Help with C6 diagnosis please

[Garbage_Mechan]

The 86 in my sig has been sitting almost a year now and I'm beginning to need it. I used to build C-6 trans all the time 25 years ago, but now I'm pressed for time and have lost a lot of my resources now that I'm working in Montana. (trucks back at the ranch in Cali, my son needs it)
A year ago, we had just dropped a completely rebuilt engine with all new supporting parts in. Then my business world imploded and it just sat because of this trans issue.
The issue is that the trans was fairly fresh and running perfectly. Shifting dialed in, shift timing perfect, shifts crisp and firm. It has been filled with synthetic fluid since the last rebuild, and has an external spin on filter and aux cooler. During the truck rehab, we dropped the pan and valve body to replace the shift sector seals which isn't an unusual leak on a C6. The VRV got broken during the engine R&R and was replaced with a new one, and of course the pump was rebuilt so the baseline settings were lost on VRV.
The problem is it has no resonse to changes to the VRV setting, and shift timing is off and mushy, even "sliding" into 3rd instead of the little "kick" it has always had. I've only driven it about 200 miles carefully since I don't want to tear up a perfectly good trans.
I have verified there is good vacuum, and that the VRV is actually controlling the vacuum. Fluid level, shifter adjustment, and kickdown adjustment OK. With a mightyvac hooked up to the modulator (no leak in mod diaphagm) it shifts the same with 0 or 20" of vacuum.

 

[Garbage_Mechan]

So last time I flew home for a couple of days, we dropped the pan on the trans. I fully expected the modulator to be misinstalled, the pin missing, or the valve stuck. Nothing wrong with any of these! There was very little clutch material and no metal or debris in the pan. So, running out of time and needing the spot in the shop, we reassembled and refilled so we could move it around. If I would have had time I probably would have tried to figure out the likely valve in the body and dissassembled.
So when I get home again on the 25th for a few days, I need a good plan. I plan to buy a couple of good test gauges and run the proceedures that are in the Motor trans book. unfortunatly my books don't cover the exact year of the truck. Some of the pressures are very specific to certain model years or enginie options. The worst part is that everyone i have talked to in the trans business said "wow I haven't done one of those in years" and were little help. Does anyone have a Motor's transmission manual that covers 86 ? I could really use the 2 or 3 pages of diagnostics and pressure testing if you do.
Thanks!

 

[towcat]

here's the vrv adjust procedures to begin with.

VACUUM REGULATOR VALVE ADJUST

NOTE: During the check and adjustment procedure, the engine must not be operating.

1. Disconnect two port vacuum connector from vacuum regulator valve located on the left side of the fuel injection pump.
2. Disconnect throttle cable from throttle lever.
3. Remove throttle return spring. Install end of spring over throttle lever ball stud and other end of spring over throttle cable support bracket.
4. Connect a suitable vacuum pump to the upper port of the vacuum regulator valve (vacuum supply side).
5. Connect a suitable vacuum gauge to lower port of vacuum regulator valve labeled TRANS.
6. Apply and maintain 20 in Hg of vacuum to the vacuum regulator valve. It will be necessary to pump the vacuum up as it bleeds off. Cycle the throttle lever 5 times from idle to the wide open throttle position with vacuum applied.

Vacuum Regulator Valve Gauge Block Installation

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1. Insert gauge block No. T83T-7B200-AH (0.515 in) between pump boss and throttle wide open stop. The throttle return spring as repositioned previously, will hold the throttle lever stop against the gauge block tool. The vacuum gauge should indicate 6-8 in Hg of vacuum. If reading obtained is not as specified, adjust vacuum regulator valve as follows:

• 1. Loosen two adjustment screws attaching vacuum regulator valve to the fuel injection pump.
  2. Rotate vacuum regulator valve until correct vacuum reading is obtained, then torque the two adjustment screws to 75-90 in lbs .
  3. If vacuum regulator valve cannot be adjusted, replace vacuum regulator valve and repeat adjustment procedure.
  4. Remove gauge block tool.
  5. Connect throttle return spring and throttle cable.
  6. Apply 20 in Hg of vacuum to the vacuum regulator valve and while maintaining vacuum, cycle throttle lever from idle to the wide open throttle position 5 times . The vacuum gauge should indicate at least 13 in Hg (throttle at idle position).
  7. If vacuum gauge indicates less than 13 in Hg the vacuum regulator valve must be replaced and the new valve adjusted.
  8. After necessary adjustment procedure has been completed, remove vacuum pump and gauge from vacuum regulator valve. Reconnect vacuum connector.
  9. Start engine and check operation of throttle. Check transmission shifts.

