fire truck needs help

[mobilevet]

Hello, Our volunteer fire department has a "brush" truck that is having problems. The truck is a 2004 GMC with the Dmax (not sure if it is the first of the 04 type or the later type). It's a 3500 4x4 dually with dual fuel tanks, 91k miles.

We bought the truck stock and drove it back a couple of hours drive to the station before sending it up to company that installs fire rigging for the truck. They installed lights, push bar & winch, radio, siren, a tank on the existing flat bed, a gas-powered electric start pump for the tank, etc.

The truck was working fine when we sent it up to them and then after getting it back we started having these problems:

The fuel gauge does not work. Sits on E all the time.
It does not appear to use fuel from the rear tank
You can be driving down the road and push the accelerator to give it fuel and instead it will begin slowing down as if it is going to go dead, but if you let up it will "come back" to some degree.

We have replaced the fuel filter a couple of times and one guy said that it seemed that if he went and pushed on the filter primer that it ran better afterward.?? Really neither made much difference.

Problem seems electrical to me, especially given that when the fire company did all the installs they had to run tons of wires everywhere and also re-routed the front fuel tank's fill line. We have since put it back in the original position (don't know why they moved it?) but that doesn't seem to have made any improvement except now you can fill the tank a lot quicker and easier.

Anyone have any ideas of where to start in tracking down this problem???

As you can imagine, we can't afford to have any of our trucks lay down on the job.

Thanks

[rapidoxidationman]

Anyone have any ideas of where to start in tracking down this problem???
Thanks

My first inclination would be to have the guys who may have screwed things up in the first place get first crack at fixing it... They'd probably like that too.

"If you like my work, tell everyone. If I screwed something up, tell me."

[mobilevet]

I'd like that, but they are not "mechanics" and I doubt they even know what they did - and, I believe we've been in contact with them and they didn't seem to interested.

[DADGLW]

was there any welding done on the trk? i have had problems with some when i didn't disconnect the bat.............

[mobilevet]

You know, I just thought of this... when they did this install they also put in a master kill switch (like on the big trucks).. would it matter if this master kill also killed everything else on the truck (like the main computer)? I'll have to go and see if the truck will start with the master switch off.. the big trucks won't but... Also, they did not install an additional battery so the two stock batteries are having to run all the new stuff also. I wonder if they could just not be able to handle the amount of voltage needed by everything? Any ideas??

and, yes, I do believe that there was some welding done.

[DmaxMaverick]

You know, I just thought of this... when they did this install they also put in a master kill switch (like on the big trucks).. would it matter if this master kill also killed everything else on the truck (like the main computer)? I'll have to go and see if the truck will start with the master switch off.. the big trucks won't but... Also, they did not install an additional battery so the two stock batteries are having to run all the new stuff also. I wonder if they could just not be able to handle the amount of voltage needed by everything? Any ideas??

and, yes, I do believe that there was some welding done.


It sounds like you have some wild grounding going on. If the truck was welded, it could have lost some dedicated grounds, which can cause much of what you describe. Also, the new additional load would do the same thing, if the grounds weren't matched to the new power wiring. In OEM trim, not much is left for additional equipment, especially if they are power hungry. If the "murphy" switch (master kill switch) was installed incorrectly, it may have something to do with it, as well. The addition of longer wiring on the same circuit increases the line impedance, which plays hell with the computer, which expects to see specific voltages under specific conditions. Power and ground wire size is marginal, before anyone messes with it. It doesn't take much to overload a circuit if it isn't done correctly.

[mobilevet]

thanks...

our plan is to check the fuel sending unit connection and see if there are any problems there and then check the master switch to see exactly what is wired to it, along with checking grounds, etc.... and then possibly (if still needed) un-wire all the new stuff to see if that makes any difference..

If so then we might put a dedicated battery somewhere back on the bed (wired to the alternator) to just run all the add-on equipment... the #3 battery will be wired to the master-kill

what are your thoughts an additional alternator for the #3 or maybe a higher out put for all three?

