Solid State Fuel Pump Relay implementation

[lpeterssen]

Dear Friends

Was thinking last night that it would be relatively easy to implement a solid state fuel pump relay on our loved classic cars. 

Fuel pump relays used on mercedes like w116/126/123 manage power to the fuel pump so that it does not run continuously if the engine doesn’t .

This kind of relay closes its circuit to fuel pump, delivering power to it, when-ever there is a cranking signal (T50) or RPM signal (- negative pulse from coil).

Once the triggering signals are not present, the relay opens its circuit one second later.

That will add one more layer of safety to our loved classic cars….

What do you think about that.?

Best regards
L.Peterssen

[Pawel66]

Leo, if there are no other conditions, e.g. the type of pump used on our cars needs some time to build up pressure, therefore it is wired on purpose so that it starts building pressure before cranking, then this could be considered, I think.

But if you use cranking signal to close the circuit, it cannot be opened when signal is gone or one second later.

It needs to stay closed. There are relays that close the circuit when they get signal impulse and keep the circuit closed until the load power is present, so they would open the circuit when the ignition is off. That would work, I think.

I am not knowledgeable enough to say how it would work with signal from coil, I do not know what kind of signal profile you get from coil.

[lpeterssen]

Dear Pawel

I have seen the diagram of these fuel relays and they, as well have a prime function, which means that when energized they let the fuel pump to operate for 5 sec and then if no cranking signal is present the energy is cut-off then fuel pump stops working.

And also when the cranking signal is not present, the coil RPM signal supersede it.

So it is a conditional loop that has the following parameters:

1. If T15 ACC on is present ===== energize pump 5 sec. OR
2. If cranking signal is present ===== energize pump OR
3. If RPM signal is detected ===== energize pump
4. If NONE of above is present, cut energy 1 sec after the last valid signal…..


I think I will buy some of these fuel pump relays and make some experimentation on the table.

Thanks for your feedback Pawel.

Best regards
l.peterssen

[mdsalemi]

SSRs not too uncommon and very cheap. Less than USD $5.00 each. One example:

APIELE 80A Relay with Harness Socket 12V 5 Pin On/Off Car Relay Heavy Duty SPDT Relay Normally Open 1NO 1NC for Car Van Truck Motor (5 Pack)

https://a.co/d/iNigas5

What is the goal? To somehow protect the fuel pump? From what?

I had many SSRs in use when I owned a car wash. Many of the control circuits used them to switch high voltage (110-220 VAC) and high current devices with low voltage (24 VAC) control circuits. They were very reliable.

[lpeterssen]

Coil signal……..


The negative post on the coil opens and closes a negative signal each time IGNITION POINTS open or closes letting the coil discharge its energy to a specific SPARK PLUG.

This signal is used normally on all the electric TACHOMETERS to show the RPM at which engine is running.   This same signal is used by FUEL PUMP SOLID STATE relays to determine if engine is running or not.

Best regards
Lp

[lpeterssen]

The goal as I see is to:

1. Avoid draining your battery if you let your ignition switch in the ACC ON position but You are not running the engine

2. avoid overheating of the pump if condition above described extends for a long period.

3. Safety, if you fatally crash your car, and engine stops running, fuel delivery is ceased

Best regards
Lp

[lpeterssen]

SSRs not too uncommon and very cheap. Less than USD $5.00 each. One example:

APIELE 80A Relay with Harness Socket 12V 5 Pin On/Off Car Relay Heavy Duty SPDT Relay Normally Open 1NO 1NC for Car Van Truck Motor (5 Pack)

https://a.co/d/iNigas5

What is the goal? To somehow protect the fuel pump? From what?

I had many SSRs in use when I owned a car wash. Many of the control circuits used them to switch high voltage (110-220 VAC) and high current devices with low voltage (24 VAC) control circuits. They were very reliable.

Dear Salemi:

I am talking about a relay with certain intelligence inside like this, not a simple 4 or 5 prong relay.

https://www.autohausaz.com/pn/0015450605

[mdsalemi]

The goal as I see is to:

1. Avoid draining your battery if you let your ignition switch in the ACC ON position but You are not running the engine

2. avoid overheating of the pump if condition above described extends for a long period.

3. Safety, if you fatally crash your car, and engine stops running, fuel delivery is ceased

If you or anyone else thinks these are problems that need solving by all means bench test your idea and offer it for sale.

For me? These are not issues I think about.

1. I use the Pertronix ignition system. The instructions specify do not let the car sit with ignition on and engine not running for any length of time to avoid damage to the unit. So, I never do this. System has been in functioning flawlessly for 12 years so I’ll continue to follow the rule.

2. Isn’t the pump being cooled by fuel running through it? If the pump is running fuel is pulled from the tank, sent to the FIP, and unused fuel returned to the tank. What are the conditions that cause the pump to overheat?

