Engine/Transmission (1998.5 - 2002) lift pump control voltage?

[99guy]

Here's a question that arose to me during troubleshooting my "dead" pump. I had a VOM attached to the pump wiring when I first found it wasn't pumping to see if it was receiving current. I noticed as I cycled the ign key to start quickly the to run the voltage supplied to lift pump read out as 12v (or batt. voltage) then after short period, say 20 - 30 sec. would drop to something less, maybe 9 or 10 v. , can't remember for sure and truck is locked in at my local stealership today.



I read in a cummins manual the procedure, but was just curious as to the reasoning of droping the voltage after inital startup.



Any of you had experience with this?

 

[Shovelhead]

The lift pump is controled by the ECM.

It does not run continuously.

 

[A Johnson]

The voltage supplied to the lift pump from the ECM is a PWM (on- off) signal. Therefore the voltage you are reading may show less than 12 VDC depending on the output of the ECM. IT is still 12 VDC but most meters cant react fast enough to read it correctly so it will show up as something less than 12VDC. At least that is how it works with my Fluke 83. The ECM will vary the signal to control lift pump pressure during engine cranking, and this signal also varies depending on what calibration code is in your ECM.



I don't know how you are monitoring the voltage but system resistence can go up as the lift pump works and wires heat up, so on and so forth.



A Johnson

 

[Chipstien]

Does anyone think that some of the probs that the 24v are experiencing are related to this constant triggering of the pump. Would we be better off to place a relay before the pump that would be triggered by the ECM. The relay wouldn't react fast enough for the ECM so there would be constant voltage and it would be getting a few extra volts, (14. 4 volts). This probably wouldn't work for those with the TSB that reduces fuel pressure at start up but for those that haven't done this then would it work? Maybe I am just beating a dead horse.

 

[99guy]

A Johnson, I follow your explanation, makes sense, yes I was just using an analog VOM to check for current.



I'm assuming the ECM dosen't physically monitor pressure, I see no sensor externally unless it does so through current draw. That leads me to think then that rather the ECM regulates the lift pump based on calculated values programed in the ECM itself?



Logically my next question is, Will a diagnostic pick up on a mechanically broken lift pump or will the truck thro a code to indicate so. Think I read a thread somewhere once that indicated that was the case.

 

[lmills]

It must monitor pressure some how. Yesterday, I had my lift pump unplugged to check the polarity so I could run a jumper wire to a relay. When I bumped the starter to get a voltage check, it gave me a code and check engine light. The code is still in the ECM. I tried disconnecting the battery and also unplugged the computer, with no luck. The code doesn't bother me, I know it is false. I will probably head to Autozone to get them cleared

 

[A Johnson]

The ECM doesn't monitor pressure, it uses calculated values, for the average system. Thats why pressures will vary a few PSI rig to rig.



I am not familiar with DC codes, but the most common Cummins code for a lift pump failure is a 278. Without looking I can't tell you if this is an open cuircut, or short to ground type of code. ( I believe it is open cuircut. ) The only thing that the ECM monitors is supply and return voltage and I believe amperage draw, but not to the extent that it can tell if there is excessive restriction in the system. It cannot tell you if the pump motor is free wheeling due to a broken coupling.



I believe it is important to have some sort of pressure gauge or warning light, with pressure gauge being the prefered method, so you can watch for performance degredation.



I believe the reflash that reduced lift pump pressure was a stop gap measure to compensate for a failing VP-44. This is only MY opinion.



lmills, usually when the ECM sees the problem corrected it will turn the check engine light off.



A Johnson

 

[lmills]

The light is off, I just can't get the code to clear. When I checked the code, it said "lift pump out of range". I am not sure how it came up with the code, or exactly what the code means. I know it was set when I had the pump disconnected, so it doesn't bother me.

 

[JTroiano]

Maybe the grid heaters came on? 9 volts seem about right.

Jon T

 

[Ncostello]

I once had a bad crankshaft position sensor that caused CHECK ENGINE light to come on. Dealers ScanTool showed CKP and Lift Pump out of Range. New CKP sensor cleared them both. Dealer tech told me the Out of Range code meant the ECM wasn't communicating properly with the lift pump... most likely because of the bad crank sensor. He was right too. So disconnecting surely would have had something to do with the communication link there... FWIW.

 

[99guy]

Originally posted by A Johnson

The ECM doesn't monitor pressure, it uses calculated values, for the average system. Thats why pressures will vary a few PSI rig to rig... ... ... ... .



... ..... I believe the reflash that reduced lift pump pressure was a stop gap measure to compensate for a failing VP-44. This is only MY opinion.

A Johnson

That confirms what I was guessing about calculated values.



I'm mixed up on your comment about a reflash to reduce lift pmp pressure. I have been thinking reduced pressures were a bad thing, or is the reflash you refer to only reducing the pressure during engine start?

 

[Bob4x4]

99guy,

There are a number of DTC's that will point to the lift pump failing... You need to put a pressure gauge on it and check it at idle,high idle and under full load. To be sure you do not have a restriction you should also check vacuum at idle and high idle.



Bob

 

[A Johnson]

99guy, the reflash was to reduce lift pump pressure during engine cranking only. After engine reaches idle rpm or higher, ecm commands 100 % duty cycle of the lift pump. I was very unclear on that and do apologize.



A Johnson