Trans Shift Points

[ssdvc]

OK, I replaced my vacuum pump and vacuum hoses the other day. Before I did this, my trans shift at 17mph (1st to 2nd) and 32mph (2nd to 3rd). After installing the new vacuum head, shift point are 30mph (1st to 2nd) and I have gotten up to 45+mph and she won't shift into 3rd.

I have moved the black "vacuum actuator") on top of the IP both forward and back and have not gotten any changes. What else should I look at? Also, which way to adjust that "vacuum actuator" thing. Towards the firewall does what? Towards the radiator does what?

Thanks.

 

[WWRD99]

OK, I replaced my vacuum pump and vacuum hoses the other day. Before I did this, my trans shift at 17mph (1st to 2nd) and 32mph (2nd to 3rd). After installing the new vacuum head, shift point are 30mph (1st to 2nd) and I have gotten up to 45+mph and she won't shift into 3rd.

I have moved the black "vacuum actuator") on top of the IP both forward and back and have not gotten any changes. What else should I look at? Also, which way to adjust that "vacuum actuator" thing. Towards the firewall does what? Towards the radiator does what?

Thanks.

Swap the hoses at the vacuum switch on the throttle...mine was swapped and figured it out after a bit of testing...vacuum goes in the bottom comes out the top...if you have a vacuum gauge it'll make setting the switch easy.,,I set mine at 11 hg at idle running and it shifts great.

 

[Skinny]

Sounds like it has no vacuum at the transmission. Classic symptom you are describing.

 

[ssdvc]

I'll check the hose tomorrow, after I get back from the VA. Thank God I have two cars (well, I have one and my wife lets me borrow hers .

 

[ssdvc]

Swap the hoses at the vacuum switch on the throttle...mine was swapped and figured it out after a bit of testing,..if you have a vacuum gauge it'll make setting the switch easy.,,I set mine at 11 hg at idle running and it shifts great.

Hoses are in the correct position, but I am going to try the swap anyway, just to see what happens. Everything worked 100% before the vacuum pump head swap.

 

[ezgn]

Hoses are in the correct position, but I am going to try the swap anyway, just to see what happens. Everything worked 100% before the vacuum pump head swap.

Get a vacuum gauge if you don't already have one. Otherwise, you are just shooting in the dark. Actron CP7803 vacuum tester at Amazon $15.00

 

[cucvrus]

Are you sure the vacuum pump head swap is creating correct amount of vacuum? With the engine idling and the hose disconnected at the vacuum pump it will chirp or suck your finger tip fast to the vacuum pump exit line. Check it out. A gauge works also but I got by without one for years and still take it a reputable transmission shop to have it properly set up if I can't get it correct. My guy passed last year so I am in search of another shop. around here they are no as hard to find as other places. But the old timers that work on these old rucks are getting far and few. Most shops plug and play and that works great on new stuff. The old stuff requires much more diagnostics and many hit and miss parts replacements. next year will mark 40 years since I seen the first CUCV. They were new and improved at the time. Now they are old and can be a real PITA after all the modifications that owners have done to them. Good Luck and Take Care.

 

[ssdvc]

OK, got a vacuum tester. And yes, without, I get a strong suction at the end of the hose. Vacuum at idle is reading 25 hg in from the pump head. What else should I check?

 

[ezgn]

OK, got a vacuum tester. And yes, without, I get a strong suction at the end of the hose. Vacuum at idle is reading 25 hg in from the pump head. What else should I check?

Test it coming out of the vacuum valve down at the modulator. At idle I believe it should be at 12-15.

 

[ssdvc]

I got vacuum at the modulator. I did the the Transmission Vacuum Valve adjustment laid out in the TM, section 3-21. I have a solid 8 HG from the valve going to the trans modulator. During the adjustment, when I moved the Vacuum valve, the vacuum did increase and decrease with movement, so the valve appears to be working as it should.

The only thing left is the modulator on the trans or the trans itself, which would not make for a good day. Could that trans modulator be the issue?

