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djkeev
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Location: Reading Pennsylvania
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 Posted: Wed Sep 13, 2017 3:31 pm Post subject: Fuel pump current draw |
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The electric fuel pump on our units is an inline "turbine" design located outside of the tank.
The power feeding this pump is controlled by a relay but if you look at the wiring diagram there is NOT a fuse.
It is fed from a 24/7 always hot line from the Alternator, then to the relay.
I've "googlefooed" but cannot find an amperage draw spec for these pumps.
A popular replacement is the Bosch 69414.
I've found a post that people have been blowing 10amp fuses, I've seen posts where they draw 13amps.
Does anyone KNOW what their electric needs really are?
Why do I ask?
Apparently theybare fed by the same power as the fuel injection system, also unfused!
Without a fuse the Power Steering idle assist circuit can short and fry the injection wiring harness!
I am adding a fuse and some suggest using a 7.5 amp fuse, which will protect the small 1.0 diameter injection wires but I worry about the fuel pumps needs and if indeed installing two fuses would be a better idea with the fuel pump getting higher amperage protection.
Here is the basis for this query.....
https://www.thesamba.com/vw/forum/viewtopic.php?t=663228
Dave
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Last edited by djkeev on Fri Sep 15, 2017 10:38 am; edited 1 time in total |
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crazyvwvanman
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| Posted: Wed Sep 13, 2017 3:53 pm Post subject: Re: Fuel pump current draw |
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Why not just measure the amp draws you are interested in?
There are some cheap new type DC amp meters available now. These have a loop and you just run the wire you want to measure through the loop and look at the digital readout.
For example:
http://www.ebay.com/itm/bayite-DC-5-120V-100A-Mini...Sw44BYk9lR
Mark |
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kamzcab86
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| Posted: Wed Sep 13, 2017 4:03 pm Post subject: Re: Fuel pump current draw |
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All I can tell you is that the other VW's of that era have their fuel pump/injection circuits fused... 15A. They use the same type of pump as well (Bosch turbine).
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djkeev
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DanHoug
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| Posted: Wed Sep 13, 2017 5:05 pm Post subject: Re: Fuel pump current draw |
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why would that circuit have been unfused? can't fathom a good reason why. really the wire from the alt to the stud in the black box should be fused too.
there's small wires to the pump, no more than 16ga., so i'd be fine with a 15amp fuse for THAT circuit alone. but i bet you'll find the draw around 2 amps.
there's a cool diagnostic test for Chev 350 Vortec fuel pumps, the notorious in-tank ones. the Vortec system is prone to drivability issues due to poor fuel pressure. regulators (located under the upper intake manifold thank-you-very-much) are always suspect but the pump can be weak too. you use a DC current probe connected to a digital storage oscilloscope and you can see the current flow for each bar on the commutator. failing pumps will show periodic dips in the current flow each time the worn bar comes around. there can be multiple bad bars. |
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djkeev
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Location: Reading Pennsylvania
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djkeev
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danfromsyr
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| Posted: Thu Sep 14, 2017 11:24 am Post subject: Re: Fuel pump current draw |
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VW never fused the ignition circuit in older VWs.
the mantra may be that it's best to operate with melty wires than pop a fuse in an unsafe location..
Dave, if your concern is the shorting of the PS wire... you can add a fuse just to the PS switch wires.. leaving the rest of the system alone.
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Vanagon Nut
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| Posted: Thu Sep 14, 2017 11:45 am Post subject: Re: Fuel pump current draw |
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Wire case cracks, plastic failure aside, I'm fairly certain that the positive wire for the PS switch (under Digifant) has a plastic cover on the terminal so to some degree, that part of the circuit is protected.
Most fuses have some "headroom" (my term) before they actually blow. "headroom" being time and Amp amount or percentage.
I wonder if demands on fuel pump (volume of fuel required) have an effect on amount of Amps pump draws?
