I read somewhere that fuel pressure has to be 7-9 p.s.i. for the engine to start and higher pressures will cause a hard start condition. I also read the ECM controls this by rapid on/off pulses in the circuit to slow the lift pump pressure down for easy starts. After the engine is running the ECM knows to keep a steady voltage to power the lift pump or in our case, to trigger the relay used to power the lift pump off the battery.

 

If this is true shouldn't we be changing the lift pump relays more often, often enough to keep the contacts in new condition? As I understand it, the constant use of a relay has a greater chance of poorly connecting, burned contacts from frequent arching between the contacts on every open and closing.

Edited April 7, 20196 yr by JAG1

6 minutes ago, JAG1 said:

I read somewhere that fuel pressure has to be 7-9 p.s.i. for the engine to start and higher pressures will cause a hard start condition.

Actually, its 7 to 12 PSI is an optimal cranking pressure range. Above the 12 PSI, it could produce hard starting because the timing is over advanced during cranking. 

 

8 minutes ago, JAG1 said:

I also read the ECM controls this by rapid on/off pulses in the circuit to slow the lift pump pressure down for easy starts.

About a one pulse per second or so. Switching off and on the power to the lift pump. As the FSM calls it a 50% duty cycle. 

 

9 minutes ago, JAG1 said:

After the engine is running the ECM knows to keep a steady voltage to power the lift pump or in our case, the relay used to run off the lift pump off the battery

Once the ECM detects idle RPM then the ECM switches to 100% duty cycle for powering the lift pump. 

 

10 minutes ago, JAG1 said:

If this is true shouldn't we be changing the lift pump relays more often, often enough to keep the contacts in new condition?

Be honest I should replace my relay being its already had two locked up pumps with enough load to pop 20 Amp fuses. I'm sure my contact could be getting weak and this is not a bad idea. It would ensure the power to the pump motor without issues.

 

12 minutes ago, JAG1 said:

the constant use of a relay has a greater chance of poorly connecting, burned contacts from frequent arching between the contacts on every open and closing.

The normal operation I don't think there is a lot of arcing for relay being if everything is working correctly the pump shouldn't be drawing 20 amp but much less. With over 10 years of service on my relay, it might be a good idea to replace it. 

 

From the 2001 Ram FSM page 14-79:

Normal current flow to the pump is 12 amperes. With the engine running, the pump has 2 modes of operation: Mode 1: 100 percent duty-cycle with a minimum pressure of 10 psi except when the engine is cranking. Mode 2: 25 percent duty-cycle with minimum pressure of 7 psi with the engine cranking The 25 percent duty-cycle is used to limit injection pump inlet pressure until the engine is running.
  

How many trucks with relayed controlled lift pumps have had the relay go bad?  I haven't heard of any but I'm sure there has been some.  Like all things, they are made to last only so long and you might want to replace it as a maintenance item. 

If relay failure was a problem due to arcing contacts  then a solid state 20 amp mini relay could be used. 

5 minutes ago, IBMobile said:

 

If relay failure was a problem due to arcing contacts  then a solid state 20 amp mini relay could be used. 

So an alternative solid state relay is more reliable and has no contacts? Wondering..... how then does it do the switching without some  sort of mechanical aspect to it?

13 minutes ago, JAG1 said:

So an alternative solid state relay is more reliable and has no contacts? Wondering..... how then does it do the switching without some  sort of mechanical aspect to it?

Read all about them here!

 

We use them at work for controlling the heaters in the process chambers.

Edited April 7, 20196 yr by hdpwipmonkey

1 hour ago, hdpwipmonkey said:

Read all about them here!

 

We use them at work for controlling the heaters in the process chambers.

Thank you that is a good read. I bookmarked it after getting  only about halfway thru. I will read it again too to make sure I understand well.

 

hdpwipmonkey, are you using SSR for your lift pump? 

Edited April 7, 20196 yr by JAG1

_{Just a question, the relay on start is closing BUT is the computer itself sending some sort of pulse with modulation to control the pressure NOT the relay opening and closing}

Edited April 7, 20196 yr by JAG1

2 hours ago, Ed ke6bnl said:

_{Just a question, the relay on start is closing BUT is the computer itself sending some sort of pulse with modulation to control the pressure NOT the relay opening and closing}

I believe the ECM is opening and closing with an electrical pulse causing the relay contacts to chatter open and shut. This why my concern with burnt contacts in a mechanical relay. I understand the motor can run hotter with bad contacts, but I'm still formulating my knowledge on this and has to deal with why Daleb's PDC may have burned from poor grid relay contacts.

Edited April 7, 20196 yr by JAG1

3 minutes ago, JAG1 said:

but I'm still formulating my knowledge on this and has to deal with why Daleb's PDC may have burned from poor grid relay

 

Not possible. The solenoid low current side is only ground to the PDC. The high current side is only between the grid heater and the battery. The solenoid low current side has no contacts and low current draw. 

 

Now like the fuel pump where the pump motor is affected by the bad contact in the relay. But the ECM will not take any damage being its a very low current draw on the ECM side. Same set up...

 

7 hours ago, JAG1 said:

Thank you that is a good read. I bookmarked it after getting  only about halfway thru. I will read it again too to make sure I understand well.

 

hdpwipmonkey, are you using SSR for your lift pump? 

No, just a plain relay that just came with the FASS pump.

I haven't checked the price of the SSR for lift pump use and I like the use of an LED light used to activate the relay inside keeping the signal circuit entirely separated from the higher amp circuit. Plus the fact there is no contacts to arc, bounce and gradually burn the points. 

17 hours ago, Mopar1973Man said:

 

 

 

Now like the fuel pump where the pump motor is affected by the bad contact in the relay. But the ECM will not take any damage being its a very low current draw on the ECM side. Same set up...

 

There was some talk about backfeed or excess draw causing possible damage to the ECM from a seized or failing lift pump motor. 

 

Maybe I don't understand, as IBMobile is credited with finding and showing us how important it is to have a relay with a protection diode to stop excessive load to the ECM. 

Edited April 8, 20196 yr by JAG1

4 hours ago, JAG1 said:

There was some talk about backfeed or excess draw causing possible damage to the ECM from a seized or failing lift pump motor. 

 

Stock lift pump sure. Being there was no fuse or relay protect in the stock setup. Now with AirDog and FASS system come with a 20 amp fuse and protection relay I seriously doubt there is any way to cause damage to the ECM unless there was a wiring fault that shorted the ECM lines. Now I can see the transistor blowing up in the ECM that switches power on and off.

Didn't @IBMobile come up with a delay relay mod, so the fuel pump relay doesn't cycle at all until the engine starts

 

7 hours ago, Dieselfuture said:

Didn't @IBMobile come up with a delay relay mod, so the fuel pump relay doesn't cycle at all until the engine starts

You don't need to do a mod.   A time delay relay installed as the lift pump relay will do.

https://www.delcity.net/store/Time-Delay-Relays/p_804415.h_804416.r_IF3003?mkwid=8TgfXoca&crid=11472933143&msclkid=244a8de769fc10e0a1ddd14a10340b58&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping - PLA's(BSC)&utm_term=1100315444392&utm_content=Ad group %231

 

Another way is to have the automatic shutdown relay control the lift pump relay.