94 SC400 Heater Control Valve?
Lexus Test Driver
Joined: Aug 2011
Posts: 1,024
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From: NM
^ make sure you have vacuum , i did the "fix" and nothing then later fiddled with it again and noticed the valve was working(by pressing on it with my finger) and then took the vacuum line off it to realize i had no vacuum at all, went straight off the manifold and viola ..heat!!
How did you notice the valve was working by pressing on it? Also which line should I be checking for vacuum, the vsv line? When you found that it didnt have it, you just t'ed off one going to the manifold you're saying? Thanks in advance. It just got a little cool out here in FL didnt it?
RE: post #3. Does anyone know the purpose of the little black vacuum tee on the hose running from the VSV to the actuator? I had been under the assumption it's a check valve, same as the bigger yellow/black one running to the VSV. But I looked at it today and it appears to just be a reducer for air volume?
Pit Crew
Joined: Aug 2007
Posts: 188
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From: ca
Mine is ice cold too, driving it out of the 40 degree high Desert, I was expecting to see snow flurries! Me and my kid had a blanket on....how about just running some long vacuum lines into the cab and putting a little valve on them? open it and you have vacuum to the pot, close it and you dont, open a little and you have a little....Dont know if the electric switch thing is variable or pulse width (oscillating). If I knew the hose reroute trick, I would have been doing it in the Barstow In-N-Out parking lot.
Pit Crew
Joined: Aug 2007
Posts: 188
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From: ca
$5 at pick-a-part, just grab the electric vacuum switch. Its a phillips head screw under the solenoid itself, kinda hard to get to unless you bend the dashpot away and unhook the arm as your only going to use the solenoid. pull the harness off the metal finger and itll come right off. Hit the contacts with 12V at the test station and see if it clicks, if it does, buy it. I pulled 3 and all 3 worked but I only bought one. I bought one off a Honda for $7 off Ebay, same switch, different plug. anyone need it? Just splice your old wires into it. hardest part about installing it is getting the screw tightened. Just use a stubby. I also got a 6 disk changer out of a 95 ES300 but then found a 12 CD changer out of a 94 SC, works! It was looking pretty bad so I got the warranty but I dusted it off and it fired right up.
Pit Crew
Joined: Jan 2011
Posts: 180
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From: MS
^^ Not to my knowledge. Pishta's post a couple of years ago about the Honda HCV VSV is the first I'd ever heard of that mix and match. He never followed up on my question either so it's very hard to say exactly what VSV from what Honda was used.
Theoretically it is possible that the same VSV with the same electrical and pressure holding/release specs was offered by Denso to another OEM besides Toyota but we would have to know what Honda that VSV is from to make a determination. The only constants are that the VSV will operate under normal vacuum pressure with a check valve along the line that feeds it from the intake manifold and that it requires +12V to either open or close the valve. That is, unless there is some other duty cycle it is designed around that we are not aware of (and Toyota+Denso certainly aren't telling us).
But in the general/universal sense it's not an overly complicated thing as far as it is designed to operate. As we look into the future for alternative aftermarket HCV's and possibly VSVs for the SC300/400 the only thing that really has to stay the same is a heater control valve VSV's ability to communicate with the computer that tells it to open or close its valve (to operate the big diaphragm and its plunger arm connected to the actual heater valve) and for how long at any given point. As far as I am aware it requires +12V just like most of them on this car.
That having been said... you either have a working OEM VSV or a working/compatible alternative VSV in your hands or you don't.
Theoretically it is possible that the same VSV with the same electrical and pressure holding/release specs was offered by Denso to another OEM besides Toyota but we would have to know what Honda that VSV is from to make a determination. The only constants are that the VSV will operate under normal vacuum pressure with a check valve along the line that feeds it from the intake manifold and that it requires +12V to either open or close the valve. That is, unless there is some other duty cycle it is designed around that we are not aware of (and Toyota+Denso certainly aren't telling us).
But in the general/universal sense it's not an overly complicated thing as far as it is designed to operate. As we look into the future for alternative aftermarket HCV's and possibly VSVs for the SC300/400 the only thing that really has to stay the same is a heater control valve VSV's ability to communicate with the computer that tells it to open or close its valve (to operate the big diaphragm and its plunger arm connected to the actual heater valve) and for how long at any given point. As far as I am aware it requires +12V just like most of them on this car.
That having been said... you either have a working OEM VSV or a working/compatible alternative VSV in your hands or you don't.
Last edited by KahnBB6; Dec 3, 2019 at 07:06 PM.
Pit Crew
Joined: Aug 2007
Posts: 188
Likes: 6
From: ca
Oh crap, necropost by me! The thing I used was just the electric servo part, not the entire thing. You must wrestle it into the SC bracket but its just an electric servo that opens and closes the vacuum path to the vacuum actuator. Sorry for the late reply. Dont know what Honda I picked it out of, I was just looking for an easy pull and test.
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