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Old 05-13-2018, 04:18 PM   #1
Macgyver
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Default Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

So I have a 95 Saturn SL in my garage at the moment which has an odd temp gauge issue. Iím aware of the separate temp sensors in the 1st gen, the ECTS itself is operating fine as far as I can tell. Itís the signal sender 1 wire setup that Iíve been narrowing in on.

The car immediately pegs to the center for the temp gauge on a cold engine which I believe means itís not getting a signal from the sensor, correct? Sort of its way of telling you thereís an open or short circuit. However, if you drive the car, it goes up in temperature as if it has started at the beginning (first peg at C on the gauge) even though it started at the center peg. The gauge will max out at the last hot peg after 3-5 minutes or so of driving. The car shows no other actual signs of overheating so the gauge reading is bogus. Unplugging the sensor will immediately return the temp gauge back to the center peg.


Things Iíve tried:
New coolant temperature sender (not an ECTS)
Disassembled the sender connector and plugged the corrosion free connecting pin into both the old and new temp sender (see photos)
Swapped the gauge cluster (the odometer was broken and multiple lights burned out so it needed to be serviced anyway)


Iím hoping someone more technical than I (maybe Oldnuc or fdryer?) can shine some light into this and save me time. Based a wiring diagram, it looks like this wire goes directly to the gauge cluster. Power and ground are daisy chained to all the gauge clusters which would make me think Iíd have other problems if it was something like that.

Is there a way to test the sensor wire with a multimeter? How about the temp gauge itself in the car?
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File Type: jpg IMG_0796 (1024x768).jpg (71.3 KB, 10 views)

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Old 05-13-2018, 04:40 PM   #2
OldNuc
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

It is an NTC thermister. The shell is the ground side of the thermistor and the 2 pins are common, zero ohms betwen them and the thermistor resistance reads between the shell and either pin. It is not uncommon to find incorrectly boxed senders. Use the ECTS chart of temp vs resistance to check out the thermistor in water being slowly heated. Look in the FSM for the resistance vs indication for the temp gauge and plug in dummy resistors. This sounds a lot like a hillbilly wiring error committed in the past.

at cold mark 1300 ohm
at mid mark 98 ohm
At hot mark 60 ohm

Last edited by OldNuc; 05-13-2018 at 04:47 PM..

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Old 05-13-2018, 04:57 PM   #3
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Measure the thermistor resistance at 77F in water and that is the type resistance of the thermistor. From that you can look up the temperature-resistance values for those temp gauge points. There are multiple beta curves for a given 25C/77F rated thermistor so some educated guessing is required.

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Old 05-14-2018, 03:33 PM   #4
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Once you have narrowed down to the sender ohms, you may need to replace the actual gauge. They are a stepper motor, and they don't normally fail, but they can. It is still moving the 1/2 mark distance, which tells me the ohms of the sender unit are being read as normal voltage changes.

I do not believe the wire is wired through the PCM on this one, but I'll refer you to find a wiring diagram to be sure.

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Old 05-14-2018, 04:32 PM   #5
OldNuc
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

These are not stepper motor etc they are just like the gas gauge, opposing magnets. Go look up Wolfman cure for in accurate gas gauge. The temp gauge functions the same way. The cluster has been replaced which incidentally replaced the gauge.

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Old 05-14-2018, 06:05 PM   #6
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Whoops. I did not see the part about the cluster being replaced.

Ok, so if they are a magnetic setup, maybe the Wolfman cure for the gas gauge would suffice in this case?

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Old 05-14-2018, 06:38 PM   #7
OldNuc
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

The Wolfman cure has an explanation of how the gauge functions, other than that it is no help for this issue.

At this point there is no information as to the possible location of the problem, sender, wiring, gauge. Until the location is known there is nothing to attempt to further troubleshoot or fix.

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Old 05-14-2018, 07:32 PM   #8
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Ok, thank you both for the input so far. I'm going to test the resistance of the thermistor later in the week when I have some time. Assuming the sender is accurately reporting, would the next step just be manually running a signal wire to the back of the gauge cluster?

