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Truck Trend Garage: Expert Advice March/ April Edition

Questions and answers from the Truck Trend Garage!

Alex Steele
Mar 11, 2008
Getting My Wires Crossed
Q: I have a 2006 GMC Sierra SLE. While plugging in the trailer lights, I crossed the plug and blew the fuse (the white and brown wires). I have searched my owner's manual to find the location of this fuse without results. It's labeled in such a way that I can't understand which one of the fuse boxes or fuses control the trailer taillights. Where can I find this fuse?
Photo 2/4   |   expert Advice rear View
A: I take it you crossed the wires coming from the trailer to the seven-way connector at the truck's hitch assembly, and you've since wired the trailer harness correctly. The white wire is a ground, simple enough. The brown wire supplies 12 volts to the trailer parking lights. As you know, connecting power to a ground without a load (light bulb, motor, etc) in between results in a blown fuse, or a fire, one of the two. Open the hood. The underhood fuse block should be mounted on the left side, on top of the wheelwell. Remove the fuse-block cover. There should be a label inside the cover designating the names and locations of all the components in the block. Look for the fuse marked "TRL PARK" on the left side of the block. My service information shows a discrepancy between the wiring diagram and the fuse-block label. One shows TRL PARK being a 10-amp fuse, and the other a 15 amp, so it could be either one. Pull it out; if it looks burned, replace it with the same amperage fuse. In the future you may want to invest in a 12-volt test light. It's a good way to find a burned-out fuse quickly, even if you're not sure where to look. Just turn on all the applicable circuits--in this case the "Park" lights. Attach the alligator-clip end of the test light to a good ground (anything metallic that's bolted to the chassis). Then touch the pointy end of the test light to each side of every fuse you can find. If the fuse is good, both sides will light up the test light. If the fuse is bad, only one side will light it, because the burned-through element within the fuse is cutting off the power source.
Photo 3/4   |   expert Advice front View
A Door that's not a Jar
Q: With all the doors securely closed and even locked manually on my 2002 Expedition, the "door ajar" indicator still appears. I was wondering if I could disable this: I currently have to pull the fuse out every time I leave the truck, or the runningboard lights and the overhead light will stay on and drain my battery. Also, normally after reaching 15 mph the lights shut off, yet once I turn the truck off they come back on and they stay on.
A: Not as simple a circuit as in the old days, when almost every manufacturer used pushbutton door-ajar switches mounted in the A- and B-pillars. The switches in those circuits simply provided ground paths for the interior lights and the door-ajar indicator lamp in the dash, if equipped. Your Expedition has ajar switches mounted in all four doors, plus the rear hatch and the rear hatch glass. These switches still provide a ground path, but not to the lights. Instead, they're wired to what we call a generic control module. The GCM observes the input data from the door-ajar switches and then decides whether to turn on the door-ajar indicator, the courtesy lights, and/or illuminate the runningboards. But these switches are subject to the wear and tear from opening and closing the doors. Therefore they do commonly fail, and replacing some of these switches will require a teardown of the door in question. With the ignition on and everything closed, run around and push, pull, and jiggle every door, hatch, and glass while keeping a close eye on the door-ajar indicator lamp. If you see it go off or flicker, while disturbing one door in particular, the ajar switch at that location is most likely the problem. Otherwise it's going to require a technician to track down the cause, beginning with the evaluation of inputs and outputs at the GCM. And I wouldn't suggest bypassing anything; it may produce problems elsewhere.
Photo 4/4   |   expert Advice rear View
Where's First Gear?
Q: I recently purchased a 2007 2WD extended-cab Z71 Silverado with the 4.8-liter V-8. Why didn't anyone ever mention that the truck limits first-gear activity? From a dead stop, I bet I'd lose a 60-foot race to a lawnmower--when you floor the truck from a stop, it either bogs or the computer limits the power so much that you barely move. I don't have traction control so I find this odd. I took it to the dealer and he said it drove like any other 4.8-liter Silverado. Is this a program just for these new trucks? My dad has a 2007 Silverado Classic with the 4.3-liter V-6 automatic and it gets up and goes, even leaving an occasional black mark.
