I don't know if this information is needed but my truck is an 85 K5 Silverado w\305 and 700r4. There are two of these things on the firewall and I don't know what their function is or even what they are called. The one in the picture is on the left side of the firewall beside the heater box. The other one is located behind the brake booster. The one behind the booster has a vacuum line attached to it but doesn't go anywhere and this one doesn't have anything on it. My truck doesn't have the vacuum set correctly and is backfiring through the carb like a popcorn machine when it's running. I don't know if these have anything to do with it or not. I have a prior thread about the defrost vacuum and finally found and attached it's vacuum line. It now goes to defrost but doesn't appear to have enough suction to completely kick it over. It is still coming out the floor vents too. If anyone can guide me in finding out what these are and whether they are important to the proper function of my truck I would be greatly appreciative. On a side note, does the oil and temperature gauges run off of a vacuum or are they completely electrical? They are both laid over as far to the right as they can go and just stay there so I'm not sure if it's tied into my problem or not. The temp sensor has 2 different ends coming off of it but I can only find the matching end for 1 part of it. Thank you for your time.

 

the sensor in the pic apparently is the lock up for your torque convertor.
look at this thread i found
Vacuum and electric thing on my firewall?? What is it?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network


also found on another thread
"that's a vac operated switch to lock up the torque converter in the old transmission. Shouldn't need it with an electronically controlled trans"


I need to go to bed lol. My head hurts.

 

Thanks for the info. According to one post in that thread, this one is for the ECM which I believe has all been removed from this truck and the other one is the lockup, but I don't know if it is needed or not. We put the entire driveline into this truck and don't even know what was originally in it. Nice to at least know what it's for. Thanks again.

 

I doubt you need it. What trans did you put in?

 

It's a 700r4 Tranny.

 

What did you do about the converter lock up?

 

I'm honestly not sure exactly what you're referring to. The only things we hooked up was the control cable from the tranny to the carburetor throttle plate and the 4 plug wire that connects to it. Is there a place where a vacuum line should be attached to it and run up to this connector?

 

700r4 shouldnt need that

 

That 4 wire plug is what controls your lockup. If that vacuum switch is hooked to the 4 wire plug, you dno't have any torque converter clutch lockup right now since the switch never senses enough vacuum to activate the TCC. You don't need TCC lockup, but your tranny can get really hot in 4th gear and your highway mpg will suffer.

The EFI trucks ('87+) control lockup with the ECM, but the vacuum switch approach works just as good if the switches are functional.