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Powertrain Q & A: Mar 2003

Originally Published in MotorHome Magazine

 Q.  I have a one-of-a-kind problem that Chevrolet’s Tech Line
people have never heard of on a 1992 Winnebago 32-foot Adventurer with a
Chevrolet 7.4-liter engine and a 4L80E transmission with a locking
torque converter. It has 67,930 miles on it and tows a 1992 Geo Metro.
While driving a distance of about 200 miles down Interstate 15, the
transmission slips out of third and fourth gear into a neutral position
about 15 times, the CHECK-ENGINE light comes on; sometimes it backfires,
and the engine rpm increases rapidly until I get off the gas pedal.
Then it slows down, goes back into gear and continues on down the road.
It seems to happen when on an incline or during heavier acceleration,
between 30 and 63 mph (I usually travel about 60 mph). Also, the
speedometer jumps from about 60 down to around 40 and slowly back to 60,
in an ever changing erratic mode, not just when the problem occurs, but
most of the time now, except at speeds over 70, where it remains steady.

I spent one week at a Chevy dealer in St. George, Utah, where they
replaced the vehicle speed sensor and wiring harness, and they removed
the transmission pan, but no filings were found and no other problems
were detected. They replaced the computer and some twisted wires from
the speed sensor, replaced the ignition switch, the distributor cap and
rotor and the fuel filter, and tested fuel pressure, which was OK.

The service manager rode with me for a road test of about 125
miles, using a Tech 1 scan tool to capture snapshots, speed-in-and-out
comparisons, etc.; codes 68, 73 and 85 showed up only in history, but
the problem happened so fast no codes were generated.

GM’s Tech Line staff said a transmission check was next, but the
service manager said it was still an electrical problem, not the
transmission itself. Reaching an impasse, I resumed my journey to Yuma,
Arizona, during which the trouble happened 15 more times. My mileage is
down about 1 mpg and the engine seems to be 10 to 20 percent lower in

Also, the cruise control quit working and the
transmission-temperature gauge reads only 135 F, while outside temp was
80 to 85 F. Engine temperature was between 180 to 190 F.

I am very hopeful that you have a solution, or your readers can provide an answer.

–Larry Stoddard, Midvale, Utah

A.  It seems that the dealer’s staff has made a decent try
at fixing this, although they got a little carried away with parts
changing. I believe the transmission is not actually shifting to
neutral; it may be downshifting to second or first so the engine can’t
rev high enough to keep up with vehicle speed, and it then feels like

The erratic shifting could cause the loss of 1 mpg. You didn’t
mention where your transmission temperature sensor is located or what
its normal readings are, so that isn’t very useful, except perhaps it
indicates that the torque-converter clutch isn’t slipping. The possible
loss of power should be checked on a chassis dyno. The erratic
speedometer may be related to the inoperative cruise control. You should
have that checked first because it also may be related to the shifting
problem. The transmission needs a reliable speed signal to shift
properly, along with a good throttle-position sensor signal.

Next, it should have a set of 12-volt DC light-emitting diodes
(LEDs) installed on the shift solenoid wires to watch the solenoid
firing order. This would tell you if the ECM (computer is signaling the
transmission to downshift, or if the signal stays the same and the
transmission is faulty internally. Leaking or defective shift solenoids
can cause the transmission to be in the wrong gear, even if the signal
sequence is correct. The shift solenoid seals a bleed hole in the
circuit, which, in turn, moves the shift valve. A solenoid can be
defective hydraulically and OK electronically. That is why you need to
monitor the solenoids with LEDs.

Have a good transmission shop get small 12-volt DC LEDs in
different colors (from RadioShack, for example) wired in parallel with
the shift solenoid wires, using thin twinconductor speaker wire so they
can be monitored from inside the vehicle. The shift solenoids are
supplied with 12 volts DC positive continuously, and the ECM grounds the
solenoid return wire to make it work. The LED willbe on until the ECM
shunts the 12 volts DC to ground. When the light goes out, the solenoid
is on. That will show if the problem is inside or outside the
transmission and give a diagnostic path to follow.

I think you’ll find a faulty solenoid and/or an erratic sensor signal.

Q.   This is in regard to a letter in the September 2002
issue titled “Severe Shimmy.” I have a 1996 Flair motorhome on a Ford
Super Duty chassis, and I have had exactly the same experience twice.
The last time was on Interstate 10, but it only happened when I hit a
bump. Luckily I was in the truck lane and was able to stop safely
without being hit by anyone. It is an extreme and violent shimmy that
makes it very difficult to keep the coach on the road. Air pressure and
lug nuts were normal.

Fleetwood was of no help and passed me off to Ford, who told me where I could find a dealer.

This problem happened only after a bump in the road; otherwise the
motorhome drove just fine. After getting home, I found the problem and
fixed it. The steering stabilizer was totally shot.

I was not able to find out from the National Highway Traffic Safety
Administration (NHTSA) if this was a known problem or an isolated one.

My vehicle had 22,000 miles on it, and I don’t think the stabilizer
should have failed in that amount of time. Either way, I was not able
to report the problem. I hope this helps someone.

