Speedometer Still Fails

[spitfire]

i don't have anything like you do but it sounds like a ground problem i would look at all you body and frame grounds make sure they are clean and tight if you have had any elecrical work done like charging or battery or dash something might be loose just my 2 cents and you are right your labor rate is a lot less than theres i do have a 1994 frieghtliner fld120 with a n14 cummings and 10 speed in it good luck

[chasfm11]

Here is an update. I've been traveling on business and haven't had any time to research this matter further. I even had to cancel my appointment with the local Cummins shop. I've driven the coach one a week for the past several weeks since our last big trip (part of my regime to keep the tires from getting flat spots and moisture in them) and haven't seen a single speedo failure.

Given that the problem happens a lot more when things are cold, either from the outside temperature or with us blasting the A/C, I'm inclinded to believe that this is a connection problem and that it surely could be a poor ground connection, affected by temperature.

After this coming weekend, I should have some more time and am going to try to trace the wiring carefully. I still plan to have the engine re-flashed to try to cure the oil sensor problem per Cummins but I'm not as optimistic that that action will cure the speedo, too. I'm going to reseat all of the ground connections and verify them with a meter.

Thanks again for your continued help.

Charlie

[wcdiamonds]

I doubt if it help but on mt american coach there is a box under dash that electrically read input from the spedo and oil and a couple other guages everybody said check ground check this/that finally had to call A. coach and found there is some problem with that box,sent me to the box mfg.and they sent new one and a spare one to boot and it fixed it,it did act just like yours sometimes taking 6 hours to cut out...hope it gives you something to think on...

[chasfm11]

With 4 successful trips under our belt with no speedometer failures, I'm declaring victory! It's finally fixed!!!

The problem turned out to be a cable running to the Vehicle Data Controller (VDC) mounted in the passenger side rear compartment along with the Allison controller. The cable was stretched taunt and held that way by the foam sprayed on the back of the compartment. While removing and examining every connector, I discovered the cable and was able to work it within the foam to create more slack. After carefully examining and then spraying both sides of the cable with DeOxit, I reseated it in the VDC. Since then, the speedometer has worked without a hitch. Further, the intermittent problem with the oil pressure alarm shutting down the engine during hard braking has also seemed to disappear. The cable carries data between the VDC and the front of the vehicle so I'm not suprised by the results. I also now believe that the speedometer problem was a problem from the time that the coach was new and just didn't start failing when I bought it.

I cannot tell you how happy I am to have a reliable cruise control in addition to being able to use the speedometer to avoid traffic tickets in the small Texas towns that we drive through. I always knew how fast I was going in 5th gear (because of the tach) but had no clue at slower speeds. Score one of the weekend mechanics!

[Hooligan]

Chasfm--Very glad to hear you have resolved both the problems with your Landau, I know they have been an aggravation for a long time. Amazing that Freightliner or Oassis didn't solve that initially. I'll keep your vacumn pump solution in mind, my pump has failed to start twice since I've had the coach, just have to walk back and kick the side of the bed where the pump lives. Take care.....and enjoy.

[chasfm11]

Thanks for your reply, Hooligan. I agree that the Freightliner dealer should have fixed my problem the first time but definitely the 2nd time (when they billed my coach dealer for 4.5 hours labor)that they worked on it. I seriously doubt that the tech had the same amount of information that I've been able to gather from Allison and FCC in the past year about how the system works, however and that was really the key. Once I knew that there would have been other symptoms on the transmission if the failure was really between the transmission's speed sensor (with Freighliner replaced) and its controller, I knew that I would have to start looking further along in the data process. Armed with the complete circuit diagram for my coach, I just had to translate the diagram's components to physical locations in the wiring harness and then start examining them. It also helps that I've spent many years working on computer mainframes.

[chasfm11]

Another Update:

I lied. The speedometer started failing again with exactly the same symptoms and my work with the VDC turned out to be way off the mark. After exhaustive work with the wiring diagram and finally getting Freightliner to help me make the corrections to that diagram, we discovered that by the time the road speed data arrives as the VDC, it is already incorporated into the data buss.

