Need advice on installing a VDO fuel pressure sender

Hi all,

I have reason to believe that the VDO, resistive, fuel pressure sender on my RV-8 is bad. I'm going to double check the connections, but if they are good the next step is to replace the sender, and I have a couple of questions:

The sender is a VDO resistive, 2 bar "automotive" type. It's screwed into a manifold block that also has the oil pressure sender. As shown by the picture below. My understanding is that the ground for the sender is provided by the thread connection into the manifold.

http://i22.photobucket.com/albums/b3...s_sensor_2.jpg

I've researched various forums and some folks say to use NO thread sealant; others say use it on a few threads closest to the sender body. Yet other documents talk about how there is a thread channel for fluids to leak out if it's not a flared fitting (which I do not believe this is) and so you ALWAYS have t use some sealant.

What is the opinion here?

Sealant? Last few threads closest to the sender body? Permatex #2 ok? No sealant?

Secondly, how is this connection tightened? a torque value? TPFT?

Thanks!

Since your sender has two terminals I would expect it to have no ground through the casing. Any good auto supply store should be able to get a proper replacement based on the part number on yours if it has one. Use fuel lube as a sealer and hold it back one thread from the end furthest from the case. Tighten to good and snug as I doubt you will find a torgue spec for it since it is on an experimental. VDO may have a spec if you contact them. You might also be able to find a torgue spec for a 1/4 inch pipe thread fitting in AC 43-13. Is your gauge not working at all or do you think it is just inaccurate. Of you just want to check its accuracy remove the blue plug just ahead of it and insert a fitting connected by a hose to an analog gauge for comparison. Obviously you do NOT want to fly it that way but a short ground run should tell the tale.

Randy

You can buy VDO pressure senders as replacements online for around $36. They do vary in resistance range so you want to find an identical replacement by P/N.
The inside of the sender is built like a watch with a diaphram and sector gear that moves an arm that wipes a wound resistance wire. Service vibration will eventually cause the wiper to break down and make the unit unusable. But because the cost is relatively low, you can afford to change the sender every year or two. Try to avoid installation places that receive vibration if possible.
The two wire posts eliminate the need for case grounding. Thread sealant on pipe threads are required to stop leaking.

Quote:

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Originally Posted by vaflier

Tighten to good and snug as I doubt you will find a torgue spec for it since it is on an experimental. VDO may have a spec if you contact them. You might also be able to find a torgue spec for a 1/4 inch pipe thread fitting in AC 43-13.

Well the thread on the sender is an NPT 1/8" - 27. So I'm wondering if I need to torque that. I have to say that I',m, REALLY concerned about drips from the sender as the exhaust is down below it. I get nightmares of engine fires. So I want to do this right.



Is your gauge not working at all or do you think it is just inaccurate. Of you just want to check its accuracy remove the blue plug just ahead of it and insert a fitting connected by a hose to an analog gauge for comparison. Obviously you do NOT want to fly it that way but a short ground run should tell the tale.

Randy

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What's happening is that when I was flying around, I saw this over the fuel pressure reading on my EFIS:

http://i22.photobucket.com/albums/b3.../FPSAuxOff.jpg

When I turn the aux fuel pump on, I see this:

http://i22.photobucket.com/albums/b3...0/FPSAuxOn.jpg

So I landed and am investigating this issue before I fly again.

I have a TruTrak EFIS (couple generations old). I contacted TruTrak and the three bars means "no contact/connection" or out of range. It absolutely does not mean 0.0 psi.

So the problem could be:

Bad wiring on the sender/EFIS (which includes a bad ground)

Bad Sender

Failing fuel pump.

I have replacement senders and I'm trying to figure out how to test it one the bench to be sure I understand how they work. The TruTrak installation manual says that one of the connectors is the "signal" and is connected to a specific pin on the EFIS; the other is ground and is connected to an EFIS ground.

I have verified that the EFIS is working properly. On a ground test, I move the fuel selector to a tank, power the electronics, and turn on the Aux pump. You can see the EFIS display 0.0, 0.1, 0.2.....1.7...1.8, 1.9. It is at 1.9 psi where the EFIS either displays the fuel pressure (if the pressure is coming up) or goes to the Three Bar "---" when the pressure is going down. So if the EFIS is presented with the right value, it will display the right number. This is also why I don't think the pressure is zero.


So the simplest thing is to:

1) check the two wires from the sender all the way to the EFIS connector

2) replace the sender and see if it all works.

If neither of those work then I have to look into replacing the fuel pump. Although I'm trying to work out a way to measure the fuel pressure after turning on the aux pump while on the ground with the engine running and see what the pressure really is.

Hi Bob,

As the threads on the replacement sender are NPT 1/8" - 27, do you recommend torque wrenching it?

I've watched the EAA Hints for Homebuilders on installing NPT fittings and the aluminium elbow fitting the guy used as a demo was only hand tightened and then turned with a wrench to point in the direction he wanted it to point. There was no talk of using a torque wrench. So I'm unsure about this.

I have nightmares about engine fires that can occur if I don't do this correctly.

I've decided to use PermaTex #2 for the thread sealant.

Have a helper watch the screen and simply jump the two terminals together. If you get a reading on the screen then you know the sender is bad.

Quote:

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Originally Posted by Saville

As the threads on the replacement sender are NPT 1/8" - 27, do you recommend torque wrenching it?

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Various sources quote torque values for steel 1/8-27 NPT threads of 7 - 12 ft-lb. They also quote anywhere from 3/4 - 3 turns past finger tight. When I tighten NPT threads, I get them finger tight and then 1 - 2 turns past that, so that (as you say) things point where I want them to - I don't use a torque wrench. Never had one leak, and never had a female fitting crack.

Quote:

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Originally Posted by Saville

I've decided to use PermaTex #2 for the thread sealant.

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Not the right sealant - that's a gasket sealant material, not a thread sealant.

Look specifically for tapered thread sealants. I'm particular to Locktite 565, for no special reason - there are a lot of NPT sealants out there that will work. Don't use teflon tape - it can tear and get into the lines. When you apply the sealant, always start one thread up from the bottom to keep any from getting into the lines, and you only need enough to cover one full thread - it'll spread by itself as you tighten the fitting.

Quote:

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Originally Posted by Marc Zeitlin

Various sources quote torque values for steel 1/8-27 NPT threads of 7 - 12 ft-lb. They also quote anywhere from 3/4 - 3 turns past finger tight. When I tighten NPT threads, I get them finger tight and then 1 - 2 turns past that, so that (as you say) things point where I want them to - I don't use a torque wrench. Never had one leak, and never had a female fitting crack.

Not the right sealant - that's a gasket sealant material, not a thread sealant.

Look specifically for tapered thread sealants. I'm particular to Locktite 565, for no special reason - there are a lot of NPT sealants out there that will work. Don't use teflon tape - it can tear and get into the lines. When you apply the sealant, always start one thread up from the bottom to keep any from getting into the lines, and you only need enough to cover one full thread - it'll spread by itself as you tighten the fitting.

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Marc,

Thanks very much for the info...just the sort of guidance I'm looking for.

On pipe threads, you usually get a feel for how much torque to apply by hand. If you are not sure, get an experienced person to tighten and then you do it so you get the proper feel. It's not very much torque.
Tapered pipe threads don't normally unscrew from vibration once tightened down which is why they are specified.

All pipe threads require sealant because the pipe thread geometry leaves a tiny gap at thread mating surfaces thru which fuel will travel and eventually leak out. There are fuel resistant sealants available from Spruce or auto sources. I use a blue Hypalon (sp?) type that works well for thin applications on threads and gaskets.