Help with Troubleshooting Problem with Chevy SUT

ErVikingo · 12-01-2011, 04:21 PM

My 03 Z66 with 70k miles from new.

All systems work, no passlock light but will not start (not even a click from the solenoid so I figure power not getting there). Checked, shifter to make sure it was in P or N, battery, fuel pump relays, starter, .... Found "Ignition E" fuse blown.

When replaced (with the same amp rating - 10a) it blows it when you try to start the car. If you put in a 20a, the car will start. I immediately replaced with the proper amperage (10a) and the car worked fine after that (for a couple of weeks).

3 times already, it will start but all idiot lights on the gauges stay light up and the gauges are otherwise dead. Today, car started but had no turn signals, a/c compressor, the tach needle was bouncing between 1500 and 3500. Got home, parked it and will not start again. Checked Ignition E fuse and its blown again.

Last time I took it to the dealer who could not find anything (I had "cycled" the 20a then 10a fuse) and there was no code. They do not think its the ignition switch. I was actually told that they just don't know.

FYI, cluster changed upon recall. All else stock except for an Ipod connection.

On that subject, the Radio shows no CD's when the changer has 6 discs inside. I cannot get CD's out.

Ideas (not looking for the obvious light a match...)

kaisen · 12-01-2011, 05:40 PM

Believe it or not, I've heard of faulty instrument panel circuit boards blowing that particular fuse on Silverado/Tahoe/Sub/Avalanche. Do you have a bad/weird/intermittent gauge?

There is so much running through that circuit that it could be lots of things

kaisen · 12-01-2011, 05:50 PM

Edit: saw you had the IC replaced already

It's possible that there is damaged wiring from that, causing increased resistance and therefore heat.... check into it. You might find a melted wire under the dash near the steering column, above the brake pedal

john70t · 07:20 PM

Not knowing the system, here is my layman thinking:
-Offhand, the 10A blown fuse indicates too many amps(short to ground) somewhere close. Perhaps within the dash electronics or start/ignition circuit.
-But the tach/turn signals/etc. indicates backfeeding through the wrong circuit because of missing ground. It may have fried the radio already.

I'm assuming the main chassis ground is o.k., but it never hurt nothing to have too many grounds. An extra jumper cable or two can serve when checking.

When those 20-40K ignition volts try to get back to it's battery source, they will try and take any path available. Dead spark plug/wires may also be suspect and backfeed through the coil.

Can you pull every fuse(dash/radio/turns/etc) except the ignition and make it run good? If so, the problem will be in an accessory circuit.

ErVikingo · 12-02-2011, 04:27 AM

Many thanks for the feedback Eric+John!!!!

Weirdest thing is that it never happened until my wife drove my truck and hit the seat memory button (that is the only thing she did different than me). Tech's solution "don't let your wife drive the car...". Ja ja

But, after that, it has happened 3 times.

I cycled through again last night (put in 20a, started, shut down and installed new 10a). Started, ran and all systems perfect on my drive to work. I restarted when I got to the office with no drama.

BTW, the starter was checked for load, etc. Must trace grounds and will get under the dash.

FYI, according to GM (I spent about 2 hours on the telephone with them) it might be related to the "passlock" system (inmobilizer) but they had me do all sorts of tests and all tested ok.

Any other suggestions please send them my way.

ErVikingo · 12-02-2011, 04:27 AM

I'll scan the schematics and will upload them for your reference.

ErVikingo · 12-02-2011, 04:52 AM

john70t · 12-02-2011, 07:27 AM

I'm guessing here...but AFAIK the electronics run on 9V, not 12V. I remember hearing that was why a battery test under cranking was so important(if it drops below 9V, the engine may still turn turn but the Igition Module won't work).

-The blowing starter fuse(120+watts consumed) indicates something big shorting in the cranking circuit through the key.

-The intermittent-ness of the problem indicates something moving(starter/relay/wire connection) that connects only part of the time.
(but it still could be electronic)

-The bouncing Tach indicates pulsating voltage backfeeding(bad plug/wires ground to eng/batt) through the guages.

Tried to simplify it. The red arrows could be 12V leakage into another circuit:

My best guess.

ErVikingo · 12-02-2011, 07:36 AM

Thanks John!

The Crank fuse does not blow but the Ignition E (top left on the diagram) is the one that blows.

I'll try to trace the condition of the plug wires.

MANY thanks!

john70t · 08:21 AM

(Blue arrows may be pointing wrong way. I thought key was put at end of circuit.)