I was able to use an ohmmeter to trace the output of the TCM (Allison controller) to the input to the ECM (Cummins controller) where the invalid road speed data was being detected and reported. I did a point to point trace of those wires and, at the end, was able to verify continutity through that loop. The speedometer has worked flawlessly since then without any sort of a hiccup when we hit bumps, particularly the bridge seams that had so badly afflicted it before.

The thing that makes the road speed data so vulnerable is the fact that it goes through a lot more connections than the other gauge data does. I'm reasonably certain that the bad connection point was on the input to the VIM (Vehicle Interface Module). For some unknown (to me) reason, the speedometer data is put in one side of the VIM and the wiring diagram shows nothing but a passthru to the other.

In the course of troubleshooting, I managed to snap one of the retention tangs on a VIM connector off. I'm ordering a replacement. When I do that replacement, I'll have to take the old connector apart and that may give me some insight about why the adjacent connector carrying the speedo signal was breaking contact. I had hoped to find physical evidence of why but it just isn't apparent. The failing connector appears no different than the other 3 wires in that same connector. There is always the possibility of a poor crimp of the connector end over the wire but I cannot physically see that without taking it apart.

The VIM is where the exhaust brake, cruise control and a couple of other relays are housed. I need to be careful about not causing myself additional headaches when messing with it. For now, however, everything including the speedometer are working.

Charlie

[Flints AR Mike and Lorrie]

I had a simular problem with my speedo, abs.etc.I called freightliner and they said to make sure that the data wire comming up to the dash area was not kinked.It is a gray triangle plug.I checked this & it was okay. One day I had the engine cover open just looking around for anything that could be the cause of this problem at the engine area. When I looked down & toward the front I found another gray triangle shaped plug. I unplugged this plug & straightened out the harness and replugged it in. I have not had any problems with it since and we have put on about 7,000 miles. I hope this helps you.

[summerwinds48]

Yuo might look for a buffer board, in a small white box under the dash or under the instrument cluster. It's about 2 x 2 inches and has two sets of plugs. It's located between the transmission sensor and the ECM computer. Find the numbers on the internal circuit board and try to find a source. Had this same problem on my 1994 Itasca, and replaced the $70 buffer from Delco. THe system worked fine after replacement. The speedometer shops wanted to start with replacing the instrument panel for big $$$ before I researched and found the bad speedometer buffer board.

[chasfm11]

Summerwinds48, your description of the "buffer board" is close to that of the VIM that I referenced. My VIM is slightly larger (roughly 5"x5"), has 4 plugs not 2 and is located in a compartment near the engine. It does, however, appear electrically between the transmission sensor (actually in my case the TCM) and the ECM. The VIM also controls my exhaust brake, cruise control and a couple of other functions. Because of the sensitivity to road bumps kicking my speedo on and off and the assumption that there is no actual function performed by the VIM on the road speed data (at least according to the chassis wiring diagram), I've long suspected a connection rather than an actual component. I agree that a bad solder joint on one of the electronic components within a box like the VIM might yield the same bump sensitive results.

The root problem here is that there is precious little information about how all of this circuitry works. I worked on large mainframe computer systems for years and we almost always had a "theory of operations" manual that described correct operation of each of the subsystems within each attached machine. It is no wonder that the truck shops that work on motor homes have little idea how to go about troubleshooting these sorts of problems. While I'm sure that there are some similarities between some truck subsystems and those found in MHs, I suspect areas like my speedo setup are quite different.

[chasfm11]

Update:
My speedometer has worked flawlessly since December. The cause was one of the two connectors on the VIM. Freightliner finally admitted that the wire used for this area was too rigid and that sometimes the connectors didn't make proper contact. It is interesting because, as I noted in a previous post, the exhaust brake and cruise control both have relays on the VIM which are controlled through the same types of wires.

I'm just glad that my problem is fixed.

[Jestme13]

CHASFM