And yes, the gauge cluster in the photos is the new(er) unit. The temp gauge pegged immediately to the center on both clusters when the ignition was turned on. It goes right to the center even without starting the engine.

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Old 05-14-2018, 07:45 PM   #9
OldNuc
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Yes, go to the input terminal on the back of the cluster and cut the installed wire, leave enough slack this cut can be repaired, and then connect a decade resistance box directly to the instrument and see if it will sweep those points listed. This problem is most likely in the wiring. It could be the connector at the sensor or the cluster so those have to be checked. No decade box? Get a big assortment of 1% 1/4W resistors.

I have a thought that this wire may route through the UHJB and that might be the issue. Yes it does, these terminals need to be checked and the internal connection resistance while wiggling the pin.

black 68 way pin D9 dark green
brown 68 way pin A9 dark green

This is a highly likely location for your problem. Check this first.

Last edited by OldNuc; 05-14-2018 at 07:54 PM..

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Old 05-19-2018, 06:22 PM   #10
Macgyver
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

At this point, I'm just going to have to assume both gauge clusters are defective. I have 2 temp senders (OEM and a Wells TU227) which give the exact same read out. I may try a 3rd gauge cluster or just leave the sender wire disconnected so it doesn't give a temp reading in the car.

I checked the junction box and didn't see any trouble or corrosion so I ran a brand new temp sensor wire right from the sensor to the back of the gauge cluster which gave the exact same result. This is the first time I've had this car in my garage, but the factory harness doesn't look tampered with anyway.

Unfortunately, I don't have a bundle of 1% 1/4W resistors to check the gauge sweep and this is starting to take way too much time dealing with a rather dumb issue.

As an added bonus since I had to pull them, here are the gauge cluster connector pin outs for the 95/96 model years for anyone that may need them in the future for various troubleshooting.
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File Type: jpg PinOut2.jpg (161.6 KB, 5 views)

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Old 05-19-2018, 06:39 PM   #11
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Both defective is not expected but due to advanced age quite possible. Those indicators are plug in units when you disassemble the cluster and were a dealer replace item.

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Old 05-19-2018, 08:17 PM   #12
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Macgyver, when you replaced the sensor for the gauge display, did you examine the sensor key and its connector? I don't own an S-series car but images from parts places like rockauto shows two different coolant sensors with two different keys. Is it possible the wrong one was used? Was the connector replaced with an incorrect one, adding to this problem?

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Old 05-19-2018, 09:05 PM   #13
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Quote:
Originally Posted by fdryer View Post
Macgyver, when you replaced the sensor for the gauge display, did you examine the sensor key and its connector? I don't own an S-series car but images from parts places like rockauto shows two different coolant sensors with two different keys. Is it possible the wrong one was used? Was the connector replaced with an incorrect one, adding to this problem?
Yes, I'm aware of the different connector types (see attachment). The 2 wire sensor doesn't fit on the 1 wire connector unless you were to force it. Used gauge clusters are $10 from the pick and pull so cost really isn't a problem, but it's an hour ordeal to drive there and remove it from a compatible year.
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Old 05-20-2018, 08:20 PM   #14
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Some closure on this issue. I ended up having 2 faulty gauge clusters...that's unbelievable odds. This one almost has 200k on the clock and still works. Although a bunch of gears (see photos) fell out after I shook it prior to install. Something was definitely loose when I pulled it at the junkyard, but this was the only compatible year present this trip. The odometer and trip meter still work so I don't know what these are apart of.
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Old 05-20-2018, 08:45 PM   #15
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Did you trade up to a functioning temperature gauge for either a faulty odometer or trip meter?

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Old 05-21-2018, 04:50 PM   #16
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Default Re: Coolant Temp Gauge 1 Wire Sensor Connector Issue (not ECTS)

Quote:
Originally Posted by fdryer View Post
Did you trade up to a functioning temperature gauge for either a faulty odometer or trip meter?
Probably. I assume they're the left most gears as the odometer and trip meter are working as far as I can tell.

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