Home » Public Forums » GMCnet » Rear Suspension Trouble Shooting.
| Rear Suspension Trouble Shooting. [message #202043] |
Mon, 18 March 2013 13:05  |
DrPepper
Messages: 80
Registered: February 2013
Location: Goose Creek,SC
Karma: 0
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By Now you know I am kind of new at this, I have been reading as much as I can on the Air Bag system
I have talked to Jim K and he was also very helpful. I have a Question on the Height Control Valve and Travel Mode.
I rebuilt the Control Values using one of JimK Kits, and that worked great, and has 0 leaks. Best $19 I ever spent .
I having issues with Travel Mode, I was thinking that you guys could shed some lite on the subject.
How do you determine correct height for travel mode? Jimk Said to locate the Oval hole on the frame on the passenger side 2 feet behind the wheels. The height should be 11 11/16. So I found the Oval Hole, but is the 11 11/16 from Ground to Bottom of Frame, or Ground to Bottom of Oval Hole? My Friend Barry said that he measures 31 ½ inches between the wheels from ground to the line that separates the Wheel Well and the Center Shirt. How do you tell what the correct Travel height should be?
So, I might Have two bad Height Control Valve’s. For my example I am going to use the Skirt Measurements.
If I raise the Coach up to 35 inches, (about as far as go at 100 psi) then put it into travel, it will drop to about 32 inches in about 5 min. If I lower it to the frame then put it to travel it will rise to about 29 ½ inches in about 15 min. Is it normal to have a 3 inch of play in the value?
The return air flow rate from the Height Control Valve to the Rotary Height Valve is VERY slow, I would say extremely slow. So maybe I have a blockage in the Valve? The Air Feeding the Height Control Valve (Red Tubes) is good, about the max you would expect from ¼ tube.
Phillip Udel - I Own a 1975 Avion
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| Re: [GMCnet] Rear Suspension Trouble Shooting. [message #202045 is a reply to message #202043] |
Mon, 18 March 2013 13:10  |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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Senior Member |
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did you read here?
http://gmcmotorhome.info/list.html#Rear
gene
On Mon, Mar 18, 2013 at 11:05 AM, Phillip Udel <admin@salemcorp.com> wrote:
>
>
> By Now you know I am kind of new at this, I have been reading as much as
> I can on the Air Bag system
> I have talked to Jim K and he was also very helpful. I have a Question on
> the Height Control Valve and Travel Mode.
> I rebuilt the Control Values using one of JimK Kits, and that worked
> great, and has 0 leaks. Best $19 I ever spent .
> I having issues with Travel Mode, I was thinking that you guys could shed
> some lite on the subject.
> How do you determine correct height for travel mode? Jimk Said to locate
> the Oval hole on the frame on the passenger side 2 feet behind the wheels.
> The height should be 11 11/16. So I found the Oval Hole, but is the 11
> 11/16 from Ground to Bottom of Frame, or Ground to Bottom of Oval Hole?
> My Friend Barry said that he measures 31 ś inches between the wheels from
> ground to the line that separates the Wheel Well and the Center Shirt. How
> do you tell what the correct Travel height should be?
> So, I might Have two bad Height Control Valve’s. For my example I
> am going to use the Skirt Measurements.
> If I raise the Coach up to 35 inches, (about as far as go at 100 psi) then
> put it into travel, it will drop to about 32 inches in about 5 min. If I
> lower it to the frame then put it to travel it will rise to about 29 ś
> inches in about 15 min. Is it normal to have a 3 inch of play in the value?
> The return air flow rate from the Height Control Valve to the Rotary
> Height Valve is VERY slow, I would say extremely slow. So maybe I have a
> blockage in the Valve? The Air Feeding the Height Control Valve (Red
> Tubes) is good, about the max you would expect from Ś tube.
>
> --
> Phillip Udel - www.gmcfabrication.com I Own a 1975 Avion, Still removing
> the Mothballs after sitting in storage for 10+ years
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
-------
http://gmcmotorhome.info/
Alternator Protection Cable
http://gmcmotorhome.info/APC.html
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| Re: Rear Suspension Trouble Shooting. [message #202047 is a reply to message #202043] |
Mon, 18 March 2013 13:29 |
jtblank
Messages: 237
Registered: June 2007
Location: Tulare, CA
Karma: 1
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Ride height is 13 - 13 1/2" at the top of the front slots, and 11 11/16" at the top of the measuring slots in the rear
Depending on the thickness of your body pads any other measurement could vary, I suggest you get the slot measurement correct then find a body reference measurement for future quick checks.
John Blankenship
'76 Palm Beach
Tulare, CA
[Updated on: Mon, 18 March 2013 13:36] Report message to a moderator |
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| Re: [GMCnet] Rear Suspension Trouble Shooting. [message #202048 is a reply to message #202045] |
Mon, 18 March 2013 13:46 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Phil, I will try to answer your email as simply as I can. On the GMC frame,
there are 4 slotted holes. 2 are located behind the rear suspension, and 2
behind the front wheel wells. If you have a generator in the rear
compartment, one of the holes may be obscured by the bracketry from the
generator, although it can still be found by looking on the inside of the
frame rail. The suspension is attached to the frame, NOT THE BODY. It is my
opinion that due to different body mounting systems that GMC used
throughout the various models, and settling that has occurred in the body
panels through the 40 years of use, that measuring from the t skirts or
other places on the BODY is not very accurate. The factory specs for ride
height are given as distance from the ground to the CENTER of the frame
slots, and the tolerance is plus or minus 1/4 ". As you have already found
out, your factory ride height sensors are not accurate enough to do this
simple task. You can get them close, but their production tolerances is
around 5/8 " or so. So, what else is involved? Consider that the GMC frame
is like a teeter totter. But a more complex one. It not only tilts front to
back, but diagonally as well. Sketch out a rectangular box that represents
the frame of your coach. Using the slotted holes as way points or set
points, draw lines front to rear, side to side, and diagonally as well.
