Sending Unit = Switch ?

[pelesl]

I just got my truck ('68 F100 4x2 with a 390) and I've started the list of parts to get it where I want to be.

I want all the gauges to work. I suspect both my oil pressure and temperature gauges/sending units are bad. I haven't gotten out to the truck yet to find out if the gauges work, but I'm browsing the catalogs in the mean time.

LMC truck (pg. 129 - http://www.lmctruck.com/icatalog/fb/full.aspx?Page=129) seems to list everything as a "switch". Is that really interchangeable with "sending unit"? Because Dennis Carpenter (pg. 125) shows for oil at least there's different sending units for gauges and just the warning light.

[Racer Z]

The sending units (both) are electric and can be bought at any auto parts store. The gauges are probably still good. Any after market gauge should work properly with the OEM style sending unit. The sending units can be checked with an volt/ohms meter, but I don't know what they should be sending. There are several connections between the sending unit and the gauge to check. If the previous owner has the wire crossed (easy to do) between these two sending units, you will have strange readings.

[TNIceWolf]

The sending units or switches ARE different for either a gauge or a warning lamp. They use a variant of the same principle to operate either the oil pressure gauge or the low oil pressure warning lamp. An aftermarket mechanical gauge uses the pressure of the oil itself transferred through a small tube to make it work. Just some food for thought.

[fordman]

i think tecnincally they could be called a switch. becasue they open and close or produce readings. anyway. these switches hardly ever go bad. but they can. before wasting money on soemthing you may not need. i would see if they work or not. as already stated they can be tested. and there are two differnet kinds depending on if you have gauges or idiot lights in the cluster. so that may be the reason they are called sensors vs switches. soem people including me may use the term switch interchangably. even if they dont seem to work it could be the gauge or the light bulb instead.

[Thunderfoot]

To check the gauges all you have to do is unhook the wire from the sending unit and ground it to the block, the gauge should go to full reading (with the key on) if the gauge is working. If it doesn't move then it could be the gauge or the wiring to the gauge.
If it is the sending units I would just get them from your local parts store as the others stated they are a common item...

[pelesl]

To check the gauges all you have to do is unhook the wire from the sending unit and ground it to the block, the gauge should go to full reading (with the key on) if the gauge is working. If it doesn't move then it could be the gauge or the wiring to the gauge.
If it is the sending units I would just get them from your local parts store as the others stated they are a common item...

Thanks; I'll check it out. What usually goes wrong with the gauges? Needle mechanisms need cleaning/lubing, or is it usually a wire that broke? Aren't they just a spring fighting against electrical current going through a coil?

Also, where's the sending unit for temperature (360 V8)? Is it on the intake manifold near the carb somewhere? I was about to pull the air filter assembly when I realized I had an oil bath unit and I didn't want to make a mess where the truck is parked right now.

[fordman]

the temp sending unit behind the dist and coil. in that area.

[Thunderfoot]

Thanks; I'll check it out. What usually goes wrong with the gauges? Needle mechanisms need cleaning/lubing, or is it usually a wire that broke? Aren't they just a spring fighting against electrical current going through a coil?

It is usually a broken wire or corrosion in the coil that gets to them... They are basically a Volt meter by measuring the current flow through the coil, which moves the needle accordingly.

[pelesl]

Thanks; I'll check it out. What usually goes wrong with the gauges? Needle mechanisms need cleaning/lubing, or is it usually a wire that broke? Aren't they just a spring fighting against electrical current going through a coil?

It is usually a broken wire or corrosion in the coil that gets to them... They are basically a Volt meter by measuring the current flow through the coil, which moves the needle accordingly.

I just ran the test. The oil sender wire terminal is pretty rusty; it's probably not making good contact. I touched the wire to the cylinder head cover and it slowly crept up to high. I also noticed it "bumped" a bit when I first put the key in "accessory" position (or whatever it is; one click counter-clockwise).

The temperature sender has a boot on the wire and I couldn't really see in what condition it's in, but in that case the gauge also moved just not all the way up. I couldn't get to any decently clean part of the block with that wire so maybe it was just bad contact again.

The temperature sender may be OK; I'm willing to bet the thermostat is stuck open. As for the oil, the truck doesn't have any major leaks and there's plenty of oil in there; but the gauge never moves. My Boss's new Tundra has an oil pressure gauge and it moves around all over the place between idle and high-load driving. The gauges on this guy seem to have one hell of a capacitor on them because they take a long time to change once they get power. So maybe I just haven't driven it long enough to see it change? (The clutch is in shambles and it barely made it back from the first trip to the gas station).

[TNIceWolf]

Could this be a possible IPVR adjustment issue? Combined with faulty connectors at the sending units?

[Thunderfoot]

I would definitely try a new oil pressure sender, I've had a couple of them go bad over the years on my rigs... Looks like they are a 10 spot at Oreilly's http://www.oreillyauto.com/site/c/detai ... &ppt=C0039

The movement you seen when you first turned the key on is normal from the current first getting to the gauge.

[Thunderfoot]

Could this be a possible IPVR adjustment issue?

Not if grounding the wire made the gauge go all the way to full... If it wasn't getting enough voltage to the gauge, it wouldn't go all the way up, as they (gauges) are calibrated for the certain voltage that it (normally) puts out (6V). So from his test it should all be good there...

[TNIceWolf]

That makes sense. Didnt think that would be the case but I would still clean the connectors no matter what to make sure I had as many bases as possible covered when I installed new sending units..just an opinion.

[pelesl]

Thanks for all the feedback.

What kind of conductive sealant do you guys like using on these guys?

[Racer Z]

I use plumbers Teflon tape.