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Discussion Starter · #1 ·
Hi,

I have a 99' Ford Escort ZX2 with 150k miles. I've never completely trusted the temp gauge on it. The engine would be hot, heater blasting, and it would still report only a quarter of the temp gauge in the winter. "It just runs cool" I'd assume. It would take some 10 minutes to produce any heat.

About a week ago, I replaced the water pump and did a coolant flush. It's bearing failed. Now in the summer, the engine warms up quicker. The temp gauge does not exceed the halfway mark. However, out of my peripheral vision, I've noticed an intermittent fluctuation with the temp reading. It will die to 0 then spring back to the half way mark in like 1 second. The car hasn't ever had overheating issues since I've owned it.

I'm no mechanic. But I suspect I need a new temp sender sensor? I have checked it's wiring and cleaned its connectors; all good... I figure it is just a matter of time until the gauge stops responding. I try not to make to many assumptions about things I don't everything about. Which is everything. :D So I thought I'd ask if anyone has a different opinion.

Thanks!
 

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The gauge is wired to a separate sensor than what the engine computer reads, live data will confirm that it isn't overheating, but unfortunately won't tell you anything to do with the gauge.

The ground wire I believe is going to a common ground connector shared with the cooling fan and brake fluid level switch, I would check/clean that ground and follow it looking for a damaged wire or wiggle test on that wire with someone watching the gauge (you might unplug the sensor and throw a resistor to substitute the temp in there so you don't have to work in a hot engine bay, but a warm engine as long as the gauge reads high enough to see it jump it works either way).

Also note that the signal wire for the gauge goes through 2 connectors (3 if you count the one going into the cluster), not just the one at the sensor.

Also, if anyone has messed with the cluster at all, a damaged trace on the back of the cluster can short to one of the traces for the gauges (ask me how I know) and cause an inaccurate reading. Doubt this is your issue, but it is a possibility.
 

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Discussion Starter · #6 ·
The gauge is wired to a separate sensor than what the engine computer reads, live data will confirm that it isn't overheating, but unfortunately won't tell you anything to do with the gauge.

The ground wire I believe is going to a common ground connector shared with the cooling fan and brake fluid level switch, I would check/clean that ground and follow it looking for a damaged wire or wiggle test on that wire with someone watching the gauge (you might unplug the sensor and throw a resistor to substitute the temp in there so you don't have to work in a hot engine bay, but a warm engine as long as the gauge reads high enough to see it jump it works either way).

Also note that the signal wire for the gauge goes through 2 connectors (3 if you count the one going into the cluster), not just the one at the sensor.

Also, if anyone has messed with the cluster at all, a damaged trace on the back of the cluster can short to one of the traces for the gauges (ask me how I know) and cause an inaccurate reading. Doubt this is your issue, but it is a possibility.
I appreciate the detailed advice! I will have to check the wire and ground out to report back.

Any idea where the ground terminates? I believe it goes into a loom. From there, I'd be guessing.
 

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Discussion Starter · #7 ·
Broke out my ODB scanner while doing some delivery gigs. I wanted to compare the gauge to a real number. The gauge appears to be reading correctly. The halfway mark is around 220 F. At about that moment, the fan kicks on, and the temps drop quickly. Everything appears well there.

Must be a dodgy connection or wire. If anyone has any hands on experience there. I'd love to hear it. I'm hoping to address it in the next two or so days.
 

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Discussion Starter · #9 ·
My ZX2 is parked at my parent's house for now, so I can't get you a picture of it, but it is joint connector #6 in grid D10 on this location diagram http://www.alldatadiy.com/alldatadi...4853741/34869954/34849301/189620386/189622346

Relevant electrical diagram

Edit: Oh hey, I just found this Ground connector #6 under battery
Cool! Thanks so much Apex. What a legend!
 

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Any chance there's still air pockets from the coolant change, so the sensor reads low when it contacts the air instead of the coolant?
I could be wrong, but I don't think this will cool off the sensor, certainly not instantaneously. I believe this usually causes the gauge to read stable, but potentially lower than an engine that is actually overheating, which doesn't really sound like the case here. This is an advantage that cylinder head temperature sensors have over engine coolant temp sensors.
 

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Discussion Starter · #13 ·
I cleaned up that #6 connection today. It was fairly rusty on the outside. The wire adapter and threads didn't look to bad. Though I know first-hand what having even a slight bit of rust can do those grounds.

