Two wire temp sending unit

[SethChr16]

My temp gauge is reading absolutely nothing when the truck is on. Getting very good heat so not a thermostat issue. Here is the part that is confusing me, my temp sending unit has 2 wires on the connector, one is red w/ white tracer and the other is black w/ white tracer. With the key on, if I ground the R/w wire the gauge pegs out full hot so I know the gauge works. Grounding the B/w wire does nothing. If I probe the B/w wire and check for continuity I get continuity to both positive and negative. What is the extra B/w wire for and why continuity to both pos and neg?

 

[BurnTheLight88]

I'm not 100% sure on this, but the b/w should be your ground for the sensor. That's why grounding it does nothing, it's just grounded twice. I'm not really sure about the continuity.

Replace the sensor and i think you'll be fine.

 

[SethChr16]

That is what I think too. Since it's mounted in plastic housing it needs a constant ground and the R/W is a signal ground that varies based on the resistance output from the sending unit. The schematic on Alldata does not show the Bk/W wire on the connector at the sending unit so I'm not trusting it. Alldata also has the sending unit and sensor swapped on their repair guide. I'm still really confused with the continuity issue though!

 

[budwich]

"If I probe the B/w wire and check for continuity I get continuity to both positive and negative. What is the extra B/w wire for and why continuity to both pos and neg?"

What???? exactly what are you testing with and exactly how are you doing your "continuity" check?

 

[2000StreetRod]

Black/white goes to ground

According to my 2000 wiring diagram the black/white wire goes to ground and the red/white wire goes to the instrument cluster. The resistance of the sender varies from 275 ohms when cold to 16 ohms when hot.

Be very careful when removing the sender. It screws into a brass fitting that is pressed into the plastic housing. If the fitting turns then the sender won't come out and the housing will leak. When I had my housing out I epoxied the fittings in place making sure not to get any epoxy on the treads.

 

[SethChr16]

I am using a digital multimeter set to an audible continuity setting. If I unplug the connector from the sending unit and probe the B/W wire with one meter lead and touch the other meter lead to either the pos or neg post on the battery I get continuity. I know it's VERY strange and that is why I am so confused. I guess I could take the sending unit out and test the resistance reading while heating the end of it and see what kind of readings I get. At least then I would know if the sending unit is good. I will still have to chase down the Bk/W wire to try and figure out why I get continuity to both positive and negative. I would think if there were a short then it would blow a fuse somewhere.

 

[ucluglee]

Having just done this last weekend, I would say that if you are going to the trouble of epoxying the fittings or replacing the sensors, go ahead and replace the entire thermostat housing and new sensors. Mine was leaking around both sensor fittings and the seal from the lower housing to the block. $170 in parts and 3 hours labor and I'm back in business.

 

[budwich]

well... your "technique" for measuring continuity isn't exactly "scientific"... You can't/shouldn't do a resistance reading (that what continuity is checking) with a battery in the circuit under test. IF you disconnect your cables and do your "check" from the end of the cable to your "selected test point", I think you may get a different result. Having said that, you might not depending on what "always hot" circuits are in your vehicle... although they should be high enough in resistance to not "trigger" continuity tone... but that depends on the meter.... but it should have also given you a digital resistance reading which may or may not be zero.

Anyways, back to your "problem", you don't have take out your sensor. Just unplug the connector, start your engine, connect / touch one test lead to one pin on the sensor and connect / touch the other lead to the other pin. Read the resistance, watch it change as the temperature of the engine heats up. Previous posts give you an idea of what to look for (ie. range of resistance). That will tell you the rest of the story... maybe.