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PRESCOTT TERRITORIAL RAMBLE TECH NOTES
The Prescott Territorial Ramble featured technical seminars
on the air suspension, car towing, living area maintenance, carburetors
and junk yard scrounging.
Duane Simmons provided a well illustrated seminar on the three
versions of the air suspension system with a number of improvements
to be considered for each of the systems. They included:
* Install the upgrade kit available from Cinnabar that has a filter,
stainless check valve, pressure relief valve and pressure gauge.
* Convert the low-air sensor indicator to a compressor-on indicator
by connecting the lamp wire through the second set of terminals
on the pressure switch.
* Connect the primary 12v power for the compressor relay to a
circuit breaker- protected power source that does not go through
the ignition switch to assure maximum voltage. (Either living
area battery or engine battery may be used).
* On Power Level Systems relocate air pressure switch to sense
tank pressure rather than compressor output.
On Electro-Level II systems, modify control logic to activate
hold solenoid only when raising or lowering coach.
Bob Cook and Chuck Garton covered the junk yards for emergency
repair and upgrade parts as well as those parts not worth scrounging.
Some of the more useful items include:
* 75-78 GM car heater control panels if you keep the original
vacuum select wafer from the GMC.
* Front suspension upper and lower "A" arms from Toronados
and Eldorados after re-enforcement by a competent welder to match
original.
* Inner and outer CV joints from Toronados and Eldos.
* Distributor and ignition modules from Olds engines.
* Cruise controls, the 73-78 Buick is reported to be an exact
match. Others will
work but lack the dash light connector.
* GM Delco radios are excellent replacements for the eight track
units.
* Toronado and Eldo steering knuckles usually have good bearing
bores. The lower ball joint hole must be reamed to match the larger
GMC ball joint.
* Final drives from the Toronado/Eldo with 10 bolt covers are
usually 3.07 gear ratios. The 66 and 67s had 3.21 (8 bolt covers).
The Cinnabar 3.46 replacement is probably a better deal than a
salvaged 3.07.
* Vacuum booster pumps used on 4 cylinder air conditioned GM cars
in the '80s
are nice to add as an auxiliary pump for the brake booster in
the event the engine quits.
* Diesel cars from 82-84 had high capacity fans to provide improved
cooling.
* Carburetors, alternators and starters are not worth the bother.
Some truck steering gear boxes will work but have different ratios
or road feel; TH425 trannys are available if you want a spare,
but because of the labor involved, I'd have it rebuilt.
The carburetor seminar highlighted a 10-12% increase in the primary
jet size
as a recommendation from Joe Mondello to compensate for reformulated
gasoline.
Generally this is an increase in the Jet Size of two. The following
are some part numbers for the Rochester Quadrajets:
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Secondary metering rod:
455 engines - 7045780 - stamped CJ
403 engines - 7045781 - stamped CK.
Secondary hanger: Identification stamp U or V (V is the only
service part).
Power piston spring: 7036019, cut-in point 8-4 inches of vacuum.)
Bob Hammerly, owner of Mile High RV in Prescott provided an
entertaining and informative seminar on living area maintenance
and repair. Bob covered heating, air conditioning, refrigeration,
propane systems and the generator in a Q and A format to the delight
of all attendees. This was one of those times when you had to
be there, since the facts, model and parts numbers and repair
hints came so quickly that one could hardly take useful notes
unless the discussion solved one of your problems.
[Secretary's note: While we normally avoid endorsing any supplier,
repair facility or mechanic who isn't a GMC owner {and then only
if they have demonstrated their expertise) we'll make an exception
for Bob Hammerly. It would definitely be worth the time and effort
to pass through Prescott if you have a problem in any of the areas
mentioned above. From every indication, Bob could save you money
and get the job done better than the average RV facility.]
Retrofitting Air Conditioners to use R. 134a
Many GMC owners may be interested in a low cost retrofit/replacement
of the original R-12 air conditioner refrigerant with EPA approved
R-134a. The least cost retrofit is simple and does not require
major component changes. Basically, the process calls for removal
of the old refrigerant, installation of new fittings, a new label
and the addition of either a polyalkylene glycol {PAG} or polyester
{POE or ester) lubricant as well as the R-134a refrigerant. O-rings,
compressors and hoses do not need to be replaced if they are in
good shape.. Obviously, if your system is flat, the leak(s) need
to be repaired.