A: Your truck's low-end performance may be as good as it gets. To make sure, the next time you put the pedal to the floor, first put the transmission in manual first gear. See if you notice any advantage as compared with starting in Drive. Then wind it out (pre-redline) and upshift manually to second gear. If it bangs into second as it should, the transmission is functioning correctly. If it doesn't, the transmission may be starting in second gear as opposed to first, resulting in sluglike acceleration. A bad torque converter can produce similar symptoms, but that happens more often on high-mileage or abused transmissions. If there were a significant performance issue with the engine itself, chances are the "service engine soon" indicator would be lit, and the boys at the service department would have noticed it. Here are a few factors that may explain performance variations between your new Silverado and Dad's Classic. It depends on the exact model and options on each truck, but in general, the new Silverado is a little heavier. The 4.8-liter puts out 100 horsepower more than the 4.3, but only offers 45 more pound-feet crucial low-end torque. Also, Dad may have gotten the optional 3.73:1 axle ratio, while your truck has the standard 3.23:1 highway gears. Plus mounting taller tires on any vehicle will lessen your jump from a start. Worst case, there are aftermarket intake, exhaust, and electronic calibration products that can improve your standings.
Surging Ford Tranny
Q: I have a 1995 Ford Explorer that won't shift correctly. When you take off from a stop you have to let off the gas to get it to shift. Once it upshifts, it's fine. Does this sound like the tranny will need to be rebuilt, or could it be a throttle or speed sensor? Also, the overdrive isn't working, and sometimes the vehicle seems to surge. It doesn't quite feel like the tranny is slipping--more like it can't decide how fast it wants to go.
A: I was ready to lay wages that "other symptom" #3 was going to be the "O/D Off" lamp on the instrument panel flashing on and off. Either way the situation warrants a qualified technician--preferably a transmission specialist--as opposed to taking a stab at it yourself. The first item on his list will be to plug in a scan tool and check for transmission trouble codes. The O/D Off lamp flashing indicates a significant problem and a diagnostic trouble code has been stored in the transmission control system's memory. The surging issue may be an unrelated symptom; it could also be the transmission going in and out of gear or the lockup torque converter turning on and off. The late upshift and lack of overdrive may be caused by a bum external electronic component or sensor, but it could be an internal mechanical failure that's going to cost you big bucks. See why it would be best to go to a technician first? Ford did have an issue with a late upshift and no fourth gear, caused by a damaged intermediate and/or overdrive servo piston seal. Depending on the mileage, that may warrant a complete overhaul.
Circling the (Battery) Drain
Q: I bought a 2007 Silverado extended-cab LTZ 4x4 with the Z71 option. Two dealerships and General Motors told me there are so many electronics operating all the time the battery will run down if sitting for an extended period. With mine, it only takes about three weeks. Why is no one investigating and reporting on the battery rundown problem with Chevy Silverado trucks?
A: Does your truck use more current while shut down than others? It's possible--a new four-wheel-drive Silverado has more than 25 computers on board. Nowadays, there's a module for almost everything. And yes, there was a battery-drain issue which was corrected by reprogramming the Body Control Module. It was a campaign affecting certain dealer inventory and in-service vehicles. And I have it from a reliable source that the reprogramming did correct the problem as intended. But there are a few other things to look for in your situation. One is the use of any and all aftermarket (or GM-sanctioned) accessories that have been wired into the courtesy-lights circuit. This may prevent that same BCM from powering down, therefore draining the battery dead. The correct testing for a parasitic draw (current used from the battery while everything is off) is a must to be sure it's not excessive. It's also important that the battery itself be tested accurately, or you may wind up with a borderline unit that drives you crazy. GM has gone as far as stressing how important it is to have the battery cables properly clamped to the battery. Last, but not least, you may very well be letting it sit too long, just like everybody's been telling you. General Motors notes that letting a vehicle sit for 30 days or driving only short distances once a week is abnormal use and can kill a battery. An engine has to run for long periods of time, above 1000 rpm, for the alternator to fully charge a battery. So going out in the garage once in a while and running it for a few minutes won't really help. Sounds like your truck is somewhere in the middle of all that. If that's the case, your choices are to alter your driving schedule, disconnect the battery when parked, or use a slow-charger specifically designed for maintaining a vehicle's battery charge while in storage.
Tracking Down a Rough Idle
Q: My 1996 Chevy Tahoe, which has about 68,000 miles on it, has a rough idle. I replaced the plugs, wires, cap, and rotor, which hasn't helped. I don't really notice any problem at acceleration or at cruising speeds. The "check engine" light isn't lit. About two years ago the "check engine" light was flashing and the truck was bucking. The dealership determined it was a misfire condition and did a fuel-injector cleaning. It ran great for a long time, and sometime after doing the tune-up it developed the rough idle.