–R. J. Durette, Vista, California

A.    Although you can notify NHTSA of vehicle problems
through its Web site or by telephone, that agency merely compiles the
data and won’t normally share the information with the public unless it
does an investigation. The first thing owners should check when they
have a severe shimmy problem is a worn-out steering stabilizer or

While I agree it seems like it didn’t last long, you didn’t mention
if the stabilizer had the same mileage as the coach, so we don’t know
for certain how many miles it had on it. Keep in mind that this is
basically a shock absorber and, as such, is a wear item.

Q.   If I may weigh in on the issue of “Steering Shake”
(August 2002) and “Steering Shake II” (November 2002), I had that
problem in my Class A motorhome. I had the front wheels balanced and the
alignment checked. I also had the front tires checked to be sure they
weren’t out of round.

I finally insisted that my dealership have one of its technicians
drive the coach with me inside. He agreed almost immediately that there
was a problem. He literally had to get on the ground and slide under the
front of the motorhome before he located the trouble. There was a
vertical brace in the front of the motorhome against the outside skin
that had broken loose from one of its welds.

Once the brace was repaired, the shake went away. It required a
little more than just rewelding, but Fleetwood, along with Beckley’s
Camping Center in Thurmont, Maryland, made it right.

–Leo Dougherty,Spring Hill, Florida

A.   Thanks for writing, Leo. Steering shake is such a
common problem and there are so many solutions that I am going to
continue sharing unusual ones like this, which most technicians don’t
look for.

Q.   This is in response to a letter about steering wheel
shaking on Ford chassis (“Doin’ the Shimmy,” April 2002). I had this
problem with a Ford F-450. After first checking and replacing worn
suspension parts with no success, I finally pulled the front-brake
rotors, turned (resurfaced on a brake lathe) and reinstalled them. The
problem has not occurred since.

–Scott Zeiler, Cheyenne, Wyoming

A.   There are many things that can cause severe front-end
shake, and this is one that is often overlooked. Thanks for writing,

Q.   I recently had a Banks exhaust system and TransCommand
installed on our 1995 Southwind 35PW motorhome with an F-53 Ford chassis
and a 7.5-liter V-8. I am extremely happy with both items.

I hear a backfire in the exhaust when I quickly step on and off the
accelerator, sitting still in park, but when I hold it a little longer
and release it, I don’t hear a backfire. I believe it existed before,
but, of course, with the stock exhaust system it was not as easy to

The vehicle has 40,000 miles on it and got new plugs at about
25,000. Mileage has never been very high, ranging from 5.7 to 7 mpg, but

with the modifications I have gotten 6.4 to 8.5 so far.

Is backfiring normal?

–Tim Bamford, Big Lake, Minnesota

A.   Since this situation involved Banks products, I
contacted Peter Treydte, director of Technical Communications at Banks
Power. Here is his reply, with which I agree: “Usually a backfire under
the conditions that you described is the result of a minor air leak
somewhere in the exhaust system. The 1995 Ford 7.5-liter engine utilizes
an air-injection system into the exhaust for emission control, and
there is the possibility that one of the threaded connections has a
small leak. This is not harmful to the engine in any way, but the
resulting backfire may be annoying. You can contact our Technical
Service Department at Gale Banks Engineering, (888)839-2700, for help in
locating the source of the problem.”

Q.   “Cummins Cooldown,” in your November 2002 issue,
concerns diesel overheating. Since this is a relatively common problem, I
would like to offer a driving technique employed by professional truck
drivers who face the same problem.

Modern turbo-diesel engines use a great deal of air, lots more than
is actually needed for the combustion process. The extra air helps keep
combustion temperatures and emissions down and provides for complete
burning of the heavy diesel fuel, along with good fuel economy. As you
start climbing a hill, engine rpm starts to drop and you tend to push
the throttle farther toward the floor and lug it. At lower rpm, however,
temperaturesstart to rise. Therefore, as you start to climb a hill, and
the engine speed drops to about 1,600 rpm, you should manually shift
the transmission into the next-lower gear. Continue downshifting until
you reach about 2,000-2,100 rpm with light throttle pressure, and you’ll
be able to maintain 2,000 rpm for the balance of the climb regardless
of vehicle speed.

Higher engine rpm (less lugging) will help cool down the combustion
chambers, and it also makes the water pump and cooling fan turn faster.

If your coach is equipped with an exhaust-temperature gauge and a
manifold-pressure gauge, you’ll find a point while downshifting where
the exhaust temperature starts to drop and you are able to maintain good
boost pressures. At that point, you have found the proper gear and
throttle settings to climb a hill without overheating.

–Anthony Steller Sr.,On the Road

A.   Well said, Anthony. As a technician, I tend to try to
solve problems from a mechanical perspective, but sometimes a simple
change in

driving technique is all that’s needed.

Q.   We have a 1990 Pace Arrow motorhome on a Chevrolet
P-chassis.The front suspension has air bags, but I can’t find the
recommended minimum or maximum air pressure. Could you please advise me?

— Cliff Mitchell, Wyoming, Ontario

A.   This information is in the chassis owner’s manual.
According to GM, the air pressure depends on the front-axle weight
rating. Recommended pressure is 40 to 50 psi for axles with weight
ratings of 4,300 pounds. The 5,000-pound-rated axle gets 50 psi, and the
air bags on the 5,300-pound-rated axle should be inflated to 70 psi.
Coaches with the optional 16,000-pound gross vehicle weight rating and
the 5,500-pound rated front axle get 80 to 90 psi.

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