[rapidoxidationman]

make sure to install a battery isolator, like what is found on an RV. That might save you the agony of dead main batteries if the "extra" stuff sucks its battery dry.

Maybe also a high idle switch in the cab, for those extended times on the scene when the christmas tree lights are spinning for hours on end?

[mobilevet]

make sure to install a battery isolator, like what is found on an RV. That might save you the agony of dead main batteries if the "extra" stuff sucks its battery dry.

Maybe also a high idle switch in the cab, for those extended times on the scene when the christmas tree lights are spinning for hours on end?

Both good ideas... we usually leave the rigs running when on the scene but it would not take long to suck a battery dry running those lights, maybe even with the truck running.

[Mark Rinker]

Your truck sounds like a good candidate for a dual alternator setup. Was it ordered that way?

[mobilevet]

well, I finally have some new info that I hope will help us get to the bottom on this issue.

some of us guys finally got time to tinker with the truck a bit. we looked over the wiring of the new add-on's and really didn't see anything that looked obviously wrong. There is a main kill switch that kills most of the add-on equipment - except maybe the radio - so it was pretty easy to turn it all off, which really did nothing for the problem.

However, I do have two new peices of information; one might not be much help but the other I think will:

1) when all this started happening we took it to the local chevy dealer. they said that it was a fuel pump issue with the rear tank and they "disconnected" it and told us to just put fuel in the front tank which we did but it didn't make the problem go away. as far as I know we haven't done anything to "un-disconnect" anything that they did. also, the fuel gauge was not working when we took it to them and it was still not working when it came back

2) SO, one of our guys got the bright idea to simply disconnect the batteries. he left them disconnected for a few minuets and the simply reconnected them and drove the truck. when he started it the fuel gauge was working and he drove it for 30-40 minutes at all speeds with no problems. when he parked it the everything seemed to be working fine. then a day or two later another guy drove it and it didn't go 1/2 a mile before it started acting up again (no power)... don't know if the fuel gauge was working when he started it or not.

So - what do you all think? Seems obviously electrical to me.. but what do I know?

THANKS

DON'T FORGET... THIS IS ABOUT AN 03 DMAX - NOT THE TRUCK IN MY SIG

[ToddMeister]

The fuel gauge issue could be a problem in the dash cluster stepper motor for the fuel gauge. Those stepper motors have been a problem in the mid 2000 year trucks.

[Mark Rinker]

Sounds like the answer is easy...only let the first guy run that truck. :D

[mobilevet]

Well, I don't know if anyone will remember this problem from so long ago (or care), but I thought I would post the answer in case someone else might search it later....

... turns out the problem was a kinked fuel line.

Apparently when the new water tank and pump were installed on the flat bed, the load of the water over-whelmed the stock springs. We had noticed that the bed was hitting a brace if you hit a bump so some of the guys "modified" the brace (above the wheels) to "solve" that problem. I guess the movement of the bed dropping so far down also kinked the fuel lines. Install of some flexible lines has resolved that problem.

So, for the most part it seems that most of the problem is solved. I say most because we have still had an intermittent issue (not sure of all the details) that might still indicate that all the extra electrical equipment (lights, siren, etc) is pulling too much load and draining the batteries causing issues.

(after going to a call and letting the truck idle with the lights on for a long time there was an issue when returning to the station)

thanks to everyone to gave their 2 cents

[BigRabbitMan]

I am in the slow process of converting my motor home to a Durmax and the first thing I did was add the second alternator due to possible electrical loads. I will have one alternator running the automotive side (loads should be similar to stock) and the other alternator handling the house side. I would recommend that you do the same. The two systems will not be connected so one cannot affect the other. I will have a manual crossover as backup.

[Kennedy]

The first step to diagnosing any fuel supply related problem is covered here:

http://www.thedieselpage.com/reviews/jkfuelpressure.htm

This little number can save a lot of time, money, and aggravation yet is often passed on in favor of changing parts like filters.

[Yukon6.2]

Nice Gauge:cool:
That would be one of the first tools to buy if you had a Dmax.