Tachometric fuel pump relay

[Jb427]

Can i set one of these relays up to prime a fuel system before cranking the engine?

I had a car that was set up that when the ignition was turned to the on position it would run the pump for 3-5 second and then turn off fuel pump to prime the fuel system before cranking and it would start the pump once the engine starts. I think it was a tachometric relay was used?

[jayb]

That's done with most EFI systems.  I've never heard of a Tachometric fuel pump relay though...

[Jb427]

That's done with most EFI systems.  I've never heard of a Tachometric fuel pump relay though...

yes there are a few types i think my last one i had was a 7 pin type https://www.fuelinjectionparts.com.au/tachometric-relay-bosch-0280230005.html 

[66FAIRLANE]

Before I went EFI I used a tachometric relay to control the Holley blue. It is out of an XE Falcon 4.1 with the Bosch J-Jetronic EFI. (jb427 will know what I am talking about). They are on the drivers side shock tower. Must be an XE as the XF went to K-Jetronic and the pump was controlled by the ECU.

When I went EFI I still used the tachometric relay to control the lift pump. I connected a separate button that I hold down to energise the relay and run the pump with the key in the acc or on position. Handy for setting fuel pressure,priming, pumping out fuel etc. I do seem to remember the pump would come on ever so briefly with key on but it was so brief I didn't think it was a feature. I believe there is a run while cranking connection.

I used one of these relays to keep the engine bay cleaner. No oil pressure switch.

[67xr7cat]

Bad old days of the early bosch EFI systems.  here is some information:

https://www.205gtidrivers.com/articles/articles-guides/engine-related/what-is-the-tachymetric-relay-r98/

It will run the fuel pump when the starter is engaged. the link above explains how it works.  You can accomplish about the same using a oil pressure switch like how the Vega was set up.  Think would be cheaper and likely more reliable.  This hotrod article explains how to do it:

https://www.hotrod.com/articles/safe-and-efficient-electric-fuel-pump-wiring/

Neither will give you that 4 second fuel pump activation when you turn the key to run.  If you want to do something like that you would want to use some type of delay timer. They are common in industrial automation.  Something like this can be made to work:

https://www.grainger.com/product/MACROMATIC-Multi-Function-Timing-Relay-23NV42?findingMethod=orderHistory&opr=ODOH

[66FAIRLANE]

Bad old days of the early bosch EFI systems.

Yes. I had one fail once. The only failure I have had. I cant remember what I was doing but I had the fuel line open and the ignition on. Shouldn't be a problem right, no run signal? Well it wasn't until I went inside for lunch. While I was inside the relay 'failed' and turned the pump on. As I am walking back out I hear a familiar whirring sound and my garage is flooded with fuel!

I will be removing it soon because its not necessary with the EFI (I will run the lift pump off the same signal) but I will keep a 'prime' button on the lift pump relay.

[Jb427]

I always ran a oil pressure switch too the car i had the set up in was a falcon historic race car it had pumps for the tank and a pump for the surge tank and 850dp because it often sat for a month or more even between runs it was great for filling the surge tank and carb when it would all dry out from long times between use. The new car is not as bad but i did like my old set up right now i just have the oil pressure safety switch. I hate when people just run a switch for a fuel pump or electric fans. I had a friend had his car getting some brake work done wile he was selling a car and was out of town they left the fuel pump on and bent rods after they filled the engine with fuel.   

[ToddK]

I’ve run a few of those relays, I have a new one in my garage at the moment. I’ll get the part number for it when I get home. It primes the fuel pump for about 4 seconds when you switch power on.

[RustyCrankshaft]

I always ran a oil pressure switch too the car i had the set up in was a falcon historic race car it had pumps for the tank and a pump for the surge tank and 850dp because it often sat for a month or more even between runs it was great for filling the surge tank and carb when it would all dry out from long times between use. The new car is not as bad but i did like my old set up right now i just have the oil pressure safety switch. I hate when people just run a switch for a fuel pump or electric fans. I had a friend had his car getting some brake work done wile he was selling a car and was out of town they left the fuel pump on and bent rods after they filled the engine with fuel.   

Same. Oil pressure switch is inexpensive and pretty reliable and available at almost any parts store. So easy even the counter guy at Autozone can do it!

[AlanCasida]

I also run my fuel pump and my cooling fans through low oil pressure switches. I have a momentary push button on dash to override the oil pressure switch to manually run the fuel pump for priming purposes. I also have a toggle switch on the dash for a manual fan override. 

[ToddK]

I’ve run a few of those relays, I have a new one in my garage at the moment. I’ll get the part number for it when I get home. It primes the fuel pump for about 4 seconds when you switch power on.

The relay I’ve used is a Hella part number 4RP 008 189-04.