Disco 2 Check Td5 fuel pump HP

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sierrafery

sierrafery

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I thought to share this for posterity :cool::

Check fuel pump HP (the easy way)
Post by sierrafery » March 23rd, 2020, 3:25 am

Hi all. Based on the interventions of vkssmika and damyan_rm here viewtopic.php?p=751638#p751638 i worked out the easy way to see if the Td5 fuel pump is working as it should or not cos it's quite common to hear it running but the HP stage to be dead while the pump is still running on LP which can cause low power, missfire or non-start

So: with multimeter set on mV measure across the pins of fuse F10(30A) in situ, (multimeter probes like this):

1703072328371.jpeg
while the pump(or engine) is running, according to the attached chart by mutiplication if the HP is working you should get around 23.mV reading(or more) which means around 13A current draw , if it's only around 10mV or below(= 5A draw) the HP is dead but the pump is still running on LP

**(i'd say that -1mV reading for HP should be acceptable)
Attachments
Fuse voltage drop.jpg
 
Last edited:
sierrafery said:
I thought to share this for posterity :cool::

Check fuel pump HP (the easy way)
Post by sierrafery » March 23rd, 2020, 3:25 am

Hi all. Based on the interventions of vkssmika and damyan_rm here viewtopic.php?p=751638#p751638 i worked out the easy way to see if the Td5 fuel pump is working as it should or not cos it's quite common to hear it running but the HP stage to be dead while the pump is still running on LP which can cause low power, missfire or non-start

So: with multimeter set on mA measure across fuse F10(30A) in situ, according to the attached chart by mutiplication if the HP is working you should get around 25.5mA reading(or more) which means around 13A current draw , if it's only around 10mA(= 5A draw) the HP is dead but the pump is still running on LP
Attachments
View attachment 203537
Click to expand...

Man, your posts are gold!!
 
This is perfect @sierrafery. Thank you.

The only issue I see using this method is if there is a grey area between >=13A (pump OK) and 5A (pump defunct). The difference in current draw is significant and I'm not sure how one would interpret a reading that falls in this range.
 
NPG said:
This is perfect @sierrafery. Thank you.

The only issue I see using this method is if there is a grey area between >=13A (pump OK) and 5A (pump defunct). The difference in current draw is significant and I'm not sure how one would interpret a reading that falls in this range.
Click to expand...
I agree, if the reading is very far from those values i have no ideea... so it better be in that range :cool: ....otherwise a gauge is needed and this test helps to avoid that if the HP is dead for good. I'd give a down to -1mV range for the HP cos if it's higher is no problem IMO... according to RAVE the pump's max draw is 15A but mine doesnt have it so high(2 years old genuine LR)
 
A friend just visited me and i was curious so measured on his vehicle, it was 23.1mV and the engine runs well so i edited the first post accordingly... his pump is some cheap aftermarket though so it seems that the range can be wider. I think that the fuel level in the tank is important too as when it's lower the pump should work harder IMO but his tank was almost full while mine below half when i measured 25.3mV.
 
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sierrafery said:
I think that the fuel level in the tank is important too as when it's lower the pump should work harder IMO but his tank was almost full while mine below half when i measured 25.3mV.
Click to expand...

Immersion pumps normally need to work harder (i.e. higher current draw) the deeper they are because fluid density goes up with depth and hence more turning resistance on the impeller. But given the depth of the fuel tank, I don't expect to see a significant increase in current as the level rises, remembering also that diesel is even less dense than water.

BTW, is F10 a 20A or a 30A fuse 'cause RAVE says it's a 20A (I know, I could always open the fuse box and just check :))
 
NPG said:
BTW, is F10 a 20A or a 30A fuse 'cause RAVE says it's a 20A (I know, I could always open the fuse box and just check :))
Click to expand...
I says 20A in the WSM description(wrong) but it's shown correctly as it is 30A in the diagram and electrical library... as about the draw, i'll check mine with full tank then we'll see cos the communicating vessels principle applies to the pump's swirl pot .
 
Last edited:
Centrifugal pumps are like centrifugal (turbo) gas compressors, the power they consume is directly proportional to the mass of liquid or gas they are moving.
As the head increases then flow decreases, ie if you pump water higher or the gas pressure in the discharge goes higher.
In theory the head would change with tank level higher but it would be extremely small.
 
raywin said:
Centrifugal pumps are like centrifugal (turbo) gas compressors, the power they consume is directly proportional to the mass of liquid or gas they are moving.
As the head increases then flow decreases, ie if you pump water higher or the gas pressure in the discharge goes higher.
In theory the head would change with tank level higher but it would be extremely small.
Click to expand...
Is this theory valid for a two stage self-priming pump too? cos that's what's about here.
 
Not quite true as that would suggest that if you dead end the pump, ie, close the discharge line then they consume no power! The pump will always have a power requirement just to run, any flow will be more power on top of that, that part is proportional to mass flow.
 
Phanc60844 said:
Not quite true as that would suggest that if you dead end the pump, ie, close the discharge line then they consume no power! The pump will always have a power requirement just to run, any flow will be more power on top of that, that part is proportional to mass flow.
Click to expand...
Yes
 
sierrafery said:
I thought to share this for posterity :cool::

Check fuel pump HP (the easy way)
Post by sierrafery » March 23rd, 2020, 3:25 am

Hi all. Based on the interventions of vkssmika and damyan_rm here viewtopic.php?p=751638#p751638 i worked out the easy way to see if the Td5 fuel pump is working as it should or not cos it's quite common to hear it running but the HP stage to be dead while the pump is still running on LP which can cause low power, missfire or non-start

So: with multimeter set on mV measure across fuse F10(30A) in situ while the pump(or engine) is running, according to the attached chart by mutiplication if the HP is working you should get around 23.mV reading(or more) which means around 13A current draw , if it's only around 10mV(= 5A draw) the HP is dead but the pump is still running on LP

**(i'd say that -1mV reading for HP should be acceptable)
Attachments
View attachment 203537
Click to expand...
OK thank you for your reply, i will give this a try tomorrow night and let you know.
 
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