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I can post the line pressure data in a couple of days. also, check your kickdown to make sure it's not binding up.

 

[Garbage_Mechan]

Towcat

Thanks for your reply. I'm sure it isn't the adjustment of the VRV since it shifts the same with 0-20 inches of vacuum. Line off or hooked up direct it still shifts the same. I'm thinking it is something that affects the modulator controlled circuit, but downstream of the modulator valve.
"I can post the line pressure data in a couple of days. also, check your kickdown to make sure it's not binding up."
Man it would be so much help to get the line pressure data specific to the 86 diesel, my Motor Manual is a 82, and i havent been able to find the 86 data anywhere.

Thanks!

 

[Wyreth]

have you disconnected vacuum to the modulator and then driven it? It should have hard hard shifts at WOT points.

Also, here is the hydro pressure chart that OldBull was kind enough to post when I rebuilt mine:

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If you're certain it is not the VRV or the modulator. Then a control pressure test is the very next thing on your list, then air pressure tests to localize where your leak is, or to tell you that you have a pump issue.

 

[Garbage_Mechan]

Yep, with hose unhooked, NO vacuum to modulator, it still shifts early and soft. Control pressure tests definitly next. But what valve can it be suddenly sticking to cause this?

 

[Wyreth]

Any of your forward clutch rings leaking will cause low control pressure, or your pump may have given up the ghost.

Sticky 1-2, 2-3, or throttle boost valve will cause shifting weirdness. Hell just over tightening the valve body can cause wacky shift points.

Test your control pressure before speculating what went wrong. That will help you narrow things down before you get it all torn down.

 

[Garbage_Mechan]

I'm thinking it has to be something caused by the R&R of the valve body, either improper torque or debris somewhere. You are absolutely correct that I need to do good testing of the pressures before assuming anything.
It is just really easy to overthink things 1200 miles away sitting in a hotel room. I get home every 2 weeks for 4-5 days and try to catch up everything on the ranch, visit my Mom, love on my wife a little and ****, I'm gone again.

 

[Garbage_Mechan]

WHERE TO GET TEST GAUGE SET?

Looking foreward to the next time I'm home, I want to buy a test gauge set up on line and have it sitting there to use when I get home. I'm thinking OTC or Star less thant $100 for 2 gauges, adaptors and hoses.

Anyone buy one lately or have any tips?

 

[OLDBULL8]

You'll have to depend on WYRETH for info, lost all my data on the C6, computer crash, my fault, went into recover instead of restore. Damn lost everything.

 

[chris142]

Is vacuum reaching the modulator? Sorry I'm auto trans stupid and can't help more.

 

[Wyreth]

Looking foreward to the next time I'm home, I want to buy a test gauge set up on line and have it sitting there to use when I get home. I'm thinking OTC or Star less thant $100 for 2 gauges, adaptors and hoses.

Anyone buy one lately or have any tips?

I got my kit off amazon, it was really really cheap. Or you could just buy a gauge, some hose, and some fittings from McMaster Carr for around $60 and build one.

You'll have to depend on WYRETH for info, lost all my data on the C6, computer crash, my fault, went into recover instead of restore. Damn lost everything.

NOOOOO!!!!

 

[Garbage_Mechan]

I ended up ordering a OTC 5610 kit off of Amazon for 68.95 shipped. I like the rubber protector around the guages. In the old days I would have just built a hose and bought a guage or borrowed one from one of my mechanics. Hopefully I'll have this one the rest om my life. Looks like it will beat me home so I check it out the trans when I get there.

 

[PwrSmoke]

I haven't touched a C6 in a long time but I think Wyreth is giving you good advice. You need to test the pressures first. Seems likely to be in the valve body but exactly what is not clear to me.

 

[trackspeeder]

I haven't touched a C6 in a long time but I think Wyreth is giving you good advice. You need to test the pressures first. Seems likely to be in the valve body but exactly what is not clear to me.

X2. Knowing vacuum is good. The next step would be a pressure test. This would verify if the pump has issues, or something is up in the valve body.