Wiring a Lockup-Converter Pressure Switch

Peek under a hot rod today and it's not uncommon to find an automatic overdrive resting between the frame rails. These transmissions are affordable, durable, and easy to install. With an overdrive transmission and 500 hp, fuel economy exceeding 20 mpg can be a reality. But, before you install that new TH2004R or TH700R4 tranny in your early hot rod, there are a few things you should know about how to make them work. Both the 2004R and 700R4 use a lockup-style torque converter that requires 12 volts and a pressure switch to lock up in Fourth gear. This eliminates the need for a manual switch. When retrofitted into early muscle cars, these trannys require the pressure switch to be changed and a few wires rerouted. Changing the switch and wiring can be done in the car without a problem. If the switch is not changed and wired properly, the converter will not lock up in fourth gear and possible trans and converter damage may occur, not counting your gas mileage will suffer.

In the original CHP article that I read, they never mentioned the low-vacuum switch or brake cancel switch. You need the low-vac switch so the converter will unlock in low vacuum situations, such as going up hill and heavy throttle. The brake cancel switch unlocks the TCC when the brakes are applied. Trucks that came stock with 700R4 trannys came with a low-vacuum switch and a brake cancel switch. The stock low-vac switch didn't let the TCC unlock soon enough with this mod. I checked the vacuum on the stock switch and it kicked in at 3.5", meaning the vacuum has to get real low before it will unlock. The vacuum switch I recommend works real well as it kicks in at 7.5", which will let it unlock sooner. The brake cancel switch on 700R4 equipped trucks also disconnects the cruise control (if equipped) when the brakes are applied.

Are you are doing this mod for one of the reasons I did, to get rid of some of the wiring and vacuum hoses that came on our trucks. When I removed the EGR bleed solenoid and related wiring, I had to rewire my 700R4. This is one of the reasons I did this. If your truck came with a 700R4 stock, the only parts you will need to buy are the normally open/closed pressure switch PN GM8627332, single pin TH400 kick-down connector switch PN GM24205373, and the low-vacuum switch PN GM14014519.

Since writing this article, I've had a lot of inquiries about it. The photos in the article are of a 2004R tranny. They both use the same principle to lock in 4th. They look different so it is confusing to some. These photos were taken from a Chevy High Performance magazine article, not by me, well except for the last one, photo D, which I did take. Also, there are many different variations of valve body's with several different types of pressure switches and wiring schemes. Early 700R4's locked in 3rd also. This is the reason they have so many pressure switches. My 89 model has only one. The 4/3 pressure switch. If you have the early type, you may have as many as 4 pressure switches, 1 beside the solenoid, and 3 at the rear. The pressure switch which needs to be changed is the center, rear one.

Now again, with so many different variations of these things, you may also have a single wire solenoid. If this is the case, instead of the GM #8627332 pressure switch listed above, which is what is needed for the two wire solenoid, you will need GM #8643710 pressure switch. It's around $4.00 at the GM dealer. To wire the single wire solenoid, go from the TH400 connector to the 8643710 pressure switch, then to the solenoid. I think this will work on any of them. I'm not 100% positive. I'm no tranny expert, but I believe this to be true. My 700R4 came from a 89 Chevy 1/2 ton pickup. It only had one pressure switch. It came with a two wire solenoid also. I changed my solenoid to a one wire type, GM #8654123, and used the GM #8643710 pressure switch. Either way works the same. There is no difference in the way it locks. I just happened to have a new one wire solenoid and was changing the fluid and decided to put it in. It didn't change a thing from the two wire one in the way it locks in 4th. If you do this mod, you change to the new pressure switch, depending on which type of solenoid you have, and leave the other switches in the valve body. Remove the old wiring, but save everything in case you want or need to put it back like it was.

Now, this is only a belief of mine. I have absolutely no proof, other than what I have seen. Early 700's locked in 3rd also. I know for sure this to be the case. Why, I don't know, but they did. I firmly believe this is one of the reasons early 700's died so quickly. People use these trucks to haul and tow. Not knowing any better, people would leave it in OD when doing this. So what happens is, it locks and unlocks, over and over again. This will kill them in hurry. It causes them to overheat, which is the number one killer of 700R4's. So, then they heard to leave it in drive (D) to keep it from doing this. This won't let it go into 4th of course, but the older ones still locked in 3rd (drive). So, in my mind, they are still locking and unlocking. Maybe not as much as being in OD, but I think they will still do it. I may be wrong, but so what, I've been wrong before. GM changed this sometime down the road. Not sure when exactly. My 89 isn't this way.

Again, just to let everyone know. I am no professional mechanic or tranny mechanic. I believe all things said to be true. They are to the best of my knowledge. I hope it all works for you.