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Discussion Starter · #1 ·
My temp gauge recently stopped working, it sits below cold when running, the needle sweeps when the car is started. If I short out the gauge wire, the gauge moves to full hot. (So the gauge isn't the problem?)

If I hook up a scanner or computer to my OBD port (OBD1 car, it's a 1995) I get correct temp readings via the PCM. (so the sender isn't the problem?).

If I unplug the sender (from the water pump) and start the car the fans come on and I get a check engine light (as expected).

Years ago I swapped my sender to a 3 wire style which feeds both the PCM and the dash gauge (when stock the dash gauge is fed by a sender in the passenger side head) I rerouted the wire & it's been working like that for years.

So I'm not sure what is causing my temp gauge to not work, since it appears that both the sender & the gauge (& wiring from the plug) have been tested & confirmed working.

I even assumed maybe half the sender was working, sending a signal to the PCM & not to the dash, so I swapped in a new sender, same issue.

What am I missing?

Thanks!

sod

Here is a link to photos of my troubleshooting temp gauge troubleshooting

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Here is a link to photos of my troubleshooting temp gauge troubleshooting
 

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You've got the wrong sender there. That one's for the PCM. The sender for that gauge is in the passenger side head. And the little green wire for it is likely broken.
 

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Discussion Starter · #3 · (Edited)
Years ago I swapped my sender to a 3 wire style which feeds both the PCM and the dash gauge (when stock the dash gauge is fed by a sender in the passenger side head) I rerouted the wire & it had been working like that for years.

ACDelco 10096181 - https://www.summitracing.com/parts/ADO-213-815
 

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Did you notice or did anything unusual happen before the gauge stopped working? It seems like something else is causing the gauge to read wrong ( I'm just not sure what).
 

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From your tests:
Key on gauge sweep, Shorting gauge pin to ground, and PCM sensor working.

To me this shows the wiring between the gauge and the sender is probably OK.

The PCM temperature uses two of the pins so there is no connection between the two systems.

Try a test light connected to the sender case and battery+. If the test light is not as bright as it is when connected to the battery your problem may be sensor to water pump grounding issue.

Another way to test a ground problem would be to set the meter to ohms and compare the gauge pin of the sensor to the sender case then the water pump. The numbers should be close.

Gauge sender pin to ground should be about 1365ohms at 100degF and 95ohms at 260degF. With a cold engine the resistance will be high on this sender.

From what you have done you seem to have a ground problem or sensor fail. I usually bet on ground problems being the problem.
 

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Discussion Starter · #6 ·
From your tests:
Key on gauge sweep, Shorting gauge pin to ground, and PCM sensor working.

To me this shows the wiring between the gauge and the sender is probably OK.

The PCM temperature uses two of the pins so there is no connection between the two systems.

Try a test light connected to the sender case and battery+. If the test light is not as bright as it is when connected to the battery your problem may be sensor to water pump grounding issue.

Another way to test a ground problem would be to set the meter to ohms and compare the gauge pin of the sensor to the sender case then the water pump. The numbers should be close.

Gauge sender pin to ground should be about 1365ohms at 100degF and 95ohms at 260degF. With a cold engine the resistance will be high on this sender.

From what you have done you seem to have a ground problem or sensor fail. I usually bet on ground problems being the problem.
Thanks! I'll ohm out those pins & see what I get. I can also swap in a spare gauge at some point, but I've got to dig it out of storage.

I'm with you, it seems like a grounding issue (it's always a grounding issue). I'll also clean my grounds under the hood & trunk.

Are there any grounds in the dash I should be aware of?

Thanks again.

sod
 

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I can also swap in a spare gauge at some point,
Your tests indicate no gauge or wiring problem

I'll also clean my grounds under the hood & trunk.
Cars always need good grounds, BUT

The ground issue I am talking about is between the sensor case and the water pump.
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You need clean threads so the sensor connects to the waterpump.

Best use of your time are to ohm the sensor (pin to waterpump and clean the sensor threads.