A: A rough idle, with no sign of a misfire during acceleration, typically eliminates a fault in the secondary ignition system (cap, rotor, wires, plugs). If that half of the ignition system were the problem, the engine would be more apt to break up under a load (accelerating). Fuel injectors are another story. An injector can be partially obstructed causing a miss just at idle, when only small amounts of fuel are needed to run the engine. Meanwhile, the symptoms aren't so noticeable during acceleration, while the injectors are dumping much larger quantities of fuel into the engine. Your previous run-in with dirty injectors may be a good hint as to the current problem. Another common cause of poor quality idle is a vacuum leak. The leak can be from a significant part of the engine, possibly getting by an intake manifold gasket, but in your case it may be simpler than that. Follow your tracks. Take a good look at every area of the engine you touched, or may have touched, while replacing parts. There could be a vacuum hose that was broken, cut, or knocked off somewhere in the process. Also, open your ears, with the hood up and the engine running. A vacuum leak can often be found by following the "hissing" noise. A technician trying to locate a hard-to-find vacuum leak may use a tool which injects smoke into the intake manifold. Wherever the smoke comes out is where the air was going into the engine, and that's the site of the vacuum leak. If you come up empty on your own, get it to a good technician. More than likely he can give you an accurate assessment of the problem for about an hour's labor time.
Water On Passenger Floor
Q: Whenever I run the A/C in my 2004 Chevy Trailblazer a pool of water collects on the floor at the front passenger seat. I assume a line is clogged somewhere. Is there an easy fix or should I have a professional look at this?
A: Give it a shot first. Condensation is formed on--and drips off--the air-conditioning system's evaporator (cooling component within the dash) and collects in the bottom of the evaporator/heater core case. The water is then routed through the firewall, into the engine compartment, and down to the street through a rubber drain tube. When this drain gets plugged up with debris, the water level will rise and eventually overflow onto the passenger-side floor. Carefully jack up and support the right front of the vehicle, crawl underneath and look for the evaporator drain tube pointing downward at the street. It's usually made of rubber, removable, and pulls off easily. But don't forget to duck. If the tube itself is obstructed and holding back the water, it's going to be a gusher. If you're lucky, and that's all there was to it, you can clear the tube of debris, reinstall it, and be on your way. But there may be more debris inside the evaporator case still plugging up the drain path, and that's going to require compressed air to blow it out. Worst-case scenario is that the drain path is clear, and condensation is entering the passenger compartment through a cracked evaporator case, or a bad seal against the firewall. Second-to-worst, there's so much debris collected in the case that it continually plugs up the drain over short periods of time. This requires removal of the evaporator case for a thorough cleaning. Either way, it's always a good idea to carefully inspect the plastic screening at the cowl, right at the bottom of the windshield beneath the wipers. This is where outside air enters the A/C system, and any missing, damaged, or misaligned protective screens will allow leaves and other debris to enter the evaporator case, consequently stopping up the evaporator drain.
Too Much Overheating
Q: I have a 2001 Dodge Durango with 92,000 miles on it. This summer, it started overheating during a road trip. It's usually okay when I'm driving on a straight road, but the minute I hook a trailer to it or start pulling hills, it overheats! No mechanics have been able to diagnose the reason. I've had the following work performed on it and it still overheats: new heater core, new thermostat (even tried running without one), new water pump, new antifreeze, new radiator cap, and cooling-system power flush. I can drive it about 60 miles max, then the temp gauge rises so much I have to stop and all the water boils out all over the place. There's no water in the oil and no steam is coming out of the tailpipe, so I'm told it's not a head gasket. The fan clutch seems to be fine.
A: There are a bunch of things to check and double-check. Let's start from the front. Be sure there's nothing obstructing airflow through the radiator. Grille ornamentation, a plastic bag pinned to the front of the A/C condenser, a missing air deflector beneath the bumper, or leaf and debris buildup between the radiator and the A/C condenser, can all cut down on the essential airflow through the radiator. Also, you had the coolant-system power flushed, but what did that really accomplish? Even the best job won't clear a severely obstructed radiator. So you should bring the radiator to a fully equipped radiator shop where it can test the radiator's flow rate in gallons per minute. This will determine if the radiator still has the capacity to cool your engine. If 92,000 miles was the first time you checked your antifreeze, there could even be "grunge" partially obstructing the coolant passages within the engine block. High ambient temperatures and load on the powertrain (towing and climbing hills) maxes out a coolant system's capacity. That's why manufacturers offer towing packages with higher-capacity radiators and auxiliary transmission coolers. Is your Durango equipped? No water in the oil and no notable steam out the tailpipe isn't an all-clear on a leaking head gasket. Checking for hydrocarbons inside the coolant system's expansion tank with an exhaust gas analyzer is a more accurate test.
How To Reach Alex
If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.

Can't wait for help with a problem you're having with your Truck or SUV? Ask the expert we trust here at Truck Trend Garage--visit Alex Steele at



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