If it were me, I'd do plugs/wires/ground and go from there.
Good luck, and get a nice alphredo sauce for that underdash spaghetti.

(edit: and start relay, occilations through coil, but I tend to throw parts at a problem)
(squirt of WD-40/electronic cleaner in the key fob?)

kaisen · 12-02-2011, 08:00 AM

The LS V8s (Avalanche, et al) are coil-fired "coil near plug" so the plug wires are just a few inches long (from the valve cover to the exhaust manifold). I doubt that's the culprit.

kaisen · 12-02-2011, 08:01 AM

My next best guess, then, is the IGNITION SWITCH.....
Not the key/tumbler, but the microswitch back that routes everything

ErVikingo · 12-02-2011, 08:17 AM

Eric,

Reading around the forums I see that being mentioned as causing a lot of problems. Apparently they are sensitive to minor corrosion.

Thanks MUCH!

ErVikingo · 12-02-2011, 08:17 AM

Quote:

Originally Posted by john70t

(Blue arrows may be pointing wrong way. I thought key was put at end of circuit.)

If it were me, I'd do plugs/wires/ground and go from there.
Good luck, and get a nice alphredo sauce for that underdash spaghetti.

It is so much easier to deal with our simple 911's...............

rick-l · 12-02-2011, 08:51 PM

This is going to be expensive.

Note in your blue line above that a short past the relay will only result in the starter activating, the PCM provides a ground to turn on the starter relay.

I cannot think of any electrical fault that would blow a 10 amp fuse yet a 20 amp fuse would clear it. Could it be something like moisture getting in some where it shouldn't.

To isolate something like this you usually isolate the individual paths and see which one draws excessive current. Here that will be hard, especially with the cluster which sounds like it was handled recently.

Maybe you could put the 20 amp fuse in and it will burn to the ground.

ErVikingo · 12-02-2011, 09:17 PM

Quote:

Originally Posted by rick-l

maybe you could put the 20 amp fuse in and it will burn to the ground.

:d

kaisen · 12-03-2011, 06:35 AM

Quote:

Originally Posted by rick-l

This is going to be expensive.

Maybe you could put the 20 amp fuse in and it will burn to the ground.

Rick hates GM, btw

boosted 89 · boosted 89's Garage

The problem may be under your underhood fuse block. Rodents like to climb inside under the fuse block and chew the wires. You may have the ignition E wire shorting out. I've seen this many times, but anything is possible. Also check the ground wire that runs behind the engine on the left bank cylinder head.Its not a large ground but a 14 gauge(I think) black wire that runs in the wire harness on the top of the intake that runs towards the back the engine.
You could also remove the A/C relay to help eliminate a possible feed back coming from the A/C compressor. In a lot of cases there is a diode to the compressor that can fault.
Any circuit that you can eliminate by either unplugging the unit or removing a fuse/relay will help pinpoint a fault. If I think of anything else I'll let you know. Unfortunetly this is what I do daily at work.....drivablilty and electrical diagnosis at a GM dealer

boosted 89 · boosted 89's Garage

Sorry forgot one thing. Also check the safety neutral switch at the transmission. they have a tendancey to melt the connector rendering it almost impossible to remove. If the connections
at the switch are burnt then it could lead to high resistance in the circuit causing a 10 amp fuse to blow but not enought to blow a 20 amp fuse. Unless I'm thinking of the older GMs but its worth the look. I have also had BCMs cause numerous issues seen as its used as a gateway for a lot of modules..ie IPC, Radio.Unless there is something that is causing the data line to fault but the radio/cd issue sounds like an internal radio fault and not a power fault, and the IPC is faulting because of the fuse being open.

john70t · 12-03-2011, 07:55 AM

Quote:

Originally Posted by rick-l

Maybe you could put the 20 amp fuse in and it will burn to the ground.

Story: Back in high school, I always wanted a VW bus. FriendA had one rotting in his backyard for cheap, but was too busy getting high and chasing tail to get it running.
-FriendA was an excellent theoretical mechanic, but just plain lazy.

I changed the oil/filter and bought a battery.
When attaching the ground wire there was a big spark, indicating a massive short.

FriendB said "hook it up and we'll just replace the bad wire"
-FriendB could fix any machine on this planet using vise grips and duct tape, but was reckless in life.

I attached the battery, and the dash promptly caught on fire. By the time we got back with an extinguisher most of the interior was gone.