Electrovalves

A recent thread about the carb airbox flap and it's controlling electrovalve made me realize how little I know about that circuit. I did some research and decided to try a drawing to make a little sense about this particular US emissions requirement. Only talking late US carb cars, 1978-1980. I might extend the effort to other electrovalve situations. Will see.

These two systems are described in the 1978 OM for 308 NA cars, 150-78. One controls gasoline vapors from fuel tanks from getting into the atmosphere. Beside the electrical circuit, there is considerable plumbing between gas tanks and carburetors/intake manifolds.

The other system shuts down air pump injection under conditions which would heat the cats to excessively high temperatures. Electrovalves are energized when the engine exceeds 4400 rpm or if either catalytic converter temperature reaches 1508° F.

Electrovalve involved have a steel plunger pushing against a spring in a 1/8" dia 3 way valve. The electro-magnetic force from +12v applied to a coil pulls a plunger back, opening the valve. The electrovalve can go bad by sticking, failure of the 'O' ring, or the coil failing.

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Nice diagram Paul -- I've certainly found it an annoyance that the schematics convey no information about what's really happening in a physical sense when current is, or is not flowing in the electrovalve coil (because how the hoses are hooked up can invert the physical action). Two uber-minor recommendations:

1. You've got the flow paths correct for the two states, and, since you show no flow in the remaining port, it's not technically incorrect, but I think it would help other people's understanding if you had a label like "this port is blocked" or "this port is closed" on the unused port for each state.

2. I know that you are just trying to show that there is some sort of voltage present on one of the terminals when the electrovalve is energized, but, to get ticky-tack, if both terminals were at voltage "V", the electrovalve would remain unenergized. The diagram also (correctly) shows that polarity shouldn't matter (i.e., the plunger is always drawn into the coil regardless of the direction of the current), but, here too, maybe a note like "polarity is not important" might help those less familiar with this stuff than yourself.

Just some suggestions...

 

Wow... Paul, thanks for taking the time to diagram this out...That really fills in the details left out of the manuals...now it all makes some sense!

Cheers
Mark