Thomas bus - all gauges don't work/read incorrectly only when engine is on

[dbsoundman]

Hi guys, here's yet another dash gauge related issue, but this one is a little different.


So when I first got my bus, all the dash gauges worked fine, EXCEPT the tachometer. When you put the key in and turned it to ON before starting, the gauges would all sweep and return to zero (except the tach), and the odometer would show 2_09, then would show the battery voltage. When you started the engine and got things running, again everything except the tach worked normally.


I took the bus to a mechanic to have the air dryer and main air line replaced, and in the process they found that my low air sensors that feed the low air alarm buzzer in the dash were bad, so they replaced those sensors (they were literally broken). However, now the gauges do something really interesting:

1. Turn key to ON, all gauges except tach sweep, odometer shows 2_09, everything goes back to zero and life is good.
2. Start engine, none of the gauges do anything. Every segment on the digital tach lights up, and the BRAKE and LOW AIR lights stay on (but no buzzers). I don't get a speedo, coolant, battery, fuel, oil pressure...nothing. BUT my BlueFire is still able to read those values just fine.
3. While driving, occasionally the warning lights on the gauges themselves will light up and I'll hear the buzzer, but the gauges never read an actual value, and then everything goes dark again.

To me, this doesn't sound like a grounding issue because I don't know why a lack of ground would make the gauges work when the engine is off, but maybe there's something obvious about this particular situation that might help provide a tip to me. I'd appreciate your thoughts!

[dbsoundman]

Just noticed something else interesting...the backlight for the speedometer and the fuel gauge won't turn off! This is with the key out/off, engine off...for several hours already.

[Shadowrunning]

I just bought a bus with this same issue did you ever find your issue?

[nikitis]

Gauge clusters are vulnerable to cracks in the solder joints. No needle reset while the key is in the ON position generally means either the lines where the harness plugs in are cracked or broken or the gauge is bad but in both circumstances it is not getting power. It could be the main ground line if other gauges are having issues, but id take the gauge cluster out and do a continuity test from all of the test points from the back side to another trace like to rule out micro cracks in the joints.

I have a failed speedo issue but I know its not my gauge or cluster because continuity are good its not getting a signal in my case. I have a bad VPM. Your bus likely only has an ecm, but the values aren't getting sent to the gauge in my case but my needle does the reset when key is ON so that means it has proper power getting to the gauge just not data. No needle movement reset is an issue of bad gauge or no power to the gauge. Power also cannot run if not completing the circuit with a ground. One of those 3 things I mentioned is your problem.

First verify power gets to the gauge, and is grounded, if so replace the gauge.

[dbsoundman]

Originally Posted by Shadowrunning

I just bought a bus with this same issue did you ever find your issue?

I think you posted about this on one of the Facebook groups too, unless there's two people with the same problem. I forgot to update this thread, but yes I did fix the issue. First, I found a complete gauge cluster of the same kind on eBay, it was a couple hundred bucks but now I have spare gauges so...yay lol. When you look at the back of the cluster, you'll see that the gauges daisy-chain, it's a digital system rather than analog. I left the key "on" and unplugged one gauge at a time until the gauges went back to normal, then I bypassed the suspected bad gauge and everything still worked. Swapped in the matching gauge (coolant temp) from the eBay cluster and put everything back together and it's worked ever since.