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Old 08-05-2004, 12:39 PM   #1
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I just returned from a 3,000 mile trip during which our speedometer failed intermittently. I've had it worked on at an Oasis Freightliner center twice now ($1,000 total) and it didn't fail for about 60 days after the last service. They replaced the speed sensor and corrected some "wiring problems" but it is back to failing the same as before.

I drove a couple of days over 500 miles during the trip and had maybe one or two failures all day. Other days, it failed consistently for hours at a time. There seem to be two symptoms:
1. While driving along, the needle drops to zero and stays there. Sometimes after a bump, it will jump up to correct speed and stay for minutes or hours or it may drop to zero again.
2. After running several hours, the needle will start oscillating toward the high end "peg". It will run up and down several times in a minute and may then maintain correct speed for a hour before oscilatting again or dropping to zero.

Sometimes, when the needle drops to zero, the check engine light comes on. Always, the cruise control responds to the needle fluctuations, by trying to speed up in the oscillations or by shutting off on the drop to zero.

I now have the complete chassis wiring diagram from Freightliner. What I lack is any insight into how the speedo circuit is supposed to work when it is working correctly (i.e. Theory of Operation). The speed sensor has a two wire output that feeds the major transmission connector. I haven't traced it carefully yet but it appears to feed the ECM.

I'm electing not to have the speedo serviced again by the "professionals". I've decided that I can "not fix it" just as well as they can and my labor rate is a whole lot cheaper. Any help that anyone can offer would be greatly appreciated. I've used multi-meters and oscilloscopes extensively in my past career.
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Old 08-05-2004, 12:39 PM   #2
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I just returned from a 3,000 mile trip during which our speedometer failed intermittently. I've had it worked on at an Oasis Freightliner center twice now ($1,000 total) and it didn't fail for about 60 days after the last service. They replaced the speed sensor and corrected some "wiring problems" but it is back to failing the same as before.

I drove a couple of days over 500 miles during the trip and had maybe one or two failures all day. Other days, it failed consistently for hours at a time. There seem to be two symptoms:
1. While driving along, the needle drops to zero and stays there. Sometimes after a bump, it will jump up to correct speed and stay for minutes or hours or it may drop to zero again.
2. After running several hours, the needle will start oscillating toward the high end "peg". It will run up and down several times in a minute and may then maintain correct speed for a hour before oscilatting again or dropping to zero.

Sometimes, when the needle drops to zero, the check engine light comes on. Always, the cruise control responds to the needle fluctuations, by trying to speed up in the oscillations or by shutting off on the drop to zero.

I now have the complete chassis wiring diagram from Freightliner. What I lack is any insight into how the speedo circuit is supposed to work when it is working correctly (i.e. Theory of Operation). The speed sensor has a two wire output that feeds the major transmission connector. I haven't traced it carefully yet but it appears to feed the ECM.

I'm electing not to have the speedo serviced again by the "professionals". I've decided that I can "not fix it" just as well as they can and my labor rate is a whole lot cheaper. Any help that anyone can offer would be greatly appreciated. I've used multi-meters and oscilloscopes extensively in my past career.
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Old 08-05-2004, 12:43 PM   #3
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Sorry, should have included that this is a 2000 Georgie Boy 36' DP with an Allison 1000 transmission in a Freightliner XCS chassis.
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Old 08-06-2004, 02:51 AM   #4
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It sounds like you may have a loose ground and possibley at the dash area if none is found around the speed sensor. You can always add an extra ground and check out the results. The best ground is always back to the negative post of the battery.
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Old 08-06-2004, 08:24 AM   #5
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Thanks for your reply, Mike. I was considering a ground problem,too, and appreciate your insight. I've also had the oil pressure light come on twice since I owned the coach but didn't want to cloud the symptoms on the speedo problem with too many facts.

