Transmission "Durability

[patrick flowers]

As promised yesterday, I dug out the GMCMM "library" and looked for the
transmission article. It wasn't easy to find because the section I
recalled was in an article titled "Hey Mister, Your Motorhome Is On
Fire!!!" (8/97).

According to this article, transmission oil overflow may be the number
one cause of fire in GMC's. When you drive the coach hard(accelerator
pedal more than halfway down), the trans pressure regulating valve moves
toward the bottom of its travel. When it reaches the bottom, it cuts
off flow to the trans cooler and torque converter. In auto
applications, this is usually a temporary situation, but in motorhomes,
if you don't drive carefully, it can be an extended condition. The lack
of cooling causes the fluid to expand, and if the trans is full, fluid
may actually spew out the vent and filler neck. The author recommends
keeping fluid level at the "ADD" mark and driving carefully to avoid
this.

So, it's not so much a transmission problem as it is a driving problem.
Many of us(I'm guilty too) push our coaches too hard, thinking "since
they'll go that fast, why not?" I'm not sure if there is a "fix" for
this or not. I'm sure the THM400 trans used in truck app's must deal
with this somehow.

Patrick
- --
Patrick Flowers
Mailtoatri63

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[patrick flowers]

>
> Patrick,
> I hate to disagree in my first post to the list,

Never apologize for telling the truth!

> but the article
> about heavy throttle running cutting off the trans, cooler was in
> error. As a burned-out Master Technician myself, I thought something
> was fishy when I read rhat article, but not being familiar with the
> THM425 trans, I bit my tongue. Sure enough, a few months later GMCMM
> published a letter from Robert E. Annis, "Retired Chief Engineer,
> Hydra-Matic Division, General Motors Corporation" stating that "High
> line pressure does not position the regulator valve such that flow to
> the torque converter ... is cut off..."
> Suggest anyone interested get the 12/97 edition of GMCMM and read Mr.
> Annis' letter on pp. 6-7. You can't get much more authoritattive than
> him!

Well Rick, you've forced a confession out of me. I procrastinated on my
GMCMM renewal and missed the 12/97 issue. I have ordered the backissue
and promise to write "mea culpa" fifty times once I have a chance to
read Mr. Annis' letter.

Since you have experience in this area, do you have any advice for
prolonging the life of the THM425?

Also, I'm curious if you have an opinion about the article "How I Almost
Trashed My Engine" in the 10/97 issue where it refers to "bulging"
torque converters damaging the thrust bearing surface of the
crankshaft. First I've ever heard of that.

> PS Glad I joined, the list is great!

And we're definitely glad to have you here.
Patrick
- --
Patrick Flowers
Mailtoatri63

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[scott woodworth]

Patrick said:

> So, it's not so much a transmission problem as it is a driving
problem.
> Many of us(I'm guilty too) push our coaches too hard, thinking "since
> they'll go that fast, why not?" I'm not sure if there is a "fix" for
> this or not. I'm sure the THM400 trans used in truck app's must deal
> with this somehow.

Patrick,

Not to worry. We have the technology. I do trannies on fairly
regular basis and I can vouch for the after market transmission repair
parts industry when I say there's always a fix.

When I overhaul an automatic transmission, I always install a
TransGo shift improver kit. These kits aren't expensive ($20-50), and
they have updates that have been deemed necessary from experience
garnered in the field. Each different model transmission has it's own
built in quirks. These updates include, but are not limited to: Full
time lubrication on certain transmissions that don't have that feature
(don't ask), erratic shifts, cooling problems like you outlined, hot and
cold slipping, etcetra.

I'm glad you told me about that fire problem, because I plan to
install a Cad 540 and a 3.42 final drive in my coach and see how many of
those little jelly bean cars I can run off the road with it. That's
where an on board halon fire suppression would come in handy I suppose.

Scott Woodworth

 

[bartz paul]

Scott:

In case you or anyone else is interested, the Cad 500 ci engine swap
into the GMC is nothing new. Fritz Slam in WI was identified to me as
the guru of the swap. I talked to him by telephone about it a year or
so ago. Fritz is one of the most respected individuals about the GMC
mechanicals and presents seminar's at each semi-annual GMC Motorhomes
International (GMCMI) rally on the GMC engine, usually regarding
overhauls.

Clarence Buskirk had a 500 ci engine, all mounted in a sub-frame, that I
saw a couple years back. Believe he told me he changed his mind about
installing it in his coach.

In any event, there are several components (e. g. alternator and A/C
compressor) which must be mounted differently than stock arrangement.

At the Mt Rushmore GMCMI rally, Sep 95, Darryl Winterfeldt, GMC engine
rebuilder from CO, when asked about the Cad 500 ci engine, responded to
me that the Cad heads would not flow as much air as the 455 ci engine
would, for whatever that's worth.

The above is just more info to add to the question of swapping the Cad
500 ci engine into the GMC coach.

Paul Bartz, Kalamazoo MI

> -----Original Message-----
> From: Scott Woodworth [SMTP:myvair]
> Sent: Thursday, January 15, 1998 2:07 AM
> To: GMC Motorhome List
> Subject: GMC: Re: "Transmission "Durability"
>
> Patrick said:
>
> > So, it's not so much a transmission problem as it is a driving
> problem. Many of us (I'm guilty too) push our coaches too hard,
> thinking "since they'll go that fast, why not?" I'm not sure if
> there is a "fix" for this or not. I'm sure the THM400 trans used in
> truck app's must deal with this somehow.
>
> Patrick,
>
> Not to worry. We have the technology. I do trannies on fairly
> regular basis and I can vouch for the after market transmission repair
> parts industry when I say there's always a fix.
>
> When I overhaul an automatic transmission, I always install a
> TransGo shift improver kit. These kits aren't expensive ($20-50), and
> they have updates that have been deemed necessary from experience
> garnered in the field. Each different model transmission has it's own
> built in quirks. These updates include, but are not limited to: Full
> time lubrication on certain transmissions that don't have that feature
> (don't ask), erratic shifts, cooling problems like you outlined, hot
> and cold slipping, etcetra.
>
> I'm glad you told me about that fire problem, because I plan to
> install a Cad 540 and a 3.42 final drive in my coach and see how many
> of those little jelly bean cars I can run off the road with it.
> That's where an on board halon fire suppression would come in handy I
> suppose.
>
> Scott Woodworth

 

[scott woodworth]

Paul,

Hey thanks for the info. I realized that the stock top mounted
Cadillac A/C compressor would have to be relocated. I have experience
with minor fabrication and welding practices that would be needed to
re-engineer the accessory mounts.

You know, if I had to start from scratch, I would go with a one
piece mounting plate to attach all the engine driven accessories to the
front of the motor. That's the way that some of the new pickup trucks
are done for NVH (noise, vibration, harmonics) reasons.

When GM engineers did a study a few years ago on engine noise
sources, they found a significant contributor of NVH was vibration of
engine driven components. When they were mounted on a thick plate, the
noise dropped quite a bit.

Scott Woodworth