Engine/Transmission (1998.5 - 2002) Pusher Pump Relay Toggles.......

[Big White Beast]

After installing my pusher pump I wired a relay to fire off the lift pump + wire. It just turned out easier to wire the relay in the cab as I finally wired my painless wiring circuit thingy up at the same time.



Anyway when I bump the starter the realy toggles very quickly for about 2 seconds. I tried it with several different realys including a really low draw one and same results. The digital meter is just to slow to capture whats happening. It reads about 6 volts then 8 and finally 12 when it stops toggling. This also happens when I start the truck. While it's turning over I hear the realy toggle.



Does anyone know if the lift pump is pulsed to get it going? Or does the ECM slowly bring the voltage up to it. Possibly avoiding a big inrush current?



My concern is quickly turning the voltage on and off to the pusher pump for those few seconds. Now most people I know with this set up haven't had any problems and they've had it on for close to a year. They have the realy mounted under the hood so they never hear it. Or God forbid there's something wrong with my truck and I'm the only one this happens to.



Anyone else out there have this symptom or am I unique?



The good news is that with a new lift pump and pusher I now get 16 ~ 17 lbs pressure at idle and it only drops to 15 at WOT. That's post filter. Prefilter reads off the scale of my pressure gauge (16 max).



Garrett

 

[Gary - K7GLD]

I think it's normal for voltage to the lift pump to "toggle" with reduced 'puter controlled voltage at startup as it builds pressure - it may be that your specific relay won't hold in at that reduced voltage due to current draw by the primary lift pump during the time it's in operation with reduced voltage at startup.

 

[jerryrigg]

I did a test similar to this except I hooked a DVM directly to the lift pump connector. (Pump was disconnected) My test showed that the second you turn the key it spikes to 12 then drops to 9 volts for starting. That would explain how the VP44 receives low pressure during starting. I'm not saying this is what is suppose to happen it is just the way my tests results turned out.



-Matt

 

[RADdodge]

The ECM runs the Lift Pump at 25% duty cycle while starting. I have a wiring diagram on my Readers Rigs page that uses the power from the starter circuit itself when the starter is NOT engaged to run the Pusher Pump. When you hit the starter, the Pusher Pump is turned off by the starter relay and lets the Lift Pump start the engine with the 25% duty cycle.

 

[Gary - K7GLD]

All of the above leads me to suspect that instead of keying relay operating voltage to an added pusher pump off the stock LP, I will simply use a separate keyed, fused 12 volt lead directly to my added pusher pump, and no relay at all... Less complicated, no danger of compromising the existing pump OR the computer...

 

[Big White Beast]

Gary, I may end up going that route also

The power for the pump is already wired for ignition hot. Not hot all the time. That also shuts off during the time the starter is running. The only advantage I see is it following the lift pump when priming. Although with the key on it would be running then too. It's funny but the relay wont toggle when you first turn on the key and get the 2 second run time on the lift pump only if you bump the starter. Brains a little fried right now. Only disadvantage I can think of is it would run if the key was on for any extended period for another reason. Toggle switch maybe? Of course I'd probably forget that too.



Garrett

 

[Gary - K7GLD]

I can't think of many instances where a person would have their ignition in the "run" position, but with the engine not running - and with the pusher pump most are using, there's only about 6 lbs pressure involved, so not enough to cause any problems *I* can think of, other than the current draw of the pump itself, which is around 4 amps at full head pressure...

 

[15w40]

There is a reason not to wire the pusher pump to come on when the ignition switch is in the run position. The ECM only powers the lift pump when the engine is started and/or running. This is to prevent fuel from being pumped in case of an accident causing fuel supply damage. If the pusher is wired to come on with the ignition, there is a small chance of being barbequed. The relay from the lift pump circuit may be a good idea.

 

[Gary - K7GLD]

I suppose there's ALWAYS some remote possibility of freak accident - but I think that one is fairly remote. But I am interested in hearing ANY comments pro or con concerning simply operating the added pump off a separate keyed 12 volt source - it's pretty easy to overlook something significant...

 

[rashwor]

My thoughts...

As some of you know, I had the TSB applied to my '99 that limits the run time of the lift pump on start up (forgot TSB number) for a hard starting condition. It didn't help and that was 30,000 miles ago. Now the lift pump runs for only 2 seconds at key on or starter bump, so priming the fuel filter is interesting.



If I ever add a pusher, it will not be keyed of the lift pump, but wired of the ignition hot so that when the ignition is on, the pump is running. This will allow me to run as much fuel after a filter change as I need to get the air out and fill the filter. I will also have a in-cab toggle switch so that I can turn it off should it actually cause a hard start condition.

 

[Big White Beast]

One last thought......

Anyone think that this could also be part of wht the lift pumps have such a limited life. It can't possibly be good to operate a DC motor at lowered voltages.



Garrett

 

[Gary - K7GLD]

No, it probably isn't good for them - but it occurs for an extremely short period of time, and probably isn't a real factor, overall...