I have two large ground posts in the front compartment, just a few feet from the back of the console. I'm going to take your suggestion and run an additional ground wired directly to one of them from the back of the console. That is where the coach batteries ground to the chassis. We'll see how things go after than.
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Old 08-07-2004, 04:41 PM   #6
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I'm not familiar with your instrument board, but most are printed circuit type panels with pin type plugins. Sometimes these traces break near the onboard plugs. After breaking they sometimes will open and close with as little as a change in temperature. They very difficult to run down as the speedometer only operates when moving. Intermittent operation is usually do to a break or poor connection. It may be as easy as disconnecting any plugs in the circuit and reconnecting them. But if the plug in pins are corroded they can be very hard to pull apart causing damage to the board area that actually has no problems. I would be very careful in what you do as you can easily cause more problems.
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Old 08-14-2004, 11:11 AM   #7
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I recently ran in to a problem with my coach which started with the battery light coming on. Since I was on the road between Nowhere and Somewhere I decided to ignore it until I could get to a FL dealer. After a while the problem got worse, so much so that all of the gauges, including the speedo, quit. It turned out that it was something called an "exciter wire" on the alternator that was causing the problem.
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Old 08-14-2004, 04:07 PM   #8
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Thanks, Canuck. Sorry to hear about your problems and they sound a whole lot worse than mine. I've been out of town on business and have not even had a chance to follow up with the ground wire that I discussed with Mike. Maybe tomorrow.

For N8XV and any others interested, my console has individual gauges. The plug to the speedo comes right off of a wiring harness and into the back of the gauge.

I'll be sure to post the solution when (if?) I find it.

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Old 08-19-2004, 01:44 PM   #9
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Chasm- I've also got the 2000 Landau Dp. My speedometer failed several times, indications were a little different than you indicate. Only speedometer went to zero. Cruise control held 65 mph. No other systems failed. Speedo returned on next restart. Same symptoms each time it happened. Freightliner replaced speedometer and tach under warranty. Gauges were slightly different with larger windows for milage and trip info) Work order showed shorted speedo. Call FTL at Gaffney, see if they will back up the work you paid for.
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Old 08-20-2004, 07:49 PM   #10
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Wow, Thanks Holligan! I'm glad that I'm not alone. How long ago did you have your speedo replaced? My understanding is that my warranty is gone on the chassis and that I'm on my own but I'll give Gaffney a try.

Boy, the window for the odometer on mine sure is SMALL!!! I'd love to have something that I could read without tilting my head all the way back to use maximum bifocal. LOL.

Thanks again.

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Old 08-24-2004, 06:39 PM   #11
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Charles: I purchased my rig new in Feb 2001, the speedo problem was repaired in July 2001, while still under warranty. Glenn Hoops at FTL is very knowledgeable and may be able to point your local Oassis people in the right direction without running your cost up chasing the problem. There's a VDC Computer that runs all the chassis info, feeds the gauges and warning lights, you may have a loose or corroded connector between the dash and that. (Of course I don't know where it is) It's shown on sheet 3 of 5 on the Chassis schematic. Hope you can find the problem...
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Old 08-26-2004, 06:19 PM   #12
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Charles- Just found this entry on the Discovery Board. http://www.discoveryowners.com/forum...earchTerms=vdc

Apparently Cummins can re-flash your computer and is aware of the problem.
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Old 08-30-2004, 06:55 AM   #13
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Had a very similar problem with a Ford V-10. Had 5 failures and speed sensor replacements before it was determined that an erroneous voltage from the ABS was causing the problem.
You might have them look at that.
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Old 08-30-2004, 12:44 PM   #14
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Larry, thanks for your suggestion. I'm not sure how related an ABS voltage problem is on you chassis (Workhorse?) versus my Freightliner XCS but it is worth looking into.

Holligan, I've contacted Cummins directly based on your Discovery link and they do have what they called a "Thursday" bulletin dated 6/21/2001 that talks about a speedometer failure. That bulletin is related to a Spartan chassis, however and the representative from Cummins did not know how applicable it might be on Freightliner chassis.

I've had a couple of transient oil sensor light problem problems, too, and Cummins did say there is a potential re-flash solution for those problems. On the hope that the speedometer would be solved at the same time, I've made an appointment at a local Dallas Cummins shop for next week. I'll be sure to post my results. We have a trip planned for September 10th so I'll have a chance to test any fixes that are applied soon.
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