OT: Electrical Relay Pros Have a Look...

[bjcsc]

I'm working on a simple circuit. My goal is to power my ignition coil when the starter is engaged, and then for the power to flow through the oil pressure safety switch to the distributor when the starter is not engaged. I would like to use a simple Bosch relay. Do you guys see any problems with this plan?

[sixto]

When you engage the starter you energize the relay and get +12V to the coil. When you release the starter and have sufficient oil pressure the relaxed relay will send +12V to the coil.

I hope the engine builds sufficient pressure quickly. It might be a challenge to get the engine running after an oil change.

Coil? Why post in a Diesel forum?

Sixto
95 S420
87 300SDL

[bjcsc]

Originally Posted by sixto

When you engage the starter you energize the relay and get +12V to the coil. When you release the starter and have sufficient oil pressure the relaxed relay will send +12V to the coil.

I hope the engine builds sufficient pressure quickly. It might be a challenge to get the engine running after an oil change.

Good Point. But if post oil change is the only problem I can live with it. I can build enough pressure cranking in about 15 seconds using only the oil pressure switch.

Originally Posted by sixto

Coil? Why post in a Diesel forum?

1) This is where the people I know hang out and 2) everyone knows diesel drivers are the most intelligent people on earth

[sixto]

I missed the OT in your subject line.

Sixto
95 S420
87 300SDL

[dannym]

Originally Posted by bjcsc

1) This is where the people I know hang out and 2) everyone knows diesel drivers are the most intelligent people on earth

OK he can stay.

[Wes Bender]

Your circuit will work just fine and I think I understand why you are doing it (engine preservation in case of rapid oil pressure loss?), but I would be a little concerned about an unexpected shutdown in a critical situation on the road. Wouldn't it be better (and simpler) to turn on a red light and sound a buzzer if you lose oil pressure? Position the light where it would get your attention.

Wes

[leathermang]

You know you are sorta reinventing the wheel...
Other gas cars had this sort of stuff.... Pinto I think .... for one.... I have one of the units in my garage I think....
It was made so the electric fuel pump would shut off if the car was in an accident and the engine was OFF....

[dannym]

Originally Posted by leathermang

You know you are sorta reinventing the wheel...
Other gas cars had this sort of stuff.... Pinto I think .... for one.... I have one of the units in my garage I think....
It was made so the electric fuel pump would shut off if the car was in an accident and the engine was OFF....

Most cars today with electric fuel pumps have inertia switches to do just that. Shut the pump off if case of accident.

Danny

[barry123400]

Perhaps I am missing something but does the oil pressure switch not open up when oil pressure is on the switch turning the indicator bulb off? When you have oil pressure the switch will be open. But then again I am not sure what is switched inside that relay you propose using. Just do not know the normally on/normally off connections.Plus I am also an advocate of the loud buzzer school as own cars with them from the factory. You still have engine power to exit the roadway plus your power steering is still working. With the buzzer rattling your brain there is absolutly no doubt. The first time mine went off after driving through a large puddle at speed and shorting it out temporarily I nearly filled my pants as did not know the car had that feature. Believe me you cannot be unaware. In your case engine is just going to die if oil pressure drops. Try to imagine being in the third traffic lane at 60 mph and the engine just dies on you. You have to still engineer a way to defeat buzzer circuit when engine is being started to the time oil pressure being initially established but can be easily done. Back to the drawing board perhaps? Anyways this is an important ideal in my opinion and even more important on cars with normal old fashioned oil pressure gauges. At least in your car you get a bright red light but in my opinion I can easily miss the oil pressure gauge dropping down to zero and from that point only takes a very short time to do damage. With all our external oil lines on our old diesels plus just having only an oil pressure gauge I think we live kind of dangerously. On occasion have guys on this site saying one of their oil lines failed and they did not catch it in time. Have never heard the response given of why did you not notice the oil pressure gauge drop. It is a given that it is subconciously expecting too much of an individual. Are you going to drive and occasionally watch the oil pressure gauge or are you going to go down the road always fixated on your oil pressure gauge? In my opinion after about one to three minutes it is far too late to notice your oil pressure is gone. But again these are just my thoughts.

[bjcsc]

Originally Posted by leathermang

You know you are sorta reinventing the wheel...
Other gas cars had this sort of stuff.... Pinto I think .... for one.... I have one of the units in my garage I think....
It was made so the electric fuel pump would shut off if the car was in an accident and the engine was OFF....

What you say is true, actually the engine in my boat does exactly that. However, this is for my CJ7, which has a factory switch that closes with oil pressure. I have an aftermarket (D.U.I.) HEI type distributor, and thus employ none of the original harness/components for all of the long gone (for good) engine/carb controls, some part of which used the signal from the oil pressure switch to interrupt the ignition. It's not the wheel I'm trying to reinvent, just this one safety feature of my engine. Lots of vehicles with electric fuel pumps have a similar system, however my fuel pump is manual so I don't have that option.

[leathermang]

"Most cars today with electric fuel pumps have inertia switches "

I have seen people in small fender benders who have not read their owners manuals.... and can't start their cars until someone tells them about the switch..typically in the trunk.... LOL

[leathermang]

Pretty complicated...
If you used the " Pinto" type deal .... which turned off the power to to the electric fuel pump (substitute ignition)... you could use a manual ' normally off ' button to override (jump) the killing effect of low oil while you crank the engine.... which I think is what you are trying to circumvent.... the fact that the power will not be going there until the oil pressure gets up to threshold for the switch...
This would be much simpler....

Hmm...you plan on offroading your CJ7?
If so, I believe you may find yourself in situations where you suck an air bubble up your oil pump and temporarily loose oil pressure. And generally that means around a corner, on a side hill, etc.
Of course there are situations where you are UPSIDE DOWN and you want the engine off.

I would have a buzzer and a warning light instead of shutting down power to your engine. A learned driver skill should be to shut your engine down in the event that something bad happens. As my bug was rolling through the air off the side of a hill, I reached out and shut the engine off mid roll.

A time delay relay would be a good idea in this application but learning how to wire one of those can get a tad more complicated.

Personally I like to have full control of my engine becuase you never know when you need to sacrafice that oil pressure for a 1/2 a second to get you out of a bad bind.

Just my 2 cents

[OMEGAMAN]

on my desk is a relay like the one in your picture and it has a diagram on it.
On all these relays power from the battery goes to terminal 30 thats what 30 means on euro diagrams like VW.
contacts 30 and 87A are normally closed so there is current flow when the relay is not energized
30 and 87 are normally open so when the relay is energized they have current flow and 30 and 87A are open.
hope this helps you. Also on bosch relays coils etc. terminal 15 means power switched from key, 30 means constant power from battery, X means power from an energized relay to a buss on a VW that would power heater fan wipers, stuff that is swithed on by the key.