Why do I have a transmission kickdown relay?

[Nutro]

I have the fuel pump relay on the firewall and next to it is a transmission kickdown relay. I have an 87 TBI truck with the 4.3L and a TH350 trans. I think (Not near the truck right now) the truck came with a TH350 new and I think that is what is in there now. It's either that or a TH350C.

I attached a pic I found on the internet to show what I am talking about.

 

[CSFJ]

The truck probably came with a 700r4. I don't think the th350 was still available in '87, let alone a 350c. Are the RPO codes still in the truck somewhere? (glovebox door, under hood on a fender well?)

 

[Nutro]

Here are the only 2 items in the glovebox

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[1987 GMC Jimmy]

MX1 = 3 speed auto so TH350C

I didn’t think they came with that in 1987, either, but maybe this one’s 11/86 but 1987 model year or something. If the sticker’s still on the driver door, that’d be interesting to see just for reference. I say C because as far as I know, all TH350s model year 1980 and newer had a locking torque converter clutch, hence the ‘C.’ The ECM controlled lockup after the vacuum controlled circuit went away. That’s a bit strange unless there’s some little known fact about these trucks I’m not aware of.

 

[bucket]

It's odd, v6 trucks often came with a th400 too. Did the later years use a relay for the electric kickdown?

 

[Nutro]

Okay this is odd. It says MX1 and M40 the M40 is a TH400. I just crawled under the truck and it is a TH400

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[chengny]

I think that in later years - when certain transmissions were being phased in and out - the MX0/1 RPO's became a catchall code for any automatic transmission.

MX0 MERCHANDISING OPTION ONLY-DO NOT CATALOG
MX1 MERCHANDISING OPTION ONLY-DO NOT CATALOG


Even the description for the M40 code (traditionally associated with the TH400), seems intentionally vague in the RPO lists.

It is generally only identified as below - that or something similar:

M40 TRANSMISSION,3-SPD AUTO

From the 1987 Service Manual:

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But the service manual ultimately identifies RPO code M40 as a THM 400:

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Daniel, this options sheet shows the exact power team as originally installed in your truck - right down to the GU4 (3.08 axle):

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And here is a description of why the relay is there - it's basically because the ECM wants to control downshifts:

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[75gmck25]

M40 often just means automatic transmission. My '75 K25 RPO sticker shows M40, but its a TH350 from the factory.

Bruce

 

[Nutro]

I posted on Facebook and everyone there says it's a TH375. Everything I've read says that GM only used those in the 75-79 range. The data tag and vacuum modulator is on the passenger side and it has a long tailshaft on it but it's obviously a TH400 case. I am so lost.

 

[Nutro]

Here are some pics

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[Keith Seymore]

It came with the TH400 (hence the kickdown switch).

MX0 Is the merchandised code for an overdrive transmission; MX1 is the merchandised code for a non-overdrive automatic transmission.

The merchandised code is the one you would pick as a customer by saying “I want an automatic transmission.”

The non-merchandised code, M40,is determined by the engineering release charts and is driven by model, powertrain, GVW, etc. and is not something you get to choose.

As an aside: My friend bought an 87 V6 2wd step side and it came with the turbo 400 trans . It was definitely available and even in a much lighter duty truck.

K

 

[Matt69olds]

The 375 is a 400 with a smaller spline like used with the 700, 350, etc. it has less clutches in the clutch pack, and is basically a light duty 400. It’s no big deal to change output shafts and extension housing, and machine the pistons to fit more clutches in the drums. There is also a 375(B, I think) which is a heavy duty TH350 trans. It has 5 clutches in the direct drum instead of the usual 4

The TH475 has straight cut gears. Usually they are found in RV, or other extreme duty applications. The advantage of the straight cut gears is less pressure of the planetary gears trying to spread the case. The disadvantage is they are noisier than the standard gears

 

[71RAT]

I have the fuel pump relay on the firewall and next to it is a transmission kickdown relay. I have an 87 TBI truck with the 4.3L and a TH350 trans. I think (Not near the truck right now) the truck came with a TH350 new and I think that is what is in there now. It's either that or a TH350C.

I attached a pic I found on the internet to show what I am talking about.

Most likely a TH375/400 kick down is PCM controlled.