LonnieChrisman

I've just acquired a 1998 Mustang V6 3.8L with a non-functional engine coolant temperature gauge, which I am now attempting to fix. There appear to be two coolant sensors, and the one to the right (driver side) has only one wire going into the connector (Red with a white stripe). Since the car runs great, I assume the left sensor is used by the computer and is fully functional while the single-wire right one is used by the dash gauge. Can anyone confirm that this is accurate?

I cut back the shrink wrap surrounding the red w/white stripe wire back to the branching harness and did not find a second wire. I expected to find the remains of a broken of wire. There is no trace of a former wire in the second hole of the connector itself either.

I'm hoping to get more information about the mysterious missing second wire before I start having to tear things apart. Can someone tell me what color I should be looking for (or point me to an on-line wiring diagram for this model?).

If I start removing engine parts to get access to the wiring harness that is down under there, do you think I'm going to find the other broken-off end of that wire? Do you think it just broke off from some point inside the harness, or is there some reason that someone would have removed it? Is there any chance I could splice off one of the wires going into the other temperature sensor?

Thanks,
Lonnie

LonnieChrisman

A part store guy informed me that some coolant temp sensors are indeed one-wire, so I looked a bit closer, and sure enough, the connector has only one conductor, and the plastic casing is solid where the mysterious second wire would have gone through. In other words, that connector never supported two wires. So this looks like a single-wire sensor.

I unplugged the connector and inserted a 70 ohm resister from the connector terminal to battery ground, and the gauge on the dash came alive. Inspired by that test, I went ahead and replaced the temp sensor itself, but the problem is not solved -- the gauge is not moving as the engine heats up. I did some comparisons with the ohm meter between the old and new sensor and found that they did differ a bit. Both have infinite resistance (open) between the single pin that seems to be used and the brass body of the sensor (at room temp). That baffles me a bit. The new sensor measures 25 ohms between the two pins, while the old one measure 350 ohms.

I am still inviting any insight from anyone who knows something about this ECT.

Thanks,
Lonnie

LonnieChrisman

I'm surprised there were no replies today. But, I managed to solve the problem.

My problem was due to a faulty coolant temperature sensor. In post #2, replacing the sensor didn't work because the parts store sold me the wrong part. Once the correct part was in there, it worked.

By the way, the good coolant temperature sensor measured 2500 ohms from the active pin to the brass body at room temperature. My bad sensor (which was the same part) measured open (infinite ohms) between the pin and the brass body. The wrong part, an coolant temperature switch, measured open/infinite between the pin and the brass body at room temp. The only visible difference between the two parts is a fin on the side, which is down the middle for the switch and offset from the middle for the sensor (which goes to the gauge). When I pointed that difference out to the parts guy, he insisted they were the same, and suggested that I just shave off the fin with a razor blade if necessary to fit into the connector (which I did).

Hope this helps someone in the future.

Lonnie

mustang newguy

im tryings a 2 wire y ony telling me it to get a pig tail for that one wire sencor n ever 98 mustang 3.8 temp sencor for gauge

mustang newguy

hey manalking bout but every dealer sayhae temp sencor ur t im trying to get a pig tail for the temp sencor the one wire one red with white strip every dealer says im nuts that its a two wire can u help me out

mustang newguy

hey man im talking bout the temp sencor to gauge same as u the ONE WIRE PIG TAIL but every dealer say temp sencor has two wires im trying to get a pig tail for the temp sencor the one wire one red with white strip every dealer says im nuts that its a two wire can u help me out

LonnieChrisman

There are two temperature sensors that look nearly identical, but are totally different. One is used by the computer to control the engine, and the other is used to drive the dash gauge. They function differently -- the dash gauge one is basically a resistor that changes its resistance as temperature changes. The one used by the engine is more like a switch, that turns on when the temperatures passes a certain point. So you have to use the correct one in the correct place - you can't swap them.

The one for the dash gauge has a single wire to it (red with white stripe). It is the one that is on the right when you are looking down into the engine. I think the part name for that one is "coolant temperature sensor".

The one that the engine computer uses is called the "coolant temperature switch" and is just to the left of the other one (towards passenger side).

When you put the two parts side-by-side, they look virtually identical. Unless you have the old part to compare to, you'd have a hard time telling which is which. But if you look at the connector on the part, there is a slight different in where a ridge appears (which pairs with a slot in the wire-side connector). That difference prevents the wrong connector from plugging in. So if you buy the part, put it in and find the connector doesn't want to plug in because of the slot being in the wrong place, it means you have the wrong one.

mustang newguy

Ya cool but im trying to find a new pig tail for the temp sencor ( im looking for the plug in with red n white wire my plug to that sencor is broke i neeed the wire to it) its called a pigtail i think

chicagoian077

Hey LonnieThanks for clearing that up man. I've been in search of this second wire for about an hour myself. Thinking someone may have ripped it completely out of the harness. But when you buy the new sensor and harness it comes with 2 wires . You said you grounded to the battery and the gauge worked correct? So couldnt that 2nd wire just be grounded to the motor?
Thanks again,
George

wbrockstar

.

If you're working on a 98 model,your harness plug should have two holes but only one metal terminal and wire in one of those holes.The other hole is empty.



Coolant temp sender for the gauge has the orange/brown harness plug shown above.