VRV and shift questions.

[lolercaust]

Before every one gets butt hurt i all ready tried using the Tm's, but my computer is acting up and wont let PDF files stay open.
So here is what going on my 84 M1028, 1-2 shift is butter 2-3 seems late and hard. I just put in a new B&M adjustable modulator (red ring), filter, inspected and replaced soft lines. All so inspected governor and modulator valve piston. The main problem im having is when pulling vacuum on VRV for proper shift adjustment it DOES NOT hold vacuum. Vacuum doesn't go above 15" and drops like a rock immediately after i stop pumping. Is this normal for the VRV to drop vacuum like that? So i have no idea what the VRV is set at because i cant hold vacuum long enough to get a good reading for a quality adjustment. All so because i cant bring up the TMs, What are the vacuum readings speced at during idle for the vac pump and at the modulator. I found alot of "my truck gets X" but not specified readings. Tomorrow im gonna dig back in and try and get a good VRV setting then adjust the modulator. My goal is to get firm predictable shifts right now the truck hits second an around 25 MPH at 3163 RMP's and third around 38MPH at 2869 RPM's.

 

[patracy]

Have you tried right clicking and downloading the file first? What version adobe reader do you have loaded? It might need an update.

 

[Gripy]

20-25 in Hg right from vac pump. Adjust regulator (VRV; EGR if buying on rockauto) on IP to 8 in Hg. Pushing vrv towards firewall increases vacuum and gives earlier shifts.


I put mine at 12 in Hg for earlier shifts (right before 20mph for 2nd and btwn 28-32mph for) 3rd

 

[Skinny]

Someone can correct me on this but I don't think the VRV is an airtight regulating assembly, I don't think it "holds" vacuum so you can't test it with a pump. All you can really do is see what reading you have after the valve with the engine running. If vacuum drops off the transmission thinks you are under load and will shift aggressively. I would replace it if you have high vacuum near low throttle and it drops off fast, seems to be losing vacuum too fast as you increase throttle angle. It isn't a dialed in electronic trans, a TH400 in the grand scheme is about as stupid as 99% of our population. It does what it is told and cannot think on it's own. It boils down to vehicle speed, kickdown signal, and vacuum to the modulator. That's all it has for inputs.

 

[Warthog]

http://www.steelsoldiers.com/showthread.php?50568-CUCV-Throttle-Vacuum-Valve-Adjustment

You can adjust the valve with the engine running but the TM has you adjust it with the engine off. The valve should hold a vacuum.

 

[Skinny]

I understand that but not everyone has the special tool. I will throw this out there and will upset the diehards because it isn't by the TM and following proper diag protocol but I think we are trying to overcomplicate a rather simple beast.

I'm also assuming most people have a stock non-adjustable modulator so your mileage may vary if you tweaked the screw.

The modulator on a gasser goes right to manifold vacuum on a gasser. This is how the TH400 senses load. That means it will be looking for around 18" near closed throttle and 0" at WOT. The CUCV should replicate the same exact signals. You may need to find tune it to suit your likings but overall that is how to get it where it should be. Your gauge should run between these values when you move the IP throttle. No road test necessary.

If your trans shifts early then late (valvebody not the issue) you have a bad vacuum source signal or the sticky governer.

Cheers

 

[lolercaust]

I adobe reader is only a few weeks past its last update, i think the whole file might be corrupt. I agree with Skinny 99% stupid yes. There is only two parts of this truck im shy about tearing apart the IP (never done one) and i haven't torn a TH apart in a couple years. iv got all the tools i just cant dial in the VRV because it wont hold vacuum. How do i get my 8'' if the VRV refuses to hold vacuum. Worthog i liked your write up on the procedure, as far as this thread goes your the subject matter expert......do i need a new VRV? And are they even worth trying to pull apart and fix? By the way i found new AC Delco regulators on amazon. If you haven't looked amazon is great i bye 90% of my parts from them often at a fraction of the stelership price.

 

[Skinny]

Doesn't hold because it is shot. Mine lacks in performance and smoke testing revealed a leak. It is probably 30 years old, just replace it. I should get one but my rig is hibernating in storage till all the road salt goes away.

 

[truck1]

Check your hard lines . Mine cracked at the firewall from rubbing and I could not see it but had to feel the crack. This resulted in same shifting that you describe.

 

[lolercaust]

Ok i gota run do some stuff but when i get back gonna dive in face first. Ill just pull vacuum on every line and make sure they all hold. So vacuum pump is 20-25" at idle and what should i draw at the modulator at idle? Trying to get every thing good for work monday.

 

[Warthog]

I have never torn a VRV apart but what others have said it is not rebuildable. Sure soulds like yours has crapped out.

 

[Gripy]

8" hg between the vrv and modulator on the trans.

 

[lolercaust]

Alright so i did find a small yet quick vacuum leak at the pumps t junction so new rubber, T and now line clamps (i recommend the clamps). The pump pulls 25Hg like a champ and the VRV wont hold vacuum still so a new one is in order. Final diagnoses is that the leak was not enough for 1-2 to be effected that much, while 2-3 was too weak of vacuum which made it wait until the last second to slam into third. With the whole system 100% leak free less the VRV the truck "shifts" like a CVT. Second is now at 12MPH 1518 RPM and third comes in at only 25MPH 1275 RPM so some fine tuning will get it to spec. I don't care for the mushy shifting and neither does the truck. There are reasons MV's usually shift like the apocalypse, its just good for the transmission. Too soft = excessive clutch ware, too hard = excessive gear ware. if i wanted comfort i wouldn't have bought a odl battle wagon, so tomorrow im gonna chase down a happy medium before im able to replace the VRV. Any one know what the shift points are supposed to be for a reference? i just got inside and haven't searched yet. Before i forget here's a link i found that is a great help for the TH400.

http://forums.highperformancepontia...-wont-shift-out-of-first-when-warm/page2.html

 

[doghead]

I would resolve your TM issue, once that is fixed you can solve the rest easily.