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DanHoug
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| Posted: Thu Sep 14, 2017 12:47 pm Post subject: Re: Fuel pump current draw |
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those fuel pumps operate with the armature in the gasoline stream for cooling and lubrication. so if anything, hotter temps will offer even less resistance tho the viscosity change with temp of gasoline is pretty small.
and since it is a centrifugal pump, the more back pressure the lower the load. an excess of fuel is pumped at all times with the extra bled back to the tank so pump load is quite consistent anyway regardless of injector demand.
there MAY be a fairly significant startup amperage before the armature gets up to speed. this is when the brushes have maximum residence time on each commutator bar and thus higher amperage draw. the faster it spins, the less amperage draw, just like a starter motor.
the 1.0 wire is slightly smaller than 16ga. ampacity charts for 16ga rate it for 10amps so i'd use a 10 amp fuse.
-dan |
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djkeev
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DanHoug
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| Posted: Fri Sep 15, 2017 8:37 am Post subject: Re: Fuel pump current draw |
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make a label for the distribution box for the "vent line heating element" 5a fuse too! i always forget that fuse is in there.
while the labeler is going, label the relays above the coil too. right one, closest to engine is fuel pump(and injectors, ICU, O2 sensor heating element, PS switch). left one, next to body, is ECU power only as far as i know.
-dan |
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crazyvwvanman
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| Posted: Fri Sep 15, 2017 8:59 am Post subject: Re: Fuel pump current draw |
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The details of which relay does exactly what depends on the version of the Digifant harness installed in a given van.
In early versions the fuel pump relay does only the fuel pump and those other items shared the ECU power relay.
Mark
| DanHoug wrote: |
.......while the labeler is going, label the relays above the coil too. right one, closest to engine is fuel pump(and injectors, ICU, O2 sensor heating element, PS switch). left one, next to body, is ECU power only as far as i know.
-dan |
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DanHoug
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| Posted: Fri Sep 15, 2017 9:39 am Post subject: Re: Fuel pump current draw |
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good to know, Mark. to me it makes sense to have all the electrically noisey stuff NOT on the relay for the ECU power, not that there is much isolation anyway. between the alternator, injectors buzzing, and the ICU pulsing the ICV, i'd want that power as far from the computer as possible. infact, the ECU power lead might be a really good place for a few wraps around a toroid or run it thru an L-C noise filter. we have enough mysterious gremlins between the AFM and idle control that clean power to the ECU might solve some issues for some.
-dan |
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dhaavers
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| Posted: Fri Sep 15, 2017 6:24 pm Post subject: Re: Fuel pump current draw |
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| Vanagon Nut wrote: |
| ...I'm fairly certain that the positive wire for the PS switch (under Digifant) has a plastic cover on the terminal... |
Perhaps '88 and later, but definitely not on the '86 early Digifant models Dave & I drive... 
Check Dave's pics posted in the thread he linked above.
- Dave
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Vanagon Nut
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| Posted: Fri Sep 15, 2017 7:01 pm Post subject: Re: Fuel pump current draw |
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| dhaavers wrote: |
Check Dave's pics posted in the thread he linked above.
- Dave |
Thanks. I see that link now and see what you mean.
When I got my '88 a wire from the PS switch was just laying there, disconnected. Upon advice from a list member on another forum, one of the first things after I bought this bus was to fuse the supply wire to the junction at the black box. I don't recall what size fuse I installed.
Neil.
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dhaavers
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| Posted: Fri Sep 15, 2017 7:59 pm Post subject: Re: Fuel pump current draw |
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^^^ Wise...I wish I had!
- Dave
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djkeev
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dhaavers
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| Posted: Sat Sep 16, 2017 7:33 pm Post subject: Re: Fuel pump current draw |
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Dave -
I'll give you a dollar for a complete set of labels like you've put throughout your van...???
- Dave
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Abscate
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| Posted: Sun Sep 17, 2017 1:01 am Post subject: Re: Fuel pump current draw |
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25% is decent headroom on the current protection for that circuit. They probably made it 50% to deal with startup current
Doubling it to 10 is ok, but might give you a little less warning about fuel pump failure if it gets old and starts drawing more current. That's a real hypothetical disadvantage.
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