 

[ezgn]

I got vacuum at the modulator. I did the Transmission Vacuum Valve adjustment laid out in the TM, section 3-21. I have a solid 8 HG from the valve going to the trans modulator. During the adjustment, when I moved the Vacuum valve, the vacuum did increase and decrease with movement, so the valve appears to be working as it should.

The only thing left is the modulator on the trans or the trans itself, which would not make for a good day. Could that trans modulator be the issue?

Is your modulator a newer one or an older one? I believe the modulator should hold at about 20 H.G. You have removed the line to the modulator and don't have any transmission leaks? l thought it should read between 12-15 HG. The valve could be your problem. Did you check the vacuum down at the modulator, or at the valve. I would disconnect the hose at the modulator and hook your gauge up there and see what you get. You may need a new valve. If your modulator is not functioning, that could indeed be your problem.

 

[chevymike]

Sounds like the trans modulator is bad at the trans. I had this same problem and if that goes bad, the TH400 will shift based on RPM's, which is very high, resulting in the shifting MPH your are seeing. I swapped mine out and replaced a couple rotted hoses and it worked perfectly.

I don't recall if there was a difference between the diesel and gas engine models but for some reason I think they were different.

 

[ezgn]

I got vacuum at the modulator. I did the the Transmission Vacuum Valve adjustment laid out in the TM, section 3-21. I have a solid 8 HG from the valve going to the trans modulator. During the adjustment, when I moved the Vacuum valve, the vacuum did increase and decrease with movement, so the valve appears to be working as it should.

The only thing left is the modulator on the trans or the trans itself, which would not make for a good day. Could that trans modulator be the issue?

If you need a modulator, and you can spend it on a good one. B&M 20234 Adjustable Vacuum Modulator

 

[cucvrus]

Are you sure the modulator is hooked up? It has a small metal line that plugs into it and a small, short rubber hose to serve as a seal. I wish I were closer.

 

[ssdvc]

Modulator is hooked up, modulator has vacuum to it. According to the TM, vacuum at the trans should be about 8 hg in. It is seeing that, so I am now trying to hunt down a modulator. I only have tomorrow to get it done, then the truck is going to sit for about two months while I recover from surgery.

Will this modulator work? https://www.napaonline.com/en/p/ATP17499?impressionRank=1

 

[ssdvc]

Or this one? https://www.oreillyauto.com/detail/...olet-k5-blazer?q=transmission+modulator&pos=0

 

[MrMikey4026]

Just put direct vacuum to the modulator, if it shifts you need a new vacuum regulator. When I worked for GM, I had a loop hose made I could just plug in place of the regulator to bypass it. If it still does not shift, replace the modulator.

 

[ssdvc]

The vacuum regulation valve next to the IP is working and I can regulate vacuum when doing the procedure outlined in the TM. I will pick up a modulator first and try that. $20 vs $140.

 

[ezgn]

The vacuum regulation valve next to the IP is working and I can regulate vacuum when doing the procedure outlined in the TM. I will pick up a modulator first and try that. $20 vs $140.

Try with a cheaper part to see if it corrects your problem, and you can upgrade to a quality part later.

 

[ezgn]

Modulator is hooked up, modulator has vacuum to it. According to the TM, vacuum at the trans should be about 8 hg in. It is seeing that, so I am now trying to hunt down a modulator. I only have tomorrow to get it done, then the truck is going to sit for about two months while I recover from surgery.

Will this modulator work? https://www.napaonline.com/en/p/ATP17499?impressionRank=1

If it says

Modulator is hooked up, modulator has vacuum to it. According to the TM, vacuum at the trans should be about 8 hg in. It is seeing that, so I am now trying to hunt down a modulator. I only have tomorrow to get it done, then the truck is going to sit for about two months while I recover from surgery.

Will this modulator work? https://www.napaonline.com/en/p/ATP17499?impressionRank=1

If it says it will fit a TH400 transmission then it will fit.