3 ported vacuum switch set up
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From: Indiana
3 ported vacuum switch set up
Truck: '78 F150 Ranger Lariat; 351m; calibration: (?) 72 11
Found vacuum diagram on forum. The green copied diagram that's difficult to see. It appears the yellow 3 ported switch on thermostat housing is set up: hot port going directly to vacuum amplifier; center port to distributor advance; cold port to 4 way tee, then to manifold vacuum.
QUESTION: Is this right? If so, understand why Vacuum amp/EGR would be hooked to hot port (EGR should open at higher temp) But if vacuum advance is on cold port, how would I get manifold vacuum on hot day at idle when cold port is closed?
Understand basic theory of 3 ported switch is to have hot port on manifold vacuum, center to advance and cold port to ported vacuum. What am I missing here?
Thanks for help.
Found vacuum diagram on forum. The green copied diagram that's difficult to see. It appears the yellow 3 ported switch on thermostat housing is set up: hot port going directly to vacuum amplifier; center port to distributor advance; cold port to 4 way tee, then to manifold vacuum.
QUESTION: Is this right? If so, understand why Vacuum amp/EGR would be hooked to hot port (EGR should open at higher temp) But if vacuum advance is on cold port, how would I get manifold vacuum on hot day at idle when cold port is closed?
Understand basic theory of 3 ported switch is to have hot port on manifold vacuum, center to advance and cold port to ported vacuum. What am I missing here?
Thanks for help.
MSEE
Joined: Apr 2004
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From: Austin, TX
You are correct that a 3-port PVS transfers one common port to one of the two others based on temperature, but I don't recall which port is which. They are most commonly used on vacuum advances. For an EGR system, the only real essential piece you need is a 2-port PVS to keep vacuum from going to it before the engine warms up.
Can you send a link to the diagram in question?
Can you send a link to the diagram in question?
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https://www.google.com/search?q=351m...qq_ZdNjFapyBM:
fmc400, here is link, I hope. If you can't see this, I'll try to add link to MilLion pic from his gallery.
fmc400, here is link, I hope. If you can't see this, I'll try to add link to MilLion pic from his gallery.
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Look at the diagram again - hot port (passenger side of pvs) there is a T at the front of the carb and connects to the S port on the carb. Then continues to the vac amplifier. Center port to the distributor. Cold port (drivers side of pvs) to manifold vacuum - 4 way T if you are using the stock air cleaner.
If you have photoshop you can sharpen the image.
Mark
If you have photoshop you can sharpen the image.
Mark
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D24X4, thanks so much for the help. On diagram, there is a short vacuum line coming from carb spark port. At the end of that line there is an engineer note that says "cap". Are you saying that's a three way splitter with a cap on the end; the other ends intercepting the line coming from hot PVS port to vacuum amplifier?
Also, regarding the cold/passenger side port on PVS, I thought you always wanted vacuum advance on dist. coming from ported vacuum when cold and manifold vacuum when hot? With this set up, vacuum advance always comes from manifold vacuum.
Lastly, I noticed a reference in the diagram to a "blue" vacuum restrictor at the top of the 3 ported PVS we're talking about. What's up with that?
Also, regarding the cold/passenger side port on PVS, I thought you always wanted vacuum advance on dist. coming from ported vacuum when cold and manifold vacuum when hot? With this set up, vacuum advance always comes from manifold vacuum.
Lastly, I noticed a reference in the diagram to a "blue" vacuum restrictor at the top of the 3 ported PVS we're talking about. What's up with that?
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From: Phoenix, AZ
For the T at the S port on the carb - I don't know why it says cap. Just use a regular 3 way T.
The pvs works off of temp and controls the vacuum source. I can't tell you exactly how it does this. Maybe FMC400 can explain it. Just run your lines the same as the diagram.
On my pvs - drivers side port is blue - restrictor is built in.
You should take the time to run all new vac lines too.
Mark
The pvs works off of temp and controls the vacuum source. I can't tell you exactly how it does this. Maybe FMC400 can explain it. Just run your lines the same as the diagram.
On my pvs - drivers side port is blue - restrictor is built in.
You should take the time to run all new vac lines too.
Mark
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