Backup Light and Clutch Questions

[jrmunn]

Both of these questions seem to fit in the Transmissions category.

First, my 1964 3/4 ton GMC pickup with a 305e V6 that I am trying to get back on the road is a very basic model. It is a no frills, working truck that has given me good service and a number of headaches over the past 35 years. I now have time to work on long delayed maintenance and improvement projects, and one of these is to add a backup light. The transmission is a four speed (three highway and low range) with the shift lever coming out of the floor. Can anyone tell me if this transmission has a reverse switch hiding under the accumulated grime, or if one can be added, to activate a backup light?

Second, the clutch catches and causes the vehicle to "buck" if the clutch is not carefully and slowly released. This is especially bad when in reverse. I had the clutch worked on at a GM truck dealer about 17,000 miles and many years ago, but the problem came back. Is this a common problem? And what is the likely cause - warped plates, worn bearings, or something else? I would like to have some idea of what is causing the problem before trying to fix it.

Thanks ahead for any information and suggestions. I enjoy reading about other people's work. It helps to know that I am not alone in trying to keep a good machine working.

JRMunn

[raycow]

67 was the only year that the SM420 had an integral reverse light switch. Earlier years, if you ordered the option, had an externally mounted switch actuated by a tab attached to the shift lever. This arrangement was somewhat fragile and finicky to keep in adjustment.
http://www.oldchevytrucks.com/blog/i...-light-switch/

Fortunately, the cover on any year SM420 can be drilled and tapped for the later type switch. This gives much more reliable operation.
http://www.kinzers.com/don/landcruiser/sm420rev_switch/
http://www.kinzers.com/don/landcruis...420_sender.jpg

As for your clutch shudder, the most likely cause is oil getting onto the disc facings, so remove the bellhousing cover and look for oil on the flywheel or pressure plate. Another possible cause is a broken mount which allows the engine to move when under load.

Ray

[jrmunn]

Ray,

Thank you for the information about a backup light switch and suggestions about my clutch problem, including the term "shudder" that better describes what is happening. Dropping the transmission to make internal modifications is more work than I want to take on for a backup light, plus I always seem to have to do things over to make them work. But this does let me know that there is not an existing switch and gives me an idea about how to connect one that would be turned on by the shift lever (different than the old style floor switch). Looks aren't that important for what I am using the truck for, but I am tired of backing up blind at night.

I can also see how oil on the clutch would be a problem, but don't know how it would get there? The truck does not leak oil at the back of the engine or under the transmission. I will look for broken mounts, but a mechanic should have seen this at some point during the years that the shuddering has been a problem.

Thank you again for the help!

JRMunn

[FetchMeAPepsi]

You could always rig up a toggle switch if it's night driving that's causing the trouble. Flip it on and BOOM Floodlight.

[raycow]

Originally Posted by jrmunn

Dropping the transmission to make internal modifications is more work than I want to take on for a backup light

You don't need to pull the transmission in order to to install a switch. All you have to remove is the transmission cover, and you can do that from above after removing the floor hump.

As for the clutch, the best way to see if oil is getting into it is to remove the bellhousing bottom cover. You want to make sure that everything inside is totally dry.

Ray

[jrmunn]

Fetch... - The toggle switch idea would work if I had an open spot on the fuse block that was only hot when ignition switch is on. Otherwise, for me, it is an invitation to a dead battery. There is also the problem of forgetting to turn it off when the truck is going forward. But this is my backup plan if I can't manage to install a backup switch.

Ray - Thank you for letting me know that the floor hump can be removed. I hadn't figured that out. And I will check the clutch as you recommend.

Thanks to all.

JRMunn

[6066gmcguy]

A very basic model in 1964 would not have Back-up lights, they didn't come standard tell 1966. but they were an option from 1960-65. Myself I just wire in a switch on the dash to run the lights and turn them one when needed. install a pilot light next to the switch so you know when you leave them on. also hooking into the ignition switch is a good ideal too.

[George Bongert]

Hey JR!

One more thing you might check as to your clutch shudder and grab is loose transmission to bell housing bolts. As I recall, you have to remove the bell housing "pan" to tighten the bottom bolts, since they screw into the lower "ears" on the transmission, and the upper bolts screw into the bell housing itself. If your bolts are loose, tighten the top bolts first. I encountered this same problem myself many moons ago, and found the described bolts to be loose, and it doesn't take much to throw things out of whack and cause the problem you are describing. Outside of that, if there is no oil leaking from either the engine or transmission, you might have a warped clutch disc, which in my experience isn't too likely.

[jrmunn]

George - Thank you. I will check the bell housing bolts along with looking for oil on the clutch disc. At the moment, I am working at replacing the bed, which has exposed several unexpected repair "opportunities." It is also encouraging to be able to see how well built the pickup is.

JRMunn