Dana Wilkes from Probuiltauotmatics.com said I should ask you folks for the guidance on troubleshooting my issue. I am working on a bone stock 1988 G20 Chevrolet sport van (it is my parents). It is a TBI 5.7L. It has a 700R4 that ate it's 3/4 clutch pack. So it got a probuilt heavy duty rebuild with an Edge HD converter. I have done a lot of TH400, 350's and powerglides so I know my way around a GM trans for the most part. But this 700R4 is kicking my a@#. It is installed and shifts great. My issues is the lockup. at 40 mph, it goes in and out of lockup. If I unplug the electrical connector to the trans, it shifts into all 4 gears without lock up just fine. With the plug connected, if I go 45+ it stays in lock up unless I tap the brakes. I have been trying to troubleshoot this electrical system on the van with no luck in solving the problem. Below is what I have done:

1. Verified TV cable geometry and set correct WOT position (is a new cable)
   
2. Changed brake light switch since van has cruise control and they share the switch for lock up.
   
3. Check power to the VSV on the instrument cluster, a good 12vdc is there. Also check chassis grounds.
   
4. There are no OBD1 codes and the check engine light is off (yes it has a bulb
   

Now, there is one defect. I don't know if this is the issue. My Dad had changed the rear end out a few years ago due to a trashed pinion. He installed a rear end out of a truck. The speedo in the van reads 5 mph slow now. Could this be messing with the VSV and causing the lock up issue? I can't find any literature on this VSV system with OBD1 vehicles. Any troubleshooting steps I am missing, please let me know. I am pulling what's left of my hair out over this one.

Thanks,

Jimmy

 

What year is the THM700-R4 Transmission?

What do you have operating the TCC Circuit? ...Stock ECM?
How do you have the TCC Circuit wired? ... Stock?

Dana did not seem to think, that you could have a hydraulic issue regarding Lock-up?

 

The trans is the original one to the van (1988). ECM is stock, no mods. The TCC should be untouched. From the looks of it, none of the wire harness is messed with. The only thing that has been changed is the rear end. The speedo is off 5 mph on the slow side (35 mph is actually 40 mph) so it's safe to assume it's not the same gear ratio.

He didn't think so. But if I did, what am i looking at? It seems to stay locked up at 45 mph or faster. You can tap the brakes and hear the engine change. One other thing I did notice today, when I started it up, I get a brief whiff of trans fluid. Nothing leaking but it made me wonder if I am not bypassing fluid internally. I pull the dip stick while running but don't see any foam on the stick. It might be nothing but I though I would mention it.

 

Ecm bcc ? Wonder if there is a bug with that $42 calibration

 

If it is staying in lockup with a lot of throttle. Then this would be an electrical issue. With more throttle (4 inches of vacuum or less) the ECM should not be sending any power to keep the torque converter clutch on. Since I am not familiar with the ECM's on these vehicles, I would not know where you should look as to solve this. Someone who is familiar with this setup should be able to diagnose this.

 

Well if testing the TCC function electrically first is what we want to do, then so be it.

Can the Car be placed on a Lift, or raised high enough off of the ground (Safely) to get under the Transmission?
If you can remove the Oil-Pan well enough to hear the TCC Solenoid activate/ de-activate (Clicking)...
I will post instructions for testing the Solenoid and the Transmission Internal Wiring-Harness.

If that goes well, you will need to have a Digital Multi-Meter to test the external Wiring.

 

Interesting. This doesn't help what so ever, but my dad had a 1987 Oldsmobile Cutlass Cierra with a 2.8l fwd. It did the same thing and some mechanic looked at it and told him to unplug the lockup connector. It drove around town fine for years. Never did find out what the problem was.

 

A few years back, [the late 1990's] I swapped a TPI and 700r4 into an S10 blazer and had the same issue . At that time there was a product called Shiftrite that plugged in between the harness from the ECM and the trans. It prevented the lockup until fourth gear and stopped the yoyo effect. I have no idea if it is still available.

