Odo showing 'no Bus'

Tiny
ON3POINTKING
  • MEMBER
  • 1997 PLYMOUTH BREEZE
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 130,000 MILES
On Friday when I first attempted to start my car it failed to function. After verifying the battery was both connected (fun story about that, btw) and charged I tried again to start my car. It worked, but only after holding the key in the start position. While starting the car, the odometer showed 'no buS', though showed the correct mileage after starting the car.

I haven't been able to learn much about this error through google; other than it being a loss of connection between the PCM and *something*. Is there anyway to verify what the PCM has lost connection to?

I should also note I've done the system checks (holding the odo reset while turning the key to on) which apparently would show a bus failure. It has 'passed' every time.
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Monday, June 6th, 2011 AT 2:30 AM

11 Replies

Tiny
CARADIODOC
  • EXPERT
That's referring to the data buss, a pair of wires that all of the computers send information back on forth on to each other. The 6 volts that appears on each wire originates in the Body Computer. Without the data buss working, the Engine Computer can run on its own but it can't share engine speed with the Transmission Computer, the Transmission Computer won't know engine load to calculate shift points, and things like that.

From what I've seen from other posts is there often is no problem associated with that message. In those cases I don't know what it takes to display it.
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Monday, June 6th, 2011 AT 3:41 AM
Tiny
ON3POINTKING
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Right, in that case, is this an issue where I should continue to use the car to gain diagnostic information?
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Monday, June 6th, 2011 AT 1:41 PM
Tiny
CARADIODOC
  • EXPERT
What was the symptom when trying to start the engine?
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Monday, June 6th, 2011 AT 5:33 PM
Tiny
ON3POINTKING
  • MEMBER
On the first attempt it failed to start. On the second it started after holding the ignition to start for a few seconds. This was Friday, I haven't attempted to start the vehicle since. I will try again in an hour when I'm home.
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Monday, June 6th, 2011 AT 10:16 PM
Tiny
ON3POINTKING
  • MEMBER
Update, the car seemed to start without hesitation.
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Monday, June 6th, 2011 AT 11:21 PM
Tiny
CARADIODOC
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The symptom I'm looking for is exactly what happened when you turned the ignition switch to "crank". A very common failure is you hear a single kind of loud clunk from under the hood but the starter doesn't spin the engine. If you release the switch and turn it to "crank" again, maybe four or five times, eventually it WILL crank the engine and it will start.

A totally different symptom would be the engine cranks fine but doesn't run. That would be the same symptom as if you had run out of gas. That could correlate with the "No Bus" message but it typically is not intermittent. The things that can cause that either fail completely and don't work at all, or they fail from getting hot from normal engine heat and cause stalling and a failure to restart until the engine has cooled down for about an hour; then the engine will start right up again.

In my first story, the failure to crank is caused by worn contacts inside the starter solenoid. They can be replaced for 20 bucks but most people just replace the entire starter. The problem will get progressively worse over many weeks or months and will take more and more attempts to get the engine started.

In the second story, the camshaft position sensor or the crankshaft position sensor fail, often by becoming heat-sensitive. The starter works fine and cranks the engine but the Engine Computer doesn't know when to fire the spark plugs or fuel injectors so the engine doesn't run. Most of the time the computer will set a diagnostic fault code in memory when it sees the lack of a signal from one of those sensors.

Those are the two most common symptoms that fall under the broad category of a "no-start" condition. From reading your descriptions, I'm inclined to think you have the first one with the starter problem but I would really like to be sure before I send you out for a new starter.
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Tuesday, June 7th, 2011 AT 12:04 AM
Tiny
ON3POINTKING
  • MEMBER
I don't recall hearing any thud, just silence. I replaced the starter about two years ago, is it possible it went bad already?
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Tuesday, June 7th, 2011 AT 12:09 AM
Tiny
CARADIODOC
  • EXPERT
It is a high-failure item, but two years is pushing it. You would hear the clunk if that was the problem. If you aren't hearing any noises, there are four things I can think of. Probably the most likely suspect would be the neutral safety switch. A clue would be it might start if you shift to neutral instead of park.

The next suspect is the ignition switch itself. They develop burned contacts and overheated terminals in the connector, although the circuit typically affected is not for starting; it's the one that feeds the heater fan and power windows. They cause more trouble for people who use the heater fan on its highest speed a lot.

The third thing is the starter relay under the hood. They don't fail real often but it's always a possibility. They can fail two ways. If the contacts are pitted or corroded, it will click lightly but not turn on the starter. That could be intermittent. If the coil of wire is broken, it will not even click. That will almost never be intermittent. That failure is permanent. You might not be able to hear the relay click. A better test is to put your finger on it and feel the click when a helper turns the ignition switch to "crank".

The last thing is a cracked cam on the end of the ignition switch key cylinder. That will prevent the switch from turning quite far enough to hit the starter contact. Everything else like dash lights and radio will still work fine. I've never seen one of those cause an intermittent problem but I suppose it is possible. The dealer has a repair kit for that problem.

A lot of diagnostics can be done right at the starter relay. First swap it with one of the other ones like it. The entire system can be split into four individual circuits corresponding to the four terminals in the socket. A real quick way to test two of them is to bypass the relay contacts. That can be done with a jumper wire when the relay is removed, or you can pop the cover off the relay, reinstall it that way, then squeeze the contact. That should make the starter crank the engine. You can also watch that contact closely to see if it clicks when a helper turns the ignition switch.
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Tuesday, June 7th, 2011 AT 2:32 AM
Tiny
ON3POINTKING
  • MEMBER
Hm. A few things I just thought of.

First, when the car failed to start, I (at least, if I recall correctly) the ODO didn't turn on or anything. When the car started on the second attempt, the ODO and other dash lights became illuminated as normal (except for the error message).

Second, the CEL light was illuminated before the incident. (It always seems to be on due to a 'large evap leak' which I haven't felt like fixing.) After, it was turned off. The clock was also at 12:00, appearing to be reset. I should mention that I play with the battery cable but power was flowing before.

So, that leads me to have a few questions:

1.) Is there a bus link between the ignition switch and the dash computer which would throw the error?

2.) Could the 'no bus' error be thrown generically after the power is cut?
If that is the case, that would make me look very closely at the starter.
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Tuesday, June 7th, 2011 AT 3:36 AM
Tiny
CARADIODOC
  • EXPERT
The clock does start out at 12:00 when you reconnect the battery. I don't know if that is related to the "bus" message. I never paid attention to that. I think that might not be a relevant clue. Even if the data buss / Body Computer had a problem, that would not prevent the starter from working.

The symptoms are a little confusing and seem to be related to different problems. If you don't fix the problem for the large leak in the fuel system, the Check Engine light will always be on and you will never know when a new problem is detected. There might be new diagnostic fault codes in memory that can point to the problem.
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Tuesday, June 7th, 2011 AT 4:40 AM
Tiny
ON3POINTKING
  • MEMBER
I will regard those symptoms as extraneous then. Time to look at some wires! Or it will be once it is light out.
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Tuesday, June 7th, 2011 AT 5:03 AM

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