Although the EPA does not have the technical expertise to recommend
procedures for each vehicle, the following recommendations are
from one service facility and parts wholesaler with extensive
experience in retrofitting vehicle air conditioners with R-134a.
Arctic Air in Fort Lauderdale, Florida, has been recognized as
a pioneer in making this retrofit/replacement.
This information is provided solely as a service to automotive technicians who should rely on their experience and judgment in determining the most appropriate steps in retrofitting any vehicle with HFC R-134a or similar substitute. Neither the EPA, Arctic Air or GMC Western States warrants or guarantees any of this information. WHAT YOU DO TO YOUR COACH IS YOUR BUSINESS!
1. Recover the R-12 according to SAE J1989 standards {this
requires a closed capture system generally available only at air
conditioning service facilities).
2. If the system is contaminated with water, oil, or other foreign
material, perform closed loop flushing.
3. Repair leaks and make any necessary parts replacements.
4. Add PAG or ester oil lubricant according to the exact specifications
for the system. Too much oil will affect system performance.
5. Replace the accumulator or dryer if this was not done during
repairs. Use an R-134a compatible desiccant such as XH-7.
6. Install the R-134a service ports. The Schrader valve cores
may have to be replaced depending on the design of the ports.
Note that the service ports for R-134a are reversed in size from
those for R-12. The small port is on the suction or low side and
the large port is on the discharge.
7. Evacuate the system for 45 minutes at 29.5 inches of mercury.
The retrofit requires a long evacuation time to ensure complete
removal of the R- 12.
8. Charge the system with R-134a using a calibrated cylinder or
weighing scale. The retrofit generally requires the weight of
R-134a to be between 75 and 90% of the original weight of the
R- 12 (it is recommended that 75% be installed and then the system
be checked for excessive clutch-cycling). Proceed in 5% increments
until the clutch-cycling is
minimized. DO NOT EXCEED 90%.9. Check for leaks.
10. Complete the R-134a label and apply over the original R-12
label.
11. Test drive the coach with the AC running.
12. Do a performance check. High-side pressures should be within
10-15% of the original R-12 pressures according to manufacturer's
specifications.
More Tips:
On most vehicles, the duct temperatures with R-134a are the
same as with R-12. On some vehicles the duct temperature may be
a few degrees cooler. Retrofit kits with complete instructions
are available from some parts houses such as Auto Parts Club.
Currently the only illegal action for the do-it-yourselfer is
to vent any of the refrigerants to the atmosphere. You must change
fittings and attach the correct label for R-134a. On clutch-cycling-orifice
tube systems, the cycling switch should be adjusted counterclockwise
to 21psi cycle off point.
[The foregoing is an edited version of material provided by the
EPA Stratospheric Protection Division. Once more: GMC Western
States is not responsible for what you do to your coach!]
Another Tip:
One of the biggest problems at we have with our old coach AC
system is that the Vacuum Select control valve in the dash climate
control develops leaks. Thus the vacuum required to operate the
Heater Water Valve and AC, heater and defrost dampers in the climate
control unit may not exist. Since this valve is not a readily
reparable part and it is difficult to find replacements, we frequently
are faced with a situation where hot engine coolant is circulated
through the heater coil when the system is set on AC. Needless
to say, this results in terribly inefficient cooling. There are
recommendations for improving the efficiency, which have been
discussed in SEPARATING HOT form COLD, by Zay Brand, April 1994
Roundup [contact Zay for copies of this excellent presentation].
A "quick and dirty" partial fix which will result in
maximum benefit for minimal time and expense is to simply shut
off the flow of hot coolant to the heater core during that time
of year when AC is desired. This may be accomplished with something
like vice grip pliers on the heater hose (very crude but effective)
or more elegantly by installing a ball valve. Just remember to
open the valve in the fall when heating is necessary.
HOW TO SMOOTH OUT THOSE PESKY RUTS
It is rumored that a solution (?) to the problem created by
the difference in track width between the front and rear wheels
on our GMC coaches is available. We have heard stories of GMCers
being pulled over for drunkenness by Smoky Bear while driving
down a severely rutted highway because the coach was zooming from
side-to-side as it climbed in and out of the ruts in the asphalt
paving. Maybe some of us have had that experience. Most of us
have noted the ruts whenever we have to drive an interstate paved
with asphalt. Those who have studied the problem criticize GM
for not increasing the track width to be in line with the rear
track when modifying the Toronado front end for use on the GMC.
