU) from Terminal 16 of the "S" Black connector to terminal R of the Push Button Connector located at the back of the display.

The Push Button Display receives its sensor power from terminal 3 of the same ECU connector to terminal N.

The battery return or ground signal is supplied from terminal 32 of the ECU connector to terminal P.

Without power and ground to the noted terminals the display will not illuminate and unfortunately the engine will not start, as this is also your Neutral Safety Device.

Some ECU's are powered through a 10amp fuse inside of the Vehicle Interface Module (VIM) out on VIM pins R1 and R2 and in on J1 and J2 from the battery, others as in my case are powered through a 10amp fuse direct from the battery.

 

[Rock-N-Ruth]

UPDATE:
I removed all the corruption in the battery box and thoroughly cleaned each contact to the batteries. Didn't solve the problem, but needed to be done.

Next day I removed the shift control panel, to see if, as suggested, the TCM might be there. It wasn't. But, since I was there I disconnected the big round cable connector, and gave it a visual inspection. Then popped it back together. Still no bingo.

Since I had the shift panel in my hand I thought I would try pushing some buttons again. One never knows, do one?

Held the mode button for the seconds. Nuttin.
Held the upshift and downshift buttons simultaneously for ten seconds. Nuttin.
Held the D and R buttons simultaneously for about five seconds, and the display lit up, R.
So, I tried N. It worked.

I'll bet you will never guess what I tried next, yup, it started.

Thank all of you guys for the mountain of good and useful input. I am sure that this thread will help others with mystery no start situations in the future.

I still have no idea what really happened here.

I had played with all the buttons in the same manner, when the problem was first discovered, with no results

The only new factor might have been, disconnecting the shift panel from the TCM with power on.

Either way, I am pleased that I didn't have to spend a bunch of time under the bus looking for broken connections, TCM, VIM, etc...

I think that some of the systematic troubleshooting suggestions you guys provided are not only valid, but will surely come in handy for more than one skoolie folk in the future.

Sometimes these gremlins are like a dried scab...ya just hafta keep picking at it, till you get results.

It's threads like this one that really present the value of the skoolie community.

 

[magnakansas]

Well, the good thing is, you’re in business. The bad thing is, we don’t really know what happened. Thanks for letting us know.

William

 

[ewo1]

I'm suspecting a "marginal" connection somewhere in the system.
Hot n cold temperature changes will tend to affect those type of issues...

Time will tell but CONGRATS on getting it resolved!
I do hope it stays working for a long time!

 

[nikitis]

What seems to have happened in my mind is what happens on the Allinson stick shifters. Your transmission somehow likely wasn't fully in Neutral. Sometimes I'll set the stick to N, but it won't actually have shifted to N. There's a ground wire on the transmissions (even on the mechanical only ones), and the ground signal isn't connected (because the trans isn't actually in Neutral), it won't start. Seems to me when you set it to Drive electronically, Reverse, then Neutral, it actually put it in neutral and it started.