I recently bought my wife a 1985 K5 Blazer. The heater core was bypassed and I wanted to get the heater working again.

I installed a new heater core and I am also replacing the radiator, water pump and hoses.

I have a few loose ends that I would like to find out their original connections.

First, I am replacing the heater hoses and I measure at the core the lower hose at 5/8” and the upper hose at 3/4”. Which hose goes to the intake manifold and which hose goes to the radiator? Does it matter which hose goes where?

Is there a vacuum or cable operated shut-off valve in one of the lines from the heater core to the intake/radiator? If so, which line gets the valve, which line is the supply line and which line is the return line?

In the picture Item 1 is a small hard plastic line coming out from the firewall along with a bunch of factory-wrapped wires. I think that this is the vacuum supply to the controls in the cab.

Item 2 is a solenoid or relay or something with a two-wire plug on one side and a vacuum line coming out of the other end, I don’t have a clue as to where this would go or what it does.

Item 3 is a vacuum line coming from a vacuum canister mounted on the far driver’s side and is a ball shaped canister. There is a vacuum line running from this ball to a tee that is opened on the other two ends. I think that one side of the open tee goes to the vacuum tap on the manifold and the other end of the open tee to Item 1, the line that runs into the cab.

 What’cha think???

The VIN is 1G8EK18H4FF*****

The GVWR as listed on the door jam 6100/2767

The GAWR is 3550/1610

The engine has just been rebuilt and is a 350 out of a 1971 Monte Carlo.

I just measured the outlets on the radiator (3/4") and intake (5/8") and I will hook them up to the same size outlets on the heater core.

I connected Item 1 to the manifold vacuum tap and the cab controls work fine. I am going to tie in Item 3 so it will run from the manifold vacuum tap to a tee, one side goes to the canister, and the other side goes inside the cab for the controls.

Item 2 eludes me. The wire colors are solid brown and solid yellow. Some kind of trip for when a certain vacuum is reached? Maybe a trip for some transmission function? How about a signal for the ECM? Hook it up to a vacuum source and see what happens? I am sifting through some wiring diagrams now, but with no luck so far…