Vacuum switching valve required? '76 280Z 5spd

[NicholasKoenig]

I have a 1976 280Z non calif, with a 5spd (one of the previous owners must have installed the trans as 1976 did not come with 5spd). 

I noticed the wires on my vacuum switching valve were completely toast and I cant find a replacement part anywhere. To my knowledge the vacuum switching valve adjusts timing for 4th gear. Yet since I have a 5spd, is this switch even necessary? Does the trans have a top gear switch that tells the vacuum switch to engage past 4th? I took it apart and can attempt to clean and resolder the connection if its still needed. The car has only idled in the driveway since I bought it in the fall so I have no idea how it behaves while running through the gears. 

Thanks

 

[Zed Head]

It actually only opens the vacuum line to the vacuum advance mechanism on the distributor.  It's an emissions thing, page EC-9.  I think that, generally, the view is that retarded timing gives cleaner exhaust.  They must let it actuate for highway cruising because it gives better mileage.  Which also gives cleaner exhaust.  I removed mine and just let the vacuum advance work in all gears.  I think it adds a little throttle response since the timing is generally more advanced at mid-range RPM.  

If your 5 speed doesn't have the top gear switch, it's useless anyway.  There are a whole range of switches on the various 5 speeds so it's impossible to guess what you might have.  The 83 ZX has the most, I think, you could check that FSM.

[NicholasKoenig]

20 minutes ago, Zed Head said:

If your 5 speed doesn't have the top gear switch, it's useless anyway.  There are a whole range of switches on the various 5 speeds so it's impossible to guess what you might have.  The 83 ZX has the most, I think, you could check that FSM.

Is there a quick way to distinguish what transmission it is while its still in the car? 

Edit: To clarify, you removed the switch itself and just ran a straight vacuum to the distributor? 

Edited April 1, 2017 by NicholasKoenig

[Zed Head]

Yes, just ran a hose straight to the distributor.  Make sure that it's the correct hose, from the bottom of the throttle body.  Ported vacuum.

You can determine your transmission, or you can look for the switch.  Find the wires sticking out from the passenger side of the transmission.  One will certainly be the reverse light switch.  If there are more, disconnect the wires (if they're connected) and use a meter to determine when the switch is closed (continuity) and when it's open.  If you have a checker that beeps you can just connect it then climb up and put the lever in to fifth and listen.  

You might that there's only one switch and the decision is easy.

 

[Captain Obvious]

Zed, On a five speed OD trans, what's the difference between a "top gear switch" and an "OD gear switch"? 

[Zed Head]

I've wondered the same myself.  Maybe a translation thing?  OD has to be 5th because it's OD right?  Maybe top is the highest before you get to OD?  The followup question might be "what would you do in 4th that you wouldn't do in 5th?"

Interesting (to me anyway) thought question, aside - why is overdrive significant?  It's really just one factor in a string of multipliers.  At the end of the full calculation, it's essentially meaningless what that single ratio is.  But it sure does look like somersetting in the advertisements.  I think it's one of those buzzy words that just sounds impactful.  Example - "Bachman-Turner Overdrive".  Much more exciting sounding than the Bachman-Turner band.  But close to ultimate meaninglessness.  They don't even play that fast.

[7tooZ]

Final drive (5 gear) on the 71B above is 0.864 or 0.773 or 0.745 depending on the build date. I always thought the OD switch was to activate a light? Is that correct?


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[NicholasKoenig]

Late on the reply but I've been wrenching on other parts of the Z lately. 

I decided to take a look at my trans tonight because I noticed my clutch pedal was way soft. Checked the reservoir and it was empty, looked under the car on jack stands and it seems as though the clutch slave clyinder leaked it all out. So I'll be replacing that! 

So my trans only has two switches I can see but I did some digging around and found helpful graphics on identification. Unfortunately it seems like my trans is the less desirable wide ratio. I'll post the graphics and my trans pic. 

 

[Captain Obvious]

I've got the same wide ratio as well. The first four gears are identical to the four speeds, and even though fifth isn't a huge change from fourth, it's still way better than not having fifth at all.