700r4 only shifts into 4th once or twice...then never again

[burbanator]

Hey all working on my 1990 1500 suburban with 700r4. It will only go into 4th gear once or twice when you 1st start driving it. Will go into 4th if you coast at highway speeds but the second you get back on throttle it drops to 3rd. All other gears and lockup work fine.
While waiting on pressure gauges, I dropped pan today and besides bit of clutch material on magnet it was spotless. Fluid was decent and not burned.
While I was in there I tried to apply air to the bleed hole right under the servo that is supposed to apply 3-4 band. I am getting a ton of air bleeding out a port on valve body when I do this "test". I stumbled across this randomly looking for anyone that has fixed this issue online.
Not sure if this is normal or if its bleeding off too much pressure. I would think it should hold more pressure.
Made a quick vid showing what I found here...

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Not sure if this is normal or maybe I found my issue?
I also tested the 4th gear pressure sensor for lockup by applying air to it and it shot fluid across my shop so it was leaking...replacing it did not fix my problem though.

I am running out of ideas...might try to replace the servo! Hoping someone might shed some light on this!

 

[burbanator]

well I hope I am on to something! finally found a vid showing this air test...and it looks like it should not bleed by so much air like mine is.

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about 26:30 shows the air test. Now...is it the servo thats leaking air by...or something screwy in the valve body????

 

[idahovette]

@NickTransmissions please help this guy out??

 

[burbanator]

hah! did not know he was on this site! been hitting his youtubes hard this week trying to figure this out!

 

[NickTransmissions]

hah! did not know he was on this site! been hitting his youtubes hard this week trying to figure this out!

I replied to your question on Youtube a little while ago...Bottom line - you need to confirm the circuit is mechanically healthy by air checking the 4th apply cirucit via the case which involves removing the valve body, spacer plate, etc...The little hole on the pan rail is an exhaust port - you can air check the band there but it's a lot easier to do w/the VB, etc out of the way...The inlet port for the 4th gear apply servo is on the case belly, near the 2-3 acc check ball capsule.

As far as the valves in the valve body, the 3-4 shift valve has to be stroked via circuit pressure to enable 4th gear - that isn't happening unless the vehicle is on the road. So no surprise you see fluid leaking from the VB, which is why you need to remove it to do the air check.

I suspect you have either a 3-4 upshift valve problem or perhaps worn teflon sealing ring in the servo piston for 4th gear (it's inside the servo cover). The other possibility is the band is simply worn though you typically have issues in 2nd gear first, before they materialize in 4th gear.

 

[burbanator]

Rad Thanks Nick! Im thinking since its cheap and way easier than dropping VB while laying under it hahah... I will rebuild the servo 1st and see if that fixes things...if not then go in deeper!
Ordered up a viton seal set...said it was better for sealing in a worn bore...seemed like a good idea...12 bucks worth a shot.

 

[burbanator]

Rebuilt servo with new seals...no dice. Must be something in valve body. Working up my courage to tear into it.

 

[NickTransmissions]

Rebuilt servo with new seals...no dice. Must be something in valve body. Working up my courage to tear into it.

I have had this issue with various 700R4 valve bodies over the years (no 3-4 upshift) and while it's likely the 3-4 shift valve, 3-4 sequencing valve and/or 4-3 relay valve, you would need to have the Sonnax reamers to ream the bore for the 3-4 relay/4-3 sequence valve train to install their oversized set of valves to fix that condition. If the problem is the 3-4 shift valve, I don't think there are repair valves available. You will likely end up swapping the valve body out.

 

[burbanator]

I have had this issue with various 700R4 valve bodies over the years (no 3-4 upshift) and while it's likely the 3-4 shift valve, 3-4 sequencing valve and/or 4-3 relay valve, you would need to have the Sonnax reamers to ream the bore for the 3-4 relay/4-3 sequence valve train to install their oversized set of valves to fix that condition. If the problem is the 3-4 shift valve, I don't think there are repair valves available. You will likely end up swapping the valve body out.

Do you have any suggestions on best place to get a reman valve body? I have been thinking thats probably best bet as I am mostly shooting in the dark here with my lack of knowledge about auto trans.

 

[Sad Sack]

Are these guys still in business:

Portland Transmission Warehouse
1016 SE Hawthorne Blvd
Portland, OR 97214

503-233-4966 or 800-444-4556

[email protected]

 

[NickTransmissions]

Do you have any suggestions on best place to get a reman valve body? I have been thinking thats probably best bet as I am mostly shooting in the dark here with my lack of knowledge about auto trans.

Call Central Valve Bodies in Oklahoma or Valve Body Exchange. I am not sure Sonnax is making those but you can try them as well.

I stock pile VBs for my builds and have the customer swap them in and out until one works. I let customers know this up front so they're aware a bit of work on their part may be necessary if the existing VB doesnt work.

 

[burbanator]

Right on thanks for leads. As much fun as I am having laying on a creeper under a dripping trans I think trying to just swap it all out is best bet for my sanity!

 

[Sad Sack]

Right on thanks for leads. As much fun as I am having laying on a creeper under a dripping trans I think trying to just swap it all out is best bet for my sanity!

Maybe a better attack angle -

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[burbanator]

I have some 4x4 around someplace...

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