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cadmanof50s 08-09-2011 10:14 PM

700r4 Vacuum Switch Cycling
I added an adjustable vacuum switch to my 700r4 lock-up system. It works just the way I want it..when under load and manifold vacuum drops the TCC disengages to give the extra RPMs needed.

However.. I have run into a problem that apparently is very common to this system. The TCC cycles back and forward when under light load...i.e. when you just want to maintain highway speeds. the TCC locks and unlocks in a constant cycle.

The answer is simple: I have read that an 8 sec vacuum delay will resolve the problem. It gives the engine time to stabilize the RPMs and vacuum after a lock or unlock.

I even have the part number: GM 14020691 ...but it is discontinued. Anyone have a good source for this obsolete part? Or how about a substitute part?

Will spark delay module work as well? Not sure how many seconds a spark delay will provide.

Any help appreciated.


Overdriv 08-10-2011 08:01 AM

Here is one, that might work. No mention on delay time, but cheap enough that it might be worth a try.

Mine does the same thing. I just adjusted the cutout point to a lower vacuum setting to about 5-6 inhg and it only disengages the TCC only when you really step on the loud pedal. My engine has enough low end torque to pass a slower moving car without TCC disengagement or downshift. But with the AC on, it does tug it down to where a TCC disengagement would be beneficial.

Please follow up on this and post your fix.

cadmanof50s 08-10-2011 03:11 PM

Thanks Overdrive,

Even with the relay adjusted to the lowest trigger point,
(turns on at 6 in of Hg), it cycles. Its really too bad as it would be a great solution if it wasn't for this cycling issue.

I purchased a BorgWarner equiv ..but it did not work as is basically a one way check valve..and there is no delay that I can see.

The way it should work is that there should be a delay in applying a high vacuum condition to the switch. A low vacuum condition would be immediate, a high vac would be delayed.

Thanks again!


jmichaelre 08-10-2011 03:29 PM

Or you could do what I do.....I have the lockup switched so that it can't come on at lower speeds and I don't have to worry about it cycling. Mine was doing the same thing to me. I have no more problems.

cadmanof50s 08-10-2011 05:05 PM

Thanks jmichaelre

I have mine connected so that it only comes on in 4th. So this typically happens at highway cruising speeds.


jmichaelre 08-10-2011 08:23 PM

I have mine switched for automatic fourth as well but I have the power to the solenoid switched so that I can turn it on only at higher speeds. If I didn't I would be driving around town between 50 and 35 and having it cycling.

It works for me. You can always give it a try.

cadmanof50s 08-11-2011 04:15 PM

OK.. I fixed the problem.

For the life of me I could not find a Vacuum Delay module that will delay the vacuum like the GM 14020691 is supposed.

So I built a small electronic circuit that basically waits 8 seconds after the vacuum switch closes before it gives the TCC the 12 volts that it needs to lock.

What this does is allows the engine time to stabilize RPM and vacuum long enough after it preventing it from locking up with the rebound vacuum. It works great..although I have to wait 8 seconds after 4th gear is reached before TCC locks....I don't think that's a bad actually feels just about I have a 5th gear.

I still need to figure out just what the right vacuum trigger point for unlocking the TCC...but the cycling is gone! I'm putting a vacuum gauge in the car tonight and taking it for a long drive to do some fine tuning.

JohnTN 08-11-2011 06:31 PM

I don't think time delay is your problem. Sounds more like the vacuum set points for ON and OFF are too close together. These are only made up numbers, but the vacuum switch should disconnect TCC (OFF) when vacuum hits some low value like 6-7 psi, and then not reconnect (ON) until vacuum reaches some higher value like 10-11 psi. This assumes normal vacuum under light load is in the 13-16 psi range.

There needs to be a significant gap between the ON and OFF triggers. If they are close together you will get cycling.

Which set point are you able to adjust? If its the turn ON point then it should be higher (like 10-11), not lower (like 6-7).


cadmanof50s 08-11-2011 09:24 PM

Thanks John,

The circuit delay definitely solved the problem. The vacuum switch that I have is adjustable but the distance between off and on set points are fixed. You are right that is the issue...however since I can't make the on/off gap wider, a delay circuit effectively widens the turn on and turn off point (electrically) to the TCC.

I tested out the circuit tonight ..put about 60 miles on the car and it works great! I put a vac gauge in the car and found the point at which I want the lock to release. This was at about 6 in hg. Now when I want to unlock the TCC, I just push the gas a little more than required to maintain speed. At that point manifold vac drops briefly to about 5 in and then rebounds. At (or below) 6 in Hg point the vac switch opens and removes 12 volts from the TCC. When the Vac switch closes again (because vacuum rebounds), my circuit waits another 8 seconds before it lets 12 volts through to the transmission. The cycle issue is cured. Going up hills is a breeze: as soon as you lean into the gas pedal a little, the vac drops, the TCC unlocks and lets you finish the hill before re-engaging...

And now I have control over the TCC lock with just the gas external switches to toggle, no touching the brake to unlock it, it doesn't jump into lock as soon as 4th is hit. I am actually very happy with the results.


JohnTN 08-11-2011 10:20 PM

I don't fully understand how the time delay solved the problem, but glad it works now.

On my 2004R I put both a time delay switch and an RPM switch in the circuit. The RPM switch allows the TCC lockup to only activate when RPMs exceed a set point, like 1700 rpm. And it unlocks the TCC when a lower set point is reached, like 1200 rpm. So in most driving situations the TCC lockups up between 45 and 50 mph and unlocks if speed drops below 40 mph.

So my circuit has 6 controls: brake switch, toggle switch, 4th gear switch, vacuum switch, rpm switch, and time delay. I haven't had to touch the toggle switch since putting in the rpm and time delay switches.

This past week I drove my 1939 Olds to the NSRA Nationals in Louisville and averaged 21 mpg on the interstate.

And, thanks for not correcting my useage of psi instead of inches Hg in my post. Should have caught that.


cadmanof50s 08-11-2011 11:14 PM

John, as you so very astutely pointed out, the problem is that the turn on and turn off points of the vacuum switch are too close together. Unfortunately that can't be changed.

The circuit holds off applying voltage to the TCC after the vac switch closes again. This has the same effect as widening the vacuum switch turn off/turn on points. I'm just doing it electrically.

I was going to add a toggle switch as you did, but I'm too lazy of a driver. I wanted it all done by the gas pedal...:-)

BTW, I am happy to post a diagram of my set up ..if any one wishes to see it...just gotta figure out how to post a pic.

Thanks again, John.


Overdriv 08-12-2011 08:00 AM

cadmanof50s, would you care to share the circuit you have made that fixes the cycling?

cadmanof50s 08-12-2011 08:02 AM

1 Attachment(s)
For sure, Overdriv.

Should be an attachment.

Let me know if you have any Q's about it.

kikkegek 07-08-2012 11:24 AM


Originally Posted by cadmanof50s (Post 1455845)
For sure, Overdriv.

Should be an attachment.

Let me know if you have any Q's about it.

man! THANK YOU very much ....I'll build this myself tomorrow and solve my problem, thanks to you dude!


kikkegek 07-08-2012 12:55 PM

@ cadmanof50s: thanks man! how do you vary the time? bigger resistor or capacitator? maybe even make it variable?

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