Last question folks ?!  If I sourced/used a 78-ZX  Oil Pressure Sender (with two spade terminals)...altering the current (“bullet) connection to “spade” on one of the terminals. Could I then just run new wire from the other spade terminal to the new relay?  Or would it be better to keep using existing (77 280) and “tap“ off of the single (“bullet”) wire to relay? Any pros or cons or just option/choice?  I plan to run relay wire across front of  engine to coil/relay area.  This is last question!  Sincerely thanks to all great input and appreciate y’all’s patience with me!!!   Doug

The first option is what you want. Replace the bullet with a insulated spade terminal. Use another spade terminal to ground the relay. You need the 78 oil pressure sender for this.

The oil sender has one terminal with a varible resistance to ground which is determined by the varying oil pressure. The other spade terminal is a switch to ground and closes when the oil pressure reaches a determined set point.

It is a safety feature. If the engine stops the pressure will drop and stop the pump.

16 minutes ago, EuroDat said:

The other spade terminal is a switch to ground and closes when the oil pressure reaches a determined set point.

Hello @EuroDat  I am pretty sure the switch opens when pressure is reached.  A person could study the relay system diagram, and get a headache, or just work from the knowledge that if you remove the sender/switch plug (creating an open switch circuit) and turn the key on the pump will run.  

So you would want to wire up your 2nd relay to be open when power is applied through the key, then close when power is removed as the pressure switch opens.  I think that leads to using 87a, normally closed when their is no power through the solenoid.  It's a rare case of using the 87a circuit.

Key on, the solenoid energizes, the 30 circuit is open, no power to the pump.  Engine starts, oil pressure builds, the switch opens, the solenoid deenergizes, 87a is actuated, 30 gets power.

I think this is right.  The fact that you can pull the oil sender/switch plug and make the pump run is a well-known flaw in the design.

EuroDat,  Some Confusion-  current wire will be converted to OPS with spade attach.  How ‘bout other spade terminal? Can either of the two spade terminals be used to connect either wire (Relay or gauge alert) or is there one specific terminal for a specific wire?  Doesn’t OPS unit ground itself when screwed into block?

If you look at yhe circuit diagram here, the terminals form a "T". The vertical spade terminal will go to the switch. The other horizontal spade terminal will go to the guage. You need two spade terminals on the oil sender. They are two completely different circuits with differant functions. Yes the oil sender goes to ground through the threaded section.

@Zed Head could be right. I'm not 100% on the 78. Mine is a 77. If that is true, instead of using the 87 terminal you can use the 87A terminal on the bosch relay.

 

 By vertical blade/wiring to “switch”- do you mean to relay then to AFM?  I am puzzled by vertical spade “switch” circuit of OPS ..:which/what switch is on Oil Pressure Circuit?  Trying NOT to make this complicated but trying to understand wire connections only!   Please; Forgive my lack of knowledge. Doug

Vertical would be the up and down part of the "T", the bottom.  Horizontal would be the bar across the top.  Follow the diagram, jump across the connection to the oil pressure portion.

I just posted this thread link in a different thread and realized that we wee describing here how to use a switch and a relay to complete a circuit that powers another relay, the original fuel pump relay.

Might be easier to just use the oil pressure switch to control a relay that controls the pump directly.  Just a thought.

Zed,  I now have 5-wire relay and two-spade terminal OPS. Now ready to connect relay.          four-wires (disregarding black (non-used ground?) What color wires go to which incoming signal

Do They connect as:   Red= to cool ballast?                                                     

Blue=butt connect to existing oil pressure gauge wiring (yellow/ black or blue stripe wire coming from  Horizontal OPS spaded terminal) through current bundle from firewall then to AFM and then connecting to AFM pin # 36?

White= out from relay internal switching  directly to pin #39 

Now; “mystery yellow”... does this wire connect to Vertical  spade connector of OPS with new/created wiring? If not, where to/from...where does Vertical spade go to?  Trying to simplify by color to signal input to finish this posting.  You guys are SO patient with my best efforts to understand all this for myself!   Doug

 

 

 

 

By vertical blade/wiring to “switch”- do you mean to relay then to AFM?  I am puzzled by vertical spade “switch” circuit of OPS ..:which/what switch is on Oil Pressure Circuit?  Are vertical and horizontal plug wires joined in any way?

Trying NOT to make this complicated but trying to understand wire connections only!   Please; Forgive my lack of knowledge. Doug

Hi Doug,

Wire the relay as follows.

Relay terminal nr. 85 (Brown) goes to the positive side of the ignition coil ballast.

Relay terminal nr. 86 (Blue) goes to the oil pressure switch. See photo with red circle for correct terminal. (Vertical terminal in "T" figure layout)

Relay terminal nr. 30 (Yellow) can go to terminal 39 in the AFM harness

Relay terminal nr. 87a (Black) then goes to terminal 36 in the AFM harness

You will then need to re-connect the oil sender' remaining terminal to the original wire (Yellow/black) going to the oil guage.

Edited June 20, 20204 yr by EuroDat