temp sender not grounding

[taylor]

Hi,

I installed a new temperature sender a few weeks ago, and it has been working only intermittently. I believe the problem is corrosion on the internal threads.

I grounded the sender wire, and the gauge pegged, so that seems to be working. I pulled the sender after two weeks, and the threads were covered in rust. I cleaned them off with a toothbrush and cleaned the internal threads as well, and the gauge gave a signal (though may have been reading low) for a week.

It is now not working again. I imagine I need to clean the internal and external threads with a wire brush. Any ideas on how to prevent further rusting?

(I was thinking a contact cleaner or dielectric grease on the threads before installing the sender, but that seems like it would insulate the sender from the block and defeat its own purpose.)

What do y'all think?

[Dave Neptune]

Contact a must!

Did you install the sender with Teflon? If so it can insulate the sender and cause your problem. It is recommended that you not use a sealer when installing a brass sender too. Also give a good look at your block ground.

If you have a leaking problem with the sender try a bit tighter or use some Aviation grade Permatex lightly on the threads. It's thin and allows far a good "contact~ground" for the sender to the block.

Dave Neptune

[JOHN COOKSON]

Just To Be Through.....

Check the wiring. There may be only an intermittent signal from sending unit to gauge.
Disconnect the sending unit wire at the gauge and sending unit. Hook a jumper wire on one end and run it to the other end. Put an ohm meter on the two wires. S\B zero ohms of course. While someone reads the meter shake the wiring around.

TRUE GRIT

[taylor]

John, wire and terminals are brand new, but I'll test that.

Dave, thanks for the tip. I didn't know permatex allowed current through.

[taylor]

wait a sec

Sorry, Dave, just realized I'm not quite clear on your instructions.

Rust on the block's internal threads (and the sender's external threads) is raising resistance. I'm going to remove that with a wire brush. Afterward should I coat the threads with Permatex to prevent future corrosion or not? (The sender is a brass ISS Pro temp sender from Moyer. )

[Dave Neptune]

Sealing

taylor, the Permatex is not conductive it does allow the threads to contact which is the ground and seals what does not contact. Teflon usually works but I have seen more than one instance that it did not. If the threads in the block are really rusty you may want to touch them with an NPT tap.

Do not wire brush the threads on the sender to vigorously it is the high sharp pointy edges of the "tapered threads" that actually do the sealing mechanical contact. The sealer just seals the rest.

Dave Neptune

[JOHN COOKSON]

You can always figure out if the temp sending unit is grounded with an ohm meter. One probe on the body of the sending unit and one probe on the head somewhere.
Also you can get an idea if the sending unit is working. Check the resistance (center terminal to ground) with the engine cold then fire up the engine and see if the resistance decreases as the engine warms up. As a guide line the values for my Teleflex sending unit are:
At 75 degrees: 550-750 ohms
At 212 degerees: 55 ohms

TRUE GRIT

[taylor]

Both of those things make total sense.

I'm working on getting the thread tap at the moment. Not sure whether anyone will know the thread size on the temp threads in the A-4 head. I'm assuming it's 1/2" - 14 NPT. Can anyone confirm that?

[edwardc]

(I was thinking a contact cleaner or dielectric grease on the threads before installing the sender, but that seems like it would insulate the sender from the block and defeat its own purpose.)

What do y'all think?

Dielectric grease will not insulate the sender from the block. It doesn't work that way. The threads will squish it out of the way, producing a metal-to-metal contact. What the grease does is fill the remaining space between the non-load-bearing faces of the threads, sealing out both oxygen and saltwater, therby inhibiting corrosion from forming.

[ndutton]

I'm thinking along the lines of John. Rather than assume the unit is not grounding (assumption is the bane of troubleshooting), it can be easily tested following either of the two methods he suggested. It is exceedingly rare for a threaded connection regardless of sealant to have no continuity. Besides, you said the sender was recently replaced so I find it nearly impossible accepting enough rust could develop over a few weeks to completely obliterate continuity.

[taylor]

Hmm... so, which is better, in this case, for preventing future corrosion?

-aviation permatex
-lanocote (or other dielectric grease)

[taylor]

Neil,

I will definitely test the resistance as John said. It will be good to rule out there being a problem with the sender itself. I'm thinking that I might also put the sender in boiling water and test the change in resistance (just from threads to central terminal). I might also test resistance from the block to the internal threads themselves.

The sender is definitely not grounding. When I installed it, the gauge gave no reading. I touched the sender wire to a screw on the engine, and the gauge pegged.

Evidence for it being corrosion is that I took the sender out, cleaned it and the threads in the block with a toothbrush, and when I reconnected it, I got a signal for a few days. Nothing else has changed, but I now have no signal.

[ndutton]

The sender is definitely not grounding. When I installed it, the gauge gave no reading. I touched the sender wire to a screw on the engine, and the gauge pegged.

Equally possible is a failure of the variable resistor internally. I don't mean to be argumentative but the reported tests thus far are not conclusive that sender grounding is definitely the issue. It's not conclusive grounding is NOT the issue either. I'm sure you'll find it.

Try touching the wire to the sender body instead of the terminal and see if the gauge pegs.

[taylor]

You're right. It's not conclusive. I'll try touching the sender wire to the sender body.

And that's not argumentative at all. I really appreciate everyone's advice and thought processes. I feel like I have such a better understanding of this engine now, compared to a few months ago when I joined the forum.

[ndutton]

And that's not argumentative at all. I really appreciate everyone's advice and thought processes.

I've been previously considered to be argumentative, ornery and grumpy. No comment on the validity though. It's a self-fueling problem. I might argue the idea I'm argumentative.