Others note that twenty plus years ago, the interstates didn't
have ruts in them so GM couldn't know that there would be a problem
with this design.
Now some GMCers are trying Ford truck wheels which have a deep
inset hub rather that the standard outset hub. This effectively
may extend the front track width by 4" or more. The result
is that the coach stays in the ruts since both front and rear
tracks are essentially the same. Two Mountainaires are currently
using this system and report excellent results. For example, the
ruts on 1-25 north of Denver are "smoothed out."
BUT, and this is a BIG BUT, is this a good idea? Two thoughts
have been presented which would indicate that the cure may be
worse than the disease. First, in theory, the center of the load
or force should lie on the vector passing though the suspension
points. In other words, the center of the front wheel tread should
be centered on a line though the upper ball joint, the wheel bearings
and the lower ball joint. This provides for what may be considered
balanced steering since the steering load at the point of contact
with the pavement is in line with this hinge point. This problem
may not be too significant since power steering can easily overcome
the additional resistance provided by the offset.
Second, and perhaps more critical, is that with the lateral offset
the wheel bearings are no longer being loaded symmetrically. This
can accelerate wear on the bearings and possible elongate the
softer metal of the surrounding knuckle. How critical is this?
Who knows? However, according to the above theory it can happen.
You decide - it's your coach.
GMC Western States, this Newsletter and the Editor and chapter
officers accept no responsibility whatsoever for what you do to
your coach. TECH CENTER information is provided as a matter of
general interest and is neither a criticism of any of the original
design features of the GMC Motorhome or an endorsement of any
potential change, correction, modification, repair or solution,
or any apparent or perceived problems with the coach.
Front Suspension and Alignment
Another solution to the rut problem was presented at GMC International
at Boerne, Texas, this spring. This involves adjusting Caster
and Camber according to the following theory.
Wheel Alignment: Safe steering control with minimum tire wear,
can be achieved on the GMC coach when certain values are set for
the planes angles and radii of the front suspension and steering
members. The terms and their meanings are discussed as follows:
Camber angle: Measured in degrees and is the angle that the wheels
are angled out at the top for positive camber or in at the top
for negative camber.
Caster angle: Measured in degrees and is the amount that the knuckle
support pivot points are tilted toward the back of the coach at
the top for positive caster or toward the front of the coach in
the case of negative caster. The effect of positive caster is
to cause the coach to steer in the direction that it goes. Positive
caster tends to steer down off the crown of a road or in the direction
of a cross wind. Negative caster will tend to cause a coach to
steer up a crown or into a cross wind.
Toe-in: The amount that the front of the wheels are closer together
than the rear of the wheels. Measured in inches on the GMC coach.
Generally, toe is set to modify the effects of camber. This is
because a cambered wheel tends to steer in the direction that
it is cambered. Toe-in is set to compensate slightly in the direction
opposite to the direction the wheel tends to steer. Camber and
caster both have an effect on toe-in, therefore toe-in is the
last setting to be made when aligning the front suspension and
steering system.
Built into the geometry of the front suspension of the OMC coach
is the inclination of the knuckle support pivots. This is called
the steering axis inclination. The pivots on the GMC are the upper
and lower ball joints and the axis through the ball joints is
inclined inward at the top. The pivot inclination angle is not
the same angle as camber, but, it changes with camber changes.
The function of pivot inclination is to cause the coach to tend
to steer in a straight line regardless of outside forces such
as crowned roads and cross winds. As the spindle is moved through
its steering from left to right turns, the spindle rises and falls.
This action forces the coach to rise and fall since the tires
are in contact with the ground and the forces are being transferred
to the coach through the suspension. The coach w/Il rise at the
extreme right turn and come to it's lowest point at the straight
ahead position and again rise for an extreme left turn. The weight
of the coach will tend to cause the wheels to come to the straight
ahead position, which is the lowest position of the coach itself.
Possible alignment settings can be obtained upon request with
the usual disclaimer.
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