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Fuel Pressure Sender Ground Loop?

 
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messydeer



Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

PostPosted: Tue Sep 14, 2010 8:42 am    Post subject: Fuel Pressure Sender Ground Loop? Reply with quote

Hi!

I have a SS firewall with a brass grounding tab forest bolted to it FWF. To this fwl ground bolt I have connected a fat battery negative lead of 6" and a fat 24" engine block ground going to the far end of my starter case bolt. The fat wires are #4 copper stranded welding cable.

A couple inches away from the firewall ground I have an MGL RDAC, where all the engine instrument positive leads connect. The RDAC has a single ground wire that will be connected to the engine block ground. So far, so good.

But I also have a Stewart Warner fuel pressure sender that is mounted to the firewall with an aluminum bracket. The SW sender is designed to be grounded through case or threads threads. In my installation this would ground it to the firewall. MGL says to also run a ground wire from the sender to the engine block ground. I believe people have had problems with senders that are grounded to the firewall instead of the engine block. It's unclear to me if any of the engine block grounded senders were also grounded to the firewall, which is my situation. Wouldn't this create a ground loop problem?

Seems I'd have three options. One, run a ground wire from the sender to the engine block and hope for the best. Two, do not connect this sender ground wire to the block. Three, electrically isolate the sender from the firewall.

This last option could be done by replacing the 2x2x1.5 090 aluminum bracket with one made from non-conductive material. Another way might be to mount the sender to the engine mount tubes, using rubber to insulate the clamps.

The SW sender is an 82504, 15 psi labeled 240 ohms. 0 psi limits are 229.0-242.5 and full rating 27-43 ohms. My engine is a Jab 3300 using the mechanical pump along with a Facet pump for backup. Pressures should be in the 1-5 psi range.

Thanks for your help Smile


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PostPosted: Tue Sep 14, 2010 9:44 am    Post subject: Fuel Pressure Sender Ground Loop? Reply with quote

Dan,

I just went through this with a VDO sender. OK, so senders are not high
voltage and the forest is already grounded to the engine block right?
The test that tipped me off was to check the ohms value between the
sender contact and ground. That tells you a couple of things, 1. The
sender is working or at least returning a signal (mine did nothing which
gave away the fact that it was bad. Dynon tells me they almost never
fail, so I guess I should play the lottery). If the engine ground is
significantly better, your ohms value will likely increase.

If MGL is worth their salt, they will know the appropriate ohms reading
you should receive in the above test. You'll need a 50 ohm resistor to
put between the sender and the positive lead on your tester. Touch the
ground side of the tester first to the forest, then to the engine block.
Record the readings.

I'm sure they're chatty about connecting the sender to the block as to
share a common ground with the RDAC. That's pretty common among vendors
- CYA. I've no idea why their instrument box has to go directly to
engine ground while the rest of the universe connects their stuff to
firewall grounds. Oh well.

Glenn E. Long

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jay(at)horriblehyde.com
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PostPosted: Tue Sep 14, 2010 10:15 am    Post subject: Fuel Pressure Sender Ground Loop? Reply with quote

Hi there Dan,

I don't think that it will be a problem; I did something similar with the
Tundra that I built and so far there have not been any noise issues (which
would be indicative of a ground loop problem). I used the firewall as a big
negative terminal, and also had dedicated 'tab forests' in the cabin to
ground all the instruments. The trick is to have a good connection between
the firewall and the engine as well- a thick wire or two that will provide a
good solid low impedance path. I used 2 separate connections made up from
braided wire from the firewall to the engine block. The negative of the
engine battery was then also grounded to the firewall and the second battery
(at the back of the aircraft) was grounded to a nearby rib. You need this
in any case to carry your cranking current...
I have also used MGl equipment and had the RDAC firewall forward; all the
instrument values came through without any problems.
A small plug for these guys- they have been fantastic in terms of service;
whenever I have a fault on one of their instruments (occasionally with their
very early line of instruments) they have been very quick with the repair;
I'll send the instrument to them on a Monday, say, and get it back on the
Thursday-normally repaired for free. Of course I am located in South
Africa, where they are based, but I have been impressed with the turnaround
time.
I also had a problem in that the ECU for the engine that is being used (a
Subaru prepared by the now defunct Crossflow) processed many of the
instrument readings and then made them available via a bus. MGL developed
an interface for this, free of charge, using their ICAN attachment, and I
can now display all the values that are processed by the ECU on the MGL
Odyssey.

If anyone here has had issues with the Crossflow Subaru engines, have a look
at my blog to see what was done with the problems that I encountered
(www.rawhyde.wordpress.com)

Jay

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messydeer



Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

PostPosted: Tue Sep 14, 2010 11:20 am    Post subject: Re: Fuel Pressure Sender Ground Loop? Reply with quote

Thanks, guys Smile

I didn't quite understand why the 50 ohm load between the sender pos and probe. Is this to mimic a load created by a fuel pressure?

I did check some ohm readings with my Fluke 83. Across the sender itself is 240.0 ohms, just what they said it should be.

If I assume the engine block as ground, then the ground path would be either through the firewall to the firewall ground plate and through the fat cable to the engine block, or it would go through the 20awg wire ground attached between the sender and engine block. So I measured 0.1 ohm for both paths. Could this mean there would be less of a ground loop problem than, for example, if one of the paths was 0.1 and the other 0.4?


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PostPosted: Tue Sep 14, 2010 1:46 pm    Post subject: Fuel Pressure Sender Ground Loop? Reply with quote

Dan,

1. Yes, the resistor mimics activity from the fuel sensor. If the sender
were bad it would not produce any reading.
2. Don't lose sleep searching for ground loops (unless the noise is
rather obvious) for something as brainless as a fuel pressure sensor. If
you are getting faulty readings, that's one thing, but if it works as
advertised you are not affecting the term life of the unit. As a rule of
thumb follow the directions and connect it to the same ground source as
the other related products. If they are all 2 ohms difference you'll
never see/hear the variance in the equipment (unless of course it's
manifested as interference in combination with other resistive building
grounds).
3. The best ground has no resistance, Rg = .01 is best. Nothing is
perfect, lower is good.
4. I'm no electronics guru but I've worked on enough junk to realize a
good ground buys more steak than a good power lead. Spend the time to
build good grounds into your system, you won't regret.

Glenn E. Long
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messydeer



Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

PostPosted: Tue Sep 14, 2010 5:30 pm    Post subject: Re: Fuel Pressure Sender Ground Loop? Reply with quote

K, Glen.

The primary purpose of my fuel pressure sender is to tell me that my boost and mechanical pumps work. They're plumbed in series. I've never started an airplane engine, but I believe on startup, I'd turn the boos pump on. I should see a rapid rise to around 3 psi. I think I'd turn the pump off momentarily after startup to check that the mechanical is working, then back on again until altitude is reached.

I don't think it would have to be accurate within 1 psi, as long as the relative changes could be seen. I've heard from only a few people, but nobody has said they've had any problem with ground loops in senders wired similar to mine. Unless I hear differently, I'll mount mine as planned. If it turns out to be a problem and requires an insulated mount, I've found a place on the firewall I could stick a fiberglass bracket without much problem.


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PostPosted: Wed Sep 15, 2010 5:52 am    Post subject: Fuel Pressure Sender Ground Loop? Reply with quote

Dan,

Sounds like you're on your way. You can easily test the boost psi by
turning on the master before starting and running the boost pump 2-3
seconds before starting. For cold starts this is a typical procedure. Do
this with the throttle wide open and then open the mixture until you see
the pressure increase.

Have Fun - that's why we're here,

Glenn

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