Place a fulcrum at the point where the lines cross under the middle of the
coach. Now visualize the frame balanced on the fulcrum, and supported by
the tires and suspension. Raise the passenger side rear of the frame. What
happens to the other side, and the opposite side in the front of the
coach?? Are you beginning to see the problem? ANY adjustment that you make
on the suspension at any point affects the other three points as well. Now,
throw in load and balance variations caused by actually using the coach and
that further complicates the issue. Now that you can see the scope of the
problem, I will try to address your questions.
1. The air passages in the original system are restricted on purpose to
prevent dynamic road forces from causing the sensors to constantly change
ride height. It is one of the unintended consequences of the original
design. Not much can be done to alter that. There is nothing wrong with
your sensors. That is as good as they are capable of working. The EL II
system, which uses electronic sensors that incorporate light emitting
diodes and photo transistors that are separated by a shutter wheel attached
to the bogie arms, have a delay built into the circuit to prevent the
constant cycling of the compressors and solenoids in response to dynamic
loading of the suspension (driving). That is why we use air pressure to
control ride height, not height sensors. It is faster, and constantly
dynamic, and user friendly.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Mon, Mar 18, 2013 at 11:10 AM, gene Fisher <mr.erfisher@gmail.com> wrote:
> did you read here?
> http://gmcmotorhome.info/list.html#Rear
>
> gene
>
>
>
>
>
>
> On Mon, Mar 18, 2013 at 11:05 AM, Phillip Udel <admin@salemcorp.com>
> wrote:
>
> >
> >
> > By Now you know I am kind of new at this, I have been reading as much as
> > I can on the Air Bag system
> > I have talked to Jim K and he was also very helpful. I have a Question
> on
> > the Height Control Valve and Travel Mode.
> > I rebuilt the Control Values using one of JimK Kits, and that worked
> > great, and has 0 leaks. Best $19 I ever spent .
> > I having issues with Travel Mode, I was thinking that you guys could shed
> > some lite on the subject.
> > How do you determine correct height for travel mode? Jimk Said to locate
> > the Oval hole on the frame on the passenger side 2 feet behind the
> wheels.
> > The height should be 11 11/16. So I found the Oval Hole, but is the 11
> > 11/16 from Ground to Bottom of Frame, or Ground to Bottom of Oval Hole?
> > My Friend Barry said that he measures 31 ś inches between the wheels
> from
> > ground to the line that separates the Wheel Well and the Center Shirt.
> How
> > do you tell what the correct Travel height should be?
> > So, I might Have two bad Height Control Valve’s. For my example I
> > am going to use the Skirt Measurements.
> > If I raise the Coach up to 35 inches, (about as far as go at 100 psi)
> then
> > put it into travel, it will drop to about 32 inches in about 5 min. If
> I
> > lower it to the frame then put it to travel it will rise to about 29 ś
> > inches in about 15 min. Is it normal to have a 3 inch of play in the
> value?
> > The return air flow rate from the Height Control Valve to the Rotary
> > Height Valve is VERY slow, I would say extremely slow. So maybe I have
> a
> > blockage in the Valve? The Air Feeding the Height Control Valve (Red
> > Tubes) is good, about the max you would expect from Ś tube.
> >
> > --
> > Phillip Udel - www.gmcfabrication.com I Own a 1975 Avion, Still
> removing
> > the Mothballs after sitting in storage for 10+ years
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
> >
>
>
>
> --
> Gene Fisher -- 74-23,77PB/ore/ca
> “Give a man a fish; you have fed him for today --- give him a URL and
> -------
> http://gmcmotorhome.info/
> Alternator Protection Cable
> http://gmcmotorhome.info/APC.html
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
_______________________________________________
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Unsubscribe or Change List Options:
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| Re: Rear Suspension Trouble Shooting. [message #202261 is a reply to message #202043] |
Wed, 20 March 2013 11:44 |
DrPepper
Messages: 80
Registered: February 2013
Location: Goose Creek,SC
Karma: 0
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Member |
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Thanks for everyone's help. I just got my new pump in today.
As soon as I weld up a bracket I can do some testing.
EL II. What is that? Anyone have a link?
Phillip Udel - I Own a 1975 Avion
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| Re: Rear Suspension Trouble Shooting. [message #202306 is a reply to message #202261] |
Wed, 20 March 2013 20:15 |
tphipps
Messages: 3005
Registered: August 2004
Location: Spanish Fort, AL
Karma: 9
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Senior Member |
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Your Avion, unless modified by a P.O., has the original air suspension system. Easy way to check this is the existence of two matching knobs, by the driver's right knee.
EL II (Electro-Level Two) has a set of three eletrical switches below the toll window, next to the driver's left arm.
I may be incorrect, but the only common components are the air bags.
Tom, MS II (Avion also)
2012 Phoenix Cruiser model 2552
KA4CSG
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