3. This sounds like a good plan. Actually some crash sensor that disconnects the battery might be another option to explore. Has something like this ever been implemented on a more modern car?

Update to #3: the answer is yes. Some BMW and others have a small crash detection relay that “in the event of an impact the relay automatically turns on hazard lights, cuts fuel delivery, and enables the airbags”. Ignore the airbag part.

[Pawel66]

The goal as I see is to:

1. Avoid draining your battery if you let your ignition switch in the ACC ON position but You are not running the engine

2. avoid overheating of the pump if condition above described extends for a long period.

3. Safety, if you fatally crash your car, and engine stops running, fuel delivery is ceased

Best regards
Lp

Oh yes, point 3 is definitely talking to me!

[roymil]

Leo, Thanks for bringing this up!  For me, safety is the main reason to consider this.  Modern cars require automatic fuel shutoffs for good reason.

There is another benefit to having the relay if it can be located nearer to the fuel pump to reduce voltage drops and allow higher voltage at the pump terminals, which would obviously provide slightly higher pressure, a problem I found when I was prototyping an electric auxiliary oil pump. I used a relay to reduce voltage drops to the  pump and allow higher oil pressure before cranking.   I did that because I noticed sometimes I would see below 12V at the oil pump and even lower while cranking, so of course the fuel pump has the same issue, being at the back of the car.    I currently see less than 10psi fuel pressure while cranking and thats not to spec. so this might be a way to help that while making it safer.   Another way is to just route thicker wire out to the pump  and not worry so much where it lives.

I would suggest considering a real solid state relay and not a traditional coil driven relay with mechanical contacts.  A couple of the links to relays above were of the mechanical type and not true solid state (SS) relays.  As an electrical engineer specializing in reliability, I can assure you that mechanical relays are among the least reliable components in existence.  They will all fail, it's just a matter of when, and I'd want to reduce the chance of abrupt fuel system failure for this cause.  True solid state relays are semiconductor switches and have no coil and no moving parts to fail, so they last much much longer, and they perform much better, with lower series resistance, and therefore more resultant fuel pressure.   For a new design like this I would consider something like this one from a known supplier like Hella : https://www.summitracing.com/parts/fch-3020004086  . 

Also I suspect there may be a way to use some clever fail-safe logic in the design to reduce any chance of fuel cutout at speed, (possibly with an oil pressure switch in the circuit?)   -Mark

[lpeterssen]

Dear Mark:

Very very interesting your comments. I will take all of them in consideration.

As you may know I am not electrical engineer, but a Civil engineer with an Oil Refining master degree at FIP. 

So my passion for car electrics came by chance, not because of my university background.

Back to the point, I consider this:

1. Placing the logical main traditional smart relay (because senses the RPM, T50 cranking signal and T15 ACC-on) next to the 12 pin connectors below the steering wheel.  That because accessibility and because it’s easy to bring there all major signals required.

2. Use that main relay, which unfortunately is based on the traditional coil and electro-mechanical switching architecture, as signaling device to another SOLID STATE RELAY as you suggest made by HELLA, which will be the one that actually cuts energy to fuel pump.   Will find the way to use the NORMALLY CLOSE 87a port, so that in the event the other  intelligent  relay fails, you will still have power on the fuel pump.

3. Have an Inertia switch in all the relay arrangement to cut ground to some of them…..

4. Regarding the voltage to fuel pump, yes the workaround will be a thicker feed cable, since car battery is too far away from it, to have any effect if we place the cut relay next to pump

Best regards
LP

[rwmastel]

3. This sounds like a good plan. Actually some crash sensor that disconnects the battery might be another option to explore. Has something like this ever been implemented on a more modern car?

Update to #3: the answer is yes. Some BMW and others have a small crash detection relay that “in the event of an impact the relay automatically turns on hazard lights, cuts fuel delivery, and enables the airbags”. Ignore the airbag part.

Didn't Pagodas sold in France (or some other European country) require this?  I thought back in the 1960's one of the countries required roll over protection that shut off the ignition somehow.  Am I making this up??   

UPDATE:  No, I'm not making it up!!
https://www.sl113.org/forums/index.php?topic=36332.msg265671#msg265671

[mdsalemi]

It’s interesting how that rather large device in the French delivered pagodas can basically be accomplished with a small relay today… if so desired.

[BobH]

The original inertia switch, if that's what it is, is designed to cut all power to the car, by open circuiting the battery feed.  This device needs to handle all of the current demands of the car, including a few hundred amps when cranking, plus the connection terminals need to be large enough to connect the battery lead in and out.  Leo's device is designed to cut the supply to the pump, just minimal amps, and small wires, hence the small size.  if you wanted to isolate all power, you would need a second relay that could handle the enormous demands of the car electrics, driven by the electronic relay, and also consider an inertia switch, which would activate in a collision

[rwmastel]

A modern version of that device.  https://www.youtube.com/watch?v=_2zvMPCcEaM

I wonder if the original part works the same.