Need Help Troubleshooting Multiplex Wiring

[Lostranger]

Our '99 Gillig has frustrating issues with the gauges, including the speedo. Sometimes one or more displays a reading, sometimes not. When they're working, the displayed values change at random. They read differently when the lights are on than when they're off. Fuel and temperature gauges can be all over the scale.

Even if I wanted to take it in — which I don't — no one in this part of the world is interested in the job. I have all the factory manuals including service and wiring. My basic problem is that I don't understand multiplexing. I know that the computer is able to read multiple signals along the same conductor, but then what? Does the computer then send a separate signal to each gauge along dedicated wiring, or are the gauges able to read multiple signals? What is an indication of trouble on a specific circuit?

Any help would be greatly appreciated.

Jim in NC

P.S. And don't ask how I know that a random reading fuel gauge can be inconvenient.

[Jon]

Multiplex is a system in which a master module communicates with other modules via a single pair of communications wires.

The various modules have specific functions such as lights, gauges, or any other electric functions, maybe including the engine. The modules replace a lot of components because internally they handle the functions of relays, switches and circuit breakers. When you turn on the lights for example the module sees you have switched the lights on and internally it closes the circuit to light the lights. But it is also looking for feedback, so if it sees a higher current draw it may open the circuit functioning as a circuit breaker, or if it sees less current draw it might send a message to the master module to create a code to tell you a light may be out.

Multiplex systems are fantastic due to their simplicity because with the wiring diagram it is usually very easy to trouble shoot problems. Additionally if a module quits, just pop it out, put in one not used or underutilized and the master module "teaches" the replacement what its job is at the new address. Most faults will display a code to isolate problems to a specific area.

A true multiplex system is a dream to operate. A driver can command a light check and as he walks around the coach every light will flash in sequence so he can see each operate, including turn signals, brake lights and high and low beams. If a driver wants to do an AC system check he can initiate it and as he walks around the coach he can hear, in order, various valves opening and closing, fans engaging, unloader valves functioning, solenoid valves operating and various relays clicking. Hard to imagine a true multiplex in a 99 coach however.

[boogiethecat]

It's exactly like your ethernet wiring for your personal computer.... the bus's main computer sends ALL data down the line in the form of small data packets that are tagged for individual instruments.
If you were to look at a few  millisecond's worth of data coming down the line, and be able to metaphorically make it visible and lay it out on the driveway, you'd see long string of data containing, for example, a packet with speedometer info followed by a packet containing engine temperature followed by a packet containing data that says "open the gas cap door" etc.
The process is bidirectional- any of these instruments can also insert and send packets of their own that can be read by either other instruments or the main computer or both.  Kinda like 15 people all on the same telephone line at once, but with stringent rules to make sure they all don't talk at the same time.
So what happens in a vehicle is they simply string one set of data wires all around the vehicle, and each instrument hooks to it and waits for a data packet with it's name on it, and acts only on data packets with it's name on it.

In your case, my guess is there is a grounding issue somewhere that is making the data bus extremely noisy, which then makes the data corrupted, and various gauges will then mis-interpret data that is not aimed at them, and use it anyway.  That would result in what you see- random working properly and not properly.  A hint is that i changes when the lights are on or off- that usually signals that a ground that is common to both the lights and the computer system is intermittent and when the lights change state, voltages on the computer line that should be at ground change, messing up the data.

From there though, I can only say "good luck"... finding a crappy ground can be crazy.  One way to do it is use a very sensitive DC voltmeter that can read 12 volts down to the millivolt range, ie a 4 digit DVM that can give you a reading that looks like 12.7530 volts.... you put one of the meter's probes at the battery's ground post and use the other probe to "follow" a ground path... for example turn the headlights on to create a power flow, and starting at the battery with your "following" probe, follow the path that ground takes to the headlight.  as you move farther away from the battery you'll see a gradual increase in the millivolt reading ( as a normal result of resistance)  but say you encounter a ground stud somewhere in that path and the voltage suddenly rises half a volt- that stud is probably dirty and creating a major resistance... and anything else connected to it will experience a bad ground.  Using this method I've chased down a lot of ground issues... but it can be awesomely difficult especially when the chassis of the vehicle is used for ground.  Bit if you "get" this process and understand what is happening, this process can effectively "map out" a vehicle's grounding system and what it's doing, very accurately.

Hope I made this clear, it's a simple process but grokking it may not be so simple for some...

