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Discussion Starter · #1 ·
Can I use a S.P.D.T. relay to turn on a N.O.Pressure sw. when a N.C vaccum sw turns off.What I have is a 700r4 trans.,the 4th. gear pressure sw. & soli.valve are n.o. & grounded to the case.Im using a ome vac.sw. that is n.c.The vac.sw. turns it self off when engine vac. gets to7.0 ins. of vac. that when I need the relay to send voltage to the pressure sw.In other words the relay sees that the vac.sw. has turned off and now switchs on the pressure sw.Can one relay do this or is it going to take two.This is the relay w/ 87 & 87a.Im just not seeing this in my head.
 

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I do not quite understand your exact needs about want you want to happen, but I think I can help.
If the switch is on, and you actually want an off condition, hook the switch to the coil, and hook the load to the 87A pin on the relay. 87A is NC, (on) without the coil energized, so if power is applied to the coil, 87A is open.
 

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Discussion Starter · #4 ·
Yes,it hard to explain.The trans needs to go into lockup at higher vac.such as above 8 ins.of vac.The OEM vaccum sw. (n.c.)will only turn on at low vac. 0 to 7ins of vac. Then when you have hammered the gas,vaccum drops below 7ins.of vac then turns on the OEM vac. Sw. thus sending voltage to relay to turn off voltage to pressure sw. & soli.valve,(n.o.) Thus turning off lockup.Then when your back to steady cruise vac. goes above 7ins. of vac. OEM vac sw. turns off,relay sees no voltage an turns on pressure sw. & soli. valve an now were locked up againe.So in short OEM vac.sw. (n.c.) im guessing will be connected to n.c. contacts of relay? And pressure sw. & soli. valve (n.o.) will be connected to the other contacts on relay.I guess I should of spended the 100.00 or so for the kit,yes...yes.. I know I should have done it,but its like everything I do ” the hard way”.
 

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....So in short OEM vac.sw. (n.c.) im guessing will be connected to n.c. contacts of relay? And pressure sw. & soli. valve (n.o.) will be connected to the other contacts on relay.
Nope. The vacuum switch switches the relay's control (coil) circuit (terminals 85 and 86). The pressure switch and solenoid get their power from the relay's NC (87a) terminal.

In the diagram above:
Relay terminals 85 and 86 are the control circuit. If you power one and ground the other, it will trip the relay.

Relay terminals 30, 87, and 87a are where the work is done. Terminal 30 is known as the "common" terminal. When the relay is not tripped, terminal 30 is internally connected through the relay's contacts to the 87a (NC - Normally Closed) terminal. When you 'trip' the relay, the 30 terminal's internal connection changes from the 87a to the 87 (NO - Normally Open) terminal.

In this case, fused key-on power is supplied to both the 30 and 86 terminals. The 85 terminal gets grounded when the normally closed vacuum switch senses vacuum less than 8 inHg. During this low vacuum condition, the relay is tripped, and sends power from the 30 terminal to the 87 (which isn't connected to anything). Meanwhile, the 87a terminal (which is connected to the trans switch) is dead.

When the vacuum increases to the point that the NC vacuum switch opens, the relay coil loses it's ground and the relay un-trips. When this happens, power supplied through the 30 terminal is passed to the 87a (NC) relay terminal, thus sending power to the transmission pressure switch. If the transmission pressure switch is also closed, power passes through it to the lockup solenoid, engaging it.

Hope that makes sense...

Good Luck :thumbup:
 

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Discussion Starter · #7 ·
Thank you JoeG.Working with relays & normaly closed & open switchs makes my head spin. Funny thing is I helped the electriction wire in the house I had built a few years ago,it hasnt burned down just yet.
 

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Discussion Starter · #8 ·
I just got around to reproducing this on my work bench & it worked,I used a blinker bulb as my pressure sw.One other question , my Camaro did come with cruise control that was turned on & off by the brake sw.To cut power to the relay should I use no.30 or no.86 on the relay.The cruise control is out of the car.Thanks for your time.
 

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Depending on how you have it wired, removing power from terminals 85 or 86 will de-energize the relay

Terminals...

30 common
85 one side of the coil (either Pwr or Gnd)
86 other side of the coil (either Pwr or Gnd)
87 Normally Open (when relay functions connects with 30)
87a Normally Closed (Is connected to 30 untiil the relay functions)
 

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lighting relay

I dig what you are doing - but there may be some people reading this that will grab a lighting relay instead of the one to which you refer....

In which case both of the terminals 87 and 87a will be NO (normally open) and will close when the coil is energized. If the one they pick up is an "unknown" - ought to check it out before having a problem.
 

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"In which case both of the terminals 87 and 87a will be NO (normally open) and will close when the coil is energized."?

It has been my expr that if a relay has dual out puts both terminals are labeled 87a or 87b or just 87........ I have never seen a relay where it had dual out puts (both terminals are NO or NC) and labeled 87a and 87
 

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mY BAD

YOU ARE CORRECT THE COMMON SWITCHED RELAY TERMINALS ON THE LIGHTING RELAY WILL SHOW AN "87" AND "87b"........

The TROUBLE usually comes from the tiny "87a" and "87b" embossed on the black plastic - and old guys like me with bifocals......
 

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Maybe if you read the post you will see, I know and agree there are relays that have

"dual out puts both terminals are labeled 87a or 87b or just 87........ I have never seen a relay where it had dual out puts (both terminals are NO or NC) and labeled 87a and 87"

To make it clearer for you.......I have never seen a dual output relay where the two NO terminals, the 1st NO terminal being labeled 87a and the 2nd NO terminal being labled being labeled 87....... in my expr the 2nd NO terminal was labeled EXACTLY like the 1st, 87a ....... or 87b........ or 87

And yes I do know what the common terminal is and it's normally labeled 30, and the coil terminals are normally labeled 86 and 85
 
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