What is RPO code M-40

[54blackhornet]

Hi guys ! I got the 71 GMC started yesterday, 1st time run since 06. Had to bypass fuel tank, installed new fuel pump, fuel filter 1 gallon of non-ethnol and it fired right up. No smoke no weird sounds no leaks My question is what trans do I have? SPID is marked M-40. When temp gauge hit half way I put it in drive and NO Nothing ? All gears BUTKUS ? WTF ? Any help would be most helpful Thanks
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[VetteVet]

The m-40 on the spid indicates that it was a th 400 from the factory.

http://67-72chevytrucks.com/vboard/s...d.php?t=543275


Here's the trans pans to ID them.




There are several reasons the transmission won't go into gear. Low fluid, valve body, front pump, linkage. torque converter, so it's hard to diagnose on the net.

[bronze 72 super]

M 40 should be th-400 trans. Check to see which way the vacuum modulator on the right side of the trans faces. If it sticks stright out its a th400. Towards the back its a th350. Check your tranny fluid make sure its full. Might also be a plugged filter, or a bad pump, had that happen before just like you described. Hope this helps Mike.

[SS Tim]

Are you sure it says M40 and not M49? M49 was the turbo hydramtic spid rpo call out be it a TH350 or a TH400. On the build sheet it would call out the trans. by its specific code like CJ would be a TH400.

[special-K]

There is no M40 RPO code in 67-72,only M20 for 4spd and M49 for 3spd automatic,and M35 for Powerglides...as Tim mentioned. The SPID doesn't distinguish between TH350 and TH400,both are the same RPO...as Tim said

[VetteVet]

I googled Gm RPO codes and came up with this----http://www.rpocodes.com/GM-RPO-codes/gmrpocodesGthroughM.html

The RPO M38 lists a TH350 automatic
The RPO M39 lists a Borg Warner 4 speed
The RPO M 40 lists a three speed automatic. Other links suggest that it just means heavy duty transmission and can be either a 350 or 400 with the 400 almost exclusively behind the big blocks.

[SS Tim]

Problem here is I think you are seeing a list out of context. Though the M38 and M40 were used later post 1972 their useage is not correct for the 67-72 MY range. Often RPO codes sites are compilations without reguard to MY or line. As an example look up the L47 we know and love as a 396/400/402 BBC is clearly not the description given. Nor is the M49 that reads as a TH350 4x4 application completely correct. If you were to go to chipflyers site about half the codes there are for heavier trucks, but a M38 TH350 is listed there. http://outintheshop.com/options.html

The only automatic transmission codes I recall seeing in a light-duty (C10-C30, K5-K20) U.S. market have been listed as M49 for turbo hydramatic transmissions and M35 for the powerglide transmissions. If this truck has an RPO code M-40 on the SPID I would really like to see it. If in fact it does, it may possibly be a Canadian market truck or a "fleet" special order vehicle.

Below is a clip from '46-72 GM parts manual showing the RPO codes in the low "M" range. Being as this is a final '79 publication errors may have crept in.

Also is a page showing the '72 powertrains in C10-C30 trucks
The turbo hydramatics are exclusively called out as M49

[yuccales]

Originally Posted by SS Tim

The only automatic transmission codes I recall seeing in a light-duty (C10-C30, K5-K20) U.S. market have been listed as M49 for turbo hydramatic transmissions and M35 for the powerglide transmissions.

Are the GMC and Chevy SPID codes different? I've got a 350 trans in my '70 C-10 and it's listed as M38 3spd auto trans.

[Lee H]

Lets see that SPID marked M-40.
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[leddzepp]

All 6 glovebox SPIDs I am looking at in front of me say M49, regardless of TH 350 or 400. The op probably misread the 9 as a 0...or the person that typed it up put in the wrong number, which is also a possibility.
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[mrein3]

Originally Posted by 54blackhornet

Hi guys ! I got the 71 GMC started yesterday, 1st time run since 06. Had to bypass fuel tank, installed new fuel pump, fuel filter 1 gallon of non-ethnol and it fired right up. No smoke no weird sounds no leaks My question is what trans do I have? SPID is marked M-40. When temp gauge hit half way I put it in drive and NO Nothing ? All gears BUTKUS ? WTF ? Any help would be most helpful Thanks
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Don't worry about it not moving after putting it in gear for a couple six years. Autos don't sit well. The chances are it puked all the tranny fluid out of a dry seal. Get it back up to temp and check the tranny dip stick. Be prepared to dump 2+ quarts in. If you don't mess around with it too much when its low on fluid you won't hurt anything.

It also wouldn't hurt to buy a bottle of tranny stop leak. If indeed one of your seals dried out this will help it a bit.

Then buy one of those large baking pan looking things at the auto parts store to catch automotive fluids. Until you pull the tranny and fix that front seal it will continue to leak when sitting.

[54blackhornet]

I Stand Corrected ! My eyes are not as good as they used to be ? What looked like M-40 IS M-49 ? Sorry for the mistake guys ! I did find a lot on the net about a M-40 trans though ? Kind of confusing to me, glad I have you all to set things right as usual.....
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[SS Tim]

Are anyone's! Thanks for the update. At least we know and this may very well help someone else. As far as the trans. goes check the fluid level and a bottle Lucas Oil's trans. conditioner my very well help the seals.

[54blackhornet]

I just wanted to update you all on my M-49 transmission situation, after putting over two qts of fluid from NAPA and 1/2 qt of Lucus for transmissions she moved under her own power! Have not driven it on road yet , still working on fuel tank. Thanks to all for the help, I thought the trans was junk
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