CHeers
Boogie

[bevans6]

Best thing to do first, stupid as it sounds, is undo all the connectors and do them back up again, maybe with an electrical cleaner spray.  Then clean and refasten any and all ground wires that you can find behind the dash.  As noted the computer is trying to send data to the instrument displays but it is getting corrupted.  I had a Kia Sorrento, the transmission went all stupid.  The fix was the tech that the service manager was mad at had to pull the entire passenger compartment wiring harness out to do exactly that - unplug and replug every single connector.  They didn't have a clue which one was the "bad" one.  That was when I really got a clue that modern cars are a total crap shoot, if they run it's a minor miracle.

Brian

[eagle19952]

actually it's a major miracle.
So many electrical devices that now rely on automotive computers are not able to deal with corrosion....and that is everywhere and as it progresses the resistances change.
And the computer probably is not equipped deal with that. It can read them, but it can't fix them.
When I worked in the Aleutians we took and coated every opening in every wiring harness and coated it with a nasty sticky sealing goop.
No fun but it definitely reduced maintenance and down time.
I love my 1995 Chevy Tahoe... 

[Lostranger]

Thanks so much for this most helpful info. I already suspected a faulty ground, and checking for that, along with pulling/spraying connectors, will be my first line of attack. I'm actually good at troubleshooting electrics, but my time is in such demand lately that I was hoping to get a leg up on this process. Sounds like I have done just that. I particularly liked the data packet explanation.

Jon, your post was extremely helpful. Right up to the moment your imagination failed. It's amazing how often we assume that if we are not familiar with it, it does not exist. My H2000LF is a thoroughly modern bus. Gillig was one of the pioneers in multiplexing buses.

Best regards to all who have posted and anyone else who cares to do the same.

Jim in NC

[Jon]

My hat is off to Gillig. It is a technology that enables so many neat things apart from making a significant reduction in the amount of wire, relays, CBs, etc. it has the potential to make a lot of problems go away. But with it we have to learn how to cope with its own problems.

The days of homebuilders getting creative with their coach wiring systems is gone with multiplexed systems unless they can get their computer to update the software to encompass their changes.

[Lostranger]

My hat is off to Gillig. It is a technology that enables so many neat things.... The days of homebuilders getting creative with their coach wiring systems is gone with multiplexed systems unless they can get their computer to update the software to encompass their changes.

Jon, I'm only beginning to realize the wisdom and scope of what you said here. We first looked at this bus in November of 2012. The PO is a techie who bought the bus a year earlier when Hertz retired it from airport shuttle service. He was justifiably proud of all the features and systems, so he gave us an extensive tour. I confess that when he got to his overview of the multiplexer — all my vehicles are old, and I was not even aware of the concept — my overwhelming response was fear of the unknown. Thanks to knowledgable folks like you, I'm realizing that I'm thankful for this new technology. I just need to get better acquainted.

Do I need some sort of code reader in addition to the built-in displays? I've not seen anything in the manuals about using a code reader, but it may be there. If I need one, what should I get?

Thanks again,

Jim in NC

[Hard Headed Ken]

Jim,
Do you think your dash could be getting it's information from the 1708 or 1939 communication lines directly from the ECM? It's only 2 wires front to rear may not be too hard to troubleshoot. Here's a plan B if you have an engine diagnostic port up front. Some folks are the using the Silverleaf VMSpc and computer screen for all their instruments.

http://www.silverleafelectronics.com/node/6

Ken

[luvrbus]

The software was available for use on a laptop from Gillig (700 dollars)you need it to even add a extra light or gauge to the bus

[Lostranger]

Whoa! This Silverleaf thing looks like a must have. As usual, I resent its "windoze only" status, since I'm a dedicated Mac man. So happens that I recently retired an Intel MacBook. It needs a new inverter cable, but after that, I believe I'll reload it with some version of windoze and get the Silverleaf stuff. Should make my life easier in several ways.

Thanks for that tip.

Jim in NC

[luvrbus]

Martin at Sliver Leaf makes a great product but it is not going to read the chassis computer I was told the Gillig used a International Truck chassis computer fwiw 

[CrabbyMilton]

I just keep my cell phone on silent at all times. I check to see if there are any messages when I come to a stop. That way there is no compulsion to pick it up when it rings. Most people know I'm on the road most of the day anyway and they know I prefer email anyway. I also keep my personal cell phone on silent at all times as well. That way, if it's anumber I don't recognize, they had no reason to call in the first place. If it's important, they can leave message. I tell people that 911 is for emergencies so it won't help to call me.

[CrabbyMilton]

Sorry guys. Wrong topic.

[Lostranger]

Martin at Sliver Leaf makes a great product but it is not going to read the chassis computer I was told the Gillig used a International Truck chassis computer fwiw 

So.... Now I'm confused. Are you saying that the Silverleaf software only reads the instrument cluster signals? If that's the case, I won't bother with it. I'd rather have the original instruments working.

Jim in NC