GMC Western States

Tech Center Number 27 - December 15, 1999

FROM UNDER THE SHADE TREE IN ALBUQUERQUE

By Chuck Aulgur

The Albuquerque "Balloon Fiesta" was another super rally.

Al Chernoff set the bar pretty high; it will be hard to top. We had a schedule full of great tech sessions. It was a tough choice to decide which ones to attend as there were so many competing activities going on at the fairgrounds and all around town.
The sessions started with a very informative presentation by Jim Staskus, the owner of a performance shop in Albuquerque that specializes in RVs. His topic was what you need to look for in a shop if you need engine work. Several of our members made appointments to get their GMC checked out after the rally, and several of us took a tour of his very impressive shop. The main thing he pointed out was the need to know who is going to do the work on your engine. Make sure they are a reputable engine builder who has the proper equipment, knowledge and experience. He stated that a high percent of repair facilities send the engines out to a job shop to be rebuilt. The job shop business is very competitive, and a lot of them cut corners to keep the cost down. That is not what you need if you want your GMC engine to have an extended trouble free life. You hear of too many GMC owners, who after having their first engine overhaul, have had to have it done again within a few thousand miles. Some never even made it home.

There were two "First Timer" sessions which were well attended as we had a lot of new people attending the rally. Many came from the southeastern states. We had an informative session on "Indispensable Upgrades", and also a session about all the information that is available on the Internet. A good place to start is the GMCWS web page. You can also search for "GMC Motorhomes" and find a multitude of information on various web sites.

There was a "Bring Questions" session where people wrote down their questions, and the "Panel of Experts" had a chance to review them. Whichever panel member thought they could answer did so. This format worked out very well and there were requests to have similar sessions at further rallies. People felt they learned a lot from these types of sessions.

We had sessions on the Onan control board, how to convert the Onan to electronic ignition, air systems, brakes, additives for your engine, and the cooling system. A lot of informative material was presented. Anyone who has technical items they would like to present, or see presented at future rallies, should contact the Tech V. P. or the Rally Master.

I would like to thank the GMCWS members for electing me to be their technical representative. I will strive to maintain the high standards set by my predecessors.

Crankshaft Failures and Torque Converters

We are reprinting below two letters of expert transmission rebuilders that may shade some light on the problem of premature failure of engine crankshafts. These letters are from some time back but we find them still pertinent today.

Letter No.1

Torque converters do not cause crankshaft failures in any instance. In every case a rebuilt engine is part of the assembly. Whenever it is just a transmission and torque converter overhaul, nothing fails.

When the engine is rebuilt and the transmission is replaced, the thrust bearing burns out. Strangely it only happens when the engine is rebuilt also. The factory designed the converter, transmission and engine assembly to live 100,000 miles or more. Why then, when rebuilding only the transmission, is there no problem? But when the engine and transmission are being rebuilt together or the engine is rebuilt alone, the engine thrust bearing fails? GM tells us to look at the engine rebuilders as poor import bearings can be a cause as well as a badly ground crankshaft or a poor finish. When I ask them about micro finishes on a crank thrust surface, they appear confused. "Burnishing! What is burnishing?" they reply.

        Reground crankshafts under a microscope look like a file to a thrust bearing when improperly finished. Also, improper line boring (bad parallel of engine block main saddles and main caps) caused by hand grinding of main caps in preparation for line boring can cause trouble.

In conclusion, concerning crankshaft finishing, Chick Wilson of Chick Wilson's Crankshaft Grinding of Pomona has already died and gone. He could have helped in this problem as one of the premier automotive machinist of the age.
Converters in themselves cannot balloon; they cause no pressure of their own. The pressure in the transmission and converter is made by the front pump alone. The converter is a slave to the transmission and cannot balloon itself. If a converter balloons, it is because of excessive pressure (stuck regulator) and this usually never results in a lost thrust engine bearing.

Analysis: to have an action (worn out crank thrust) you must have an equal reaction (a destroyed front pump stator support). This has never happened in my life as a rebuilder. Front pump regulators do stick, converters do balloon, rarely to be sure (not the fault of the converter but high pressure), but when this happens, the motor thrust bearing never fails as a result.
In reality excessive main pressure together with poorly reground crankshafts will cause premature failure of crankshaft thrust bearings. The torque converter "problem" came up only when the engine rebuilder would not accept the blame and the transmission rebuilder would not either. So they blamed the part they do not understand, the converter.

In every case of crankshaft thrust failure I have examined, many people blamed converters ballooning. Upon examination they were not ballooned and never had been. Front pumps were not damaged from lateral growth of converters and transmission pressure was acceptable (shift kits included). However, engines with their reground crankshaft were worn out.

In second conclusion, use a genuine GM crankshaft kit (crank and bearing) and your problem will go away.

John White Vice President RPM Merit Inc. Azusa CA

Letter No. 2

        It is not uncommon to have crankshaft failure on GM cars with THM 400 Transmissions after the engine has been rebuilt.
Here is what happens: the torque converter has a hub that fits the transmission pump and turns the pump gear. Since the hub is open, there is less surface area on the transmission side of the torque converter than the engine side (approximately 1.76 square inch less). When the torque converter is pressurized it wants to move towards the engine. Example: if the torque converter were pressurized to 100 psi, it would exert 176 pounds of force on the crankshaft.

This is not unique to the THM 400. All transmissions do this. The difference is: most transmissions have a limiting system that will not allow torque converter pressure to exceed 90 psi. The THM 400 does not have a pressure limit system for the torque converter. Because of this the THM 400 puts a lot of force on the crankshaft during high-pressure conditions (heavy throttle). This is normal. This is not a malfunction of the transmission. Dogs bark, cats meow and the THM 400's push on the crankshaft with a lot of force.

For this reason, the crankshaft must be machined VERY precisely. Crankshaft grinding methods that work on other applications will not work when the engine is to be used in front of a THM 400. This why the transmission is always blamed for this failure, you don't see the problem with other applications.

To answer your question about coolers: yes, a restricted cooler can aggravate this situation. When cooler flow is restricted torque converter pressure goes up. Here how you test it. Tee a pressure gauge into the outgoing cooler line (top line). If cooler pressure stays below 120 psi you are OK. This equates to 211 lbs of force.

If the engine cannot withstand this force it is an engine problem. We have found this to be the case most of the time.
We have found that getting a factory rebuilt or used (never rebuilt) engine usually fixed the problem.

Dennis Madden
ATRA Technical Director
Ventura, CA

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