 

Crane Cams also had one. IIRC, inside is a diode.
I will look for the link.
Found it: https://www.thirdgen.org/forums/tran...ml#post6207190

 

Since the TCC (Torque-Converter Clutch) is operated by a 12v Solenoid (simple Electric-Circuit) and nothing Electronic...
The TCC Solenoid can be wired in many different configurations.

For a street-car, I personally would connect the Positive side of the Solenoid to a 4th-Gear N.O. Pressure-Switch, the Case Connector, through the Brake-Pedal Switch, and to any switched (ignition) power-source.
The Negative side goes through the Case Connector, through an adjustable Vacuum-Switch (Ported-Vacuum), and to the "clean" Ground-Buss.

-Now the TCC applies (locks-up) when the Transmission shifts into 4th-Gear, once I let off of the Throttle.
-If the Throttle is pushed, the TCC releases.
-If I push the Brake-Pedal, the TCC releases.
-If the Transmission down-shifts, or is down-shifted, the TCC releases.

It works wonderfully!
An override-Switch could even be wired in a few different ways...
Then the TCC could be applied in 2nd, 3rd, or 4th-Gear, when the Driver wants to apply the TCC.

 

Thanks for the info. Good stuff. Do any of you think this speedo being off could be causing this?

 

The speedo sensor reads on the output shaft of the trans, so changing the rear end ratio wont effect the speed that the sensor is reading. Does the ratio effect your actual speed? Most definitely, but the change is after the sensor. No worries there

 

I'm gonna shoot for the stars here, but do any of you have the wiring diagram for an 88 van? If not, do you know what wires out of the case plug do what? Being a van, I can drive it and have a meter on the trans case wiring harness to see if I have some sort of voltage problem coming from the van to the trans.

 

An 88 Van, no I haven't got that... But, I wouldn't be at all surprised in the least if the wire colors are the same between a GM van with a 700R4 built in 1988, and a third gen F body with a 700R4 built by GM in 1988.

Here is a 1988 F body diagram showing the TCC wiring. You should have +12V at the purple wire at the transmission plug whenever the key is on and your not stepping in the brake pedal, and the ECM will ground the tan/black wire when it deems it appropriate for the TCC to engage.....

 

My guess is that whomever rebuilt the transmission changed the wiring under the pan. With the TCC lockup now being controlled via the 4th gear (OD) switch. I'd start by dropping the pan and looking at the actual wiring.

In stock form the TCC lockup is ECM controlled. With the 4th gear switch going to the ECM as an input to it. This way the ECM knows which set of lockup tables/values to use.

RBob.

 

I was the one who rebuilt it. I took pictures of the wiring prior to removing the VB and reassembled it via the pictures. But at this point, I agree I will drop the pan and give it another look. It's defiantly something I screwed up since it worked for many years until it was rebuilt. See the attached picture, it's prior to disassembly. I assume the orange plug is the 4th gear switch and the silver one is the temp switch. Not sure what the green one is... Any ideas?

 

I had to do some look-see, but have a good idea of the situation.

From the top of the pic, the blue-greenish connector is the 4th gear (OD) switch. It goes to both the ECM (as an input) and to the thermo switch. To lock the TCC with over temperature conditions.

The center switch with the two red terminals is a 4-3 downshift pulse switch. It unlocks the converter for a moment on a down shift from 4th to 3rd.

The lowest switch with the multi-wires is the thermo switch. It's purpose is to lock up the TCC when the trans is overheating. It works in conjunction with the 4th gear switch. In affect the trans needs to be in 4th (OD) for the thermo switch to be able to lockup the TCC.

If you don't find a pinched wire or such, check that the thermo switch is open. If closed that will cause lockup in 4th.

The 4th gear switch should also be open, until the transmission actually shifts into 4th gear.

RBob.

 

Cool deal, thanks for the info. Would it be advisable to just replace the 4 gear OD pressure switch and the 3/4 downshift pressure switch? A quick look online shows about $10 each. The temp switch I can test. Do you know at what temp the temp switch engages? NO or NC? Do you have GM P/N's for the switches. I kind of guessed using Rock Auto but there seemed to be a lot of variations in model years going down in the 80's.

 

The 4th gear switch and the thermo switch are both NO.

RBob.