Lift pump cycle, is the ECM smart
03-12-2009, 07:02 AM
#1
Lift pump cycle, is the ECM smart
I don't know if I am imagining things or if my lift pump is not acting normal.
I have 98.5 24V, stock style replacement pump, using relay kit from Vulcan. Also have low pressure light installed. Nothing has changed lately, no ECM re-flashes recently.
Lately I've been noticing that I'm not getting the 2 second run of the lift pump when turning the key on. When I try to trouble shoot it, everything seems OK. I'm leaning towards a wiring issue. Before I go looking for that, is the ECM and/or the factory lift pump timing "smart". More or less, should I always get the same pre-start cycle and 25 second run after a bump of the starter? Sometimes I do, sometimes I don't. What about if I do multiple key on or bump starts, should I always get the same lift pump action?
Gathered this from some other posts.
I have 98.5 24V, stock style replacement pump, using relay kit from Vulcan. Also have low pressure light installed. Nothing has changed lately, no ECM re-flashes recently.
Lately I've been noticing that I'm not getting the 2 second run of the lift pump when turning the key on. When I try to trouble shoot it, everything seems OK. I'm leaning towards a wiring issue. Before I go looking for that, is the ECM and/or the factory lift pump timing "smart". More or less, should I always get the same pre-start cycle and 25 second run after a bump of the starter? Sometimes I do, sometimes I don't. What about if I do multiple key on or bump starts, should I always get the same lift pump action?
Gathered this from some other posts.
Yes the driver is in the ECM. Bosch uses a MOSFET (Metal Oxide Field Effect Transistor) to power the lift pump. The ECM was designed to drive the Carter lift pump, which draws no more than 4 amps. Using a relay to drive an aftermarket lift pump makes good sense unless you are sure the aftermarket pump draws no more current than the Carter. The reason Bosch used the MOSFET was to allow an easy way to duty cycle the lift pump using the ECM computer to keep pressure low for starting.
If your MOSFET is smoked, it may fail shorted causing the lift pump to not turn off or it could fail open causing the lift pump to never come on. Generally there is no middle ground.
If your MOSFET is smoked, it may fail shorted causing the lift pump to not turn off or it could fail open causing the lift pump to never come on. Generally there is no middle ground.
The ‘98.5 and ‘99 ECMs will cycle the lift pump for about 2 seconds before a start. I have found if I let the 2-second cycle happen and do not start the engine, but turn the ignition off, it will not repeat the 2-second cycle. If your ECM has been updated with a new flash you will lose the 2 second lift pump cycle and get the 0.1 second cycle. The short time was done to help starting and I believe it was when the HO engines were introduced. Since I have been running the Smarty in my truck I have the short cycle time now.
If you are wondering about the health of your lift pump try this test:
*Turn ignition key on and wait for the lift pump to cycle.
*Start truck and then turn the ignition all the way off.
*Turn ignition key on and see if you get the long cycle.
Now if you don’t start the engine the lift pump may not give the long cycle.
If you are wondering about the health of your lift pump try this test:
*Turn ignition key on and wait for the lift pump to cycle.
*Start truck and then turn the ignition all the way off.
*Turn ignition key on and see if you get the long cycle.
Now if you don’t start the engine the lift pump may not give the long cycle.
03-12-2009, 07:26 AM
#2
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Location: Grainfield, Kansas
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I just went through something very similar your return check valve could be sticking.
Take out your return line banjo bolt it has a check valve in it.
I cleaned mine up and lubed it then used air pressure to make sure it opens.
I was suggested this by dozer12216 and airdog and it worked
My truck actually sounds and runs better than it has in a long time.
you can read the post it is a page back called airdog and no purge
Take out your return line banjo bolt it has a check valve in it.
I cleaned mine up and lubed it then used air pressure to make sure it opens.
I was suggested this by dozer12216 and airdog and it worked
My truck actually sounds and runs better than it has in a long time.
you can read the post it is a page back called airdog and no purge
03-12-2009, 06:11 PM
#3
Registered User
My ears were getting warm!
The ECM is kind of dumb when it comes to controlling the lift pump. My guess is when Cummins found a hard start situation with the newly made VP44, some engineer decided to simply cycle the lift pump for 2 seconds before a cold start to remove air, and then run the pump at half speed to keep the pressure low for a start. With the advent of the HO injection pump the start problem got bad and the pre-start cycle was reduced but still ran at half pressure as the engine cranks.
With my evaporating memory of the stock ECM program, it seems like the lift pump would not always behave in the same way. The 2 second pre-start time may have also been shorter if the engine was warm. It would always be 2 seconds cold.
Maybe one of DTR’s members, HOHN will respond. He works for Cummins and may be able to add something. I wonder if his ears are burning.
The ECM is kind of dumb when it comes to controlling the lift pump. My guess is when Cummins found a hard start situation with the newly made VP44, some engineer decided to simply cycle the lift pump for 2 seconds before a cold start to remove air, and then run the pump at half speed to keep the pressure low for a start. With the advent of the HO injection pump the start problem got bad and the pre-start cycle was reduced but still ran at half pressure as the engine cranks.
With my evaporating memory of the stock ECM program, it seems like the lift pump would not always behave in the same way. The 2 second pre-start time may have also been shorter if the engine was warm. It would always be 2 seconds cold.
Maybe one of DTR’s members, HOHN will respond. He works for Cummins and may be able to add something. I wonder if his ears are burning.
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