I looked around for bad wires. Flickered the sensors wires while someone watched the gauge. Nothing to report there though.

Fingers crossed. I'll report back if the problem persists. Only time will tell. If so, then onto testing the wires.

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Thanks again everyone for taking time and helping me!

For anyone else trying to find that connector. It is underneath the battery box and right behind the air filter. You can see the ring clamp in the second image. The screw is like a 7mm? I tried imperial and metric.
 

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It does seem unlikely that low coolant level would cause a gauge fluctuation, but that happened to a coworker years ago on his 5.0. We noticed the gauge fluctuating, and the heater output as well. This was in fall, so cool weather.
On investigation he found the coolant level was way low, and on filling it up, the symptoms disappeared.
 

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I have an ODB scanner bluetooth adapter. It uses the Torque app which I've purchased the full version. The app has some data and charts. I haven't messed with it to much. Would that work?
It should. I use an app called OBD fusion and it shows data and can be configured to show whatever you want to see so torque should be the same
 

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Discussion Starter · #16 ·
It does seem unlikely that low coolant level would cause a gauge fluctuation, but that happened to a coworker years ago on his 5.0. We noticed the gauge fluctuating, and the heater output as well. This was in fall, so cool weather.
On investigation he found the coolant level was way low, and on filling it up, the symptoms disappeared.
For what it is worth, the gauge hasn't dropped since. The car has been running better in fact! It used to surge whenever the heater or AC would be on in the past. It hasn't since... I actually paid a mechanic back in the day to fix that and they failed to. They denied it surged when I picked it up from their shop. I then showed them it surging and they refunded me.

It should. I use an app called OBD fusion and it shows data and can be configured to show whatever you want to see so torque should be the same
I set it up to record coolant temps while doing a test drive. Line graph. There weren't any dips and normal readings.

If it was to dip, would that be indicative of a bad temp sensor or?
 

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Discussion Starter · #17 ·
Welp... Luck only lasts for so long. Yesterday the coolant gauge read cold after an hour drive. Today, whenever I switch on the ignition, the needle drops below zero.

I inspected the wires, flicked them while recording the dash, nothing changed. So then I shorted the sensor's connector using a resistor and the gauge moved to 3/4 the gauge. I cleaned the sensors connections with alcohol. I ran a grounding wire from the sensors housing to the negative of the battery. Neither worked.

So looks like I'll be needing a new sensor. If anyone can assist me with picking the correct one? I usually go with the cheapest options for my car. There are two wires for both the top and bottom sensors. From the outside, the main difference I can see is the space between the threads and flat faces of the nut of the sensor. But when you look on Rock Auto, there are like 5+ different designs. :oops:
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It is the bottom sensor for the gauge. Double check that the contacts on the harness side connector are clean and tight

They should be spring loaded so they keep good contact with the connector on the sensor itself.

I'm trying to remember how I figured out the exact sensor I needed before I ordered it, I know I got the right one, but I have a 98 model so it has the older (superior) thermostat housing.

Edit: I think I just went off of the Ford OEM part number from AllDataDIY, or out of my factory service manual, either way this shows 98-2003 ZX2. F7CZ10884BA Which means I would have bought the Motorcraft SW5157 (just confirmed on my Amazon purchase history)
 

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Discussion Starter · #20 · (Edited)
It is the bottom sensor for the gauge. Double check that the contacts on the harness side connector are clean and tight

They should be spring loaded so they keep good contact with the connector on the sensor itself.

I'm trying to remember how I figured out the exact sensor I needed before I ordered it, I know I got the right one, but I have a 98 model so it has the older (superior) thermostat housing.

Edit: I think I just went off of the Ford OEM part number from AllDataDIY, or out of my factory service manual, either way this shows 98-2003 ZX2. F7CZ10884BA Which means I would have bought the Motorcraft SW5157 (just confirmed on my Amazon purchase history)
Yeah, the bottom. I did clean the connectors and stuff. Check my last post?

Thanks for the sensor recommendation. That looks to be it. Though $20. I'll probably attempt a cheaper sensor if I can confirm the thread size and pitch. I would buy Motorcraft for the top sensor.

That Motorcraft sensor ($20) is very similar in design to this Ultra sensor ($3).
"Alternate/OEM Part Number(s): F77F10884BB, F7CZ10884BA"
 
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