Are there any grounds in the dash I should be aware of?
If your voltage gauge and oil pressure work(and the rest of the cluster). No they share grounds so it would not make sense to play with things that work.
Again you have done the FSM tests and they point to a sensor or ground problem with a less than 1% chance of some other problem.
 

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The temp gauge gets it's signal from a sensor/sender in the passenger side head. Not the PCM sensor on the WP.

I've dealt with this many times. I may have an answer for you.

If you turn on the key into the run position, car cold (sitting for 12+ hours) and the needle sits below cold, the gauge itself is the problem. Even if the sensor is bad, open or shorted, the gauge should never sit below cold.

The fix is to disassemble the cluster and pop the needle off the pin/shaft, and reclock it to the "C" position. You need to do the needle reclock procedure with the cluster back in the car, the key on, the car cold (sitting for 12+ hours), engine not running.

Root cause:
If the needle bangs hard to H when you crank the car, it will slowly slip on the shaft again until it reads below cold sometime in the future.

I have yet to find a permanent solution to this problem. GM had a dampening oil in the gauge shaft that dries up after many years. If you can stop the gauge from pegging hard when you crank, it will help solve the problem. I considered installing a resistor on the crank circuit but it will affect other functions. Maybe just disabling the bulb test circuit altogether is the best option, but a big compromise. The bulb test circuit is what's damaging the gauge over time.

PS: this behavior is very prominent on 9C1 cars. Not so prominent on Impala SS or Civvy Caprices. I own several 94-96 9C1 cars and several more spare gauges and they all have done this at one point in life. I had a write up about this on 9c1.net 10 years ago. But that forum is long gone.
 

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The temp gauge gets it's signal from a sensor/sender in the passenger side head. Not the PCM sensor on the WP.
Years ago I swapped my sender to a 3 wire style which feeds both the PCM and the dash gauge (when stock the dash gauge is fed by a sender in the passenger side head) I rerouted the wire & it's been working like that for years.
This sender is from a Camaro or Corvette.

OP has done the standard tests and gauge and wiring seems fine. Bulb test works. FSM and mechanic's text books use the same diagnostic procedure the OP employed. Wiring to the gauge works, the gauge works, so the normal procedure is to test the sender and it's ground connection.
 

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OP has done the standard tests and gauge and wiring seems fine. Bulb test works. FSM and mechanic's text books use the same diagnostic procedure the OP employed. Wiring to the gauge works, the gauge works, so the normal procedure is to test the sender and it's ground connection.
Normally, yes. This is absolutely correct.

But there is a missing detail here... "the gauge works". Are we sure about that? The OP reports that it reads below C. This is hint #1. And this is something specific to our cars. Why? Because the state of reading below C cannot not electrically exist with our gauges. On a nominally functioning car, if you short the sensor wire directly to ground, it will peg the gauge (this is what the bulb test does). If you open it completely, as in unplugging it, that will be infinite resistance and the gauge will still read C. But it will never go below C. That's an important detail.

Why? This is not a bi-metallic thermal gauge as in older cars, it does not need any base resistance. Nor does it contain a spring returning it to zero. Our cars use a magnetic motor system to swing the gauge. Ergo, there will never be a case where the gauge will read below C electrically. Perhaps you can short it to a hot wire, and maybe it will go below C (I haven't tried that). There will be other problems if it were shorted to hot, ie: the bulb test circuit will fail or blow a fuse, so let's eliminate that edge case.

Since we can safely say that there isn't a case that electrically it will read below C (no matter if the sensor is bad or if the wire is open or shorted), then the next probable cause is the gauge itself. And since I've already identified a flaw in the gauges where the needle will spin on the shaft due to excessive force exerted during the bulb test, I suggest that being the next point of focus.

Reclock the needle on the shaft as instructed in my previous post. Pull the needle off and push it back on at the C position with the engine running (but still ice cold). Then see if the gauge operates normally from there. I'm suggesting doing it with the engine started to bypass the next bulb test, which could still affect the outcome.

If this solves the problem, then we need to collectively come up with a solution to stop the darn thing from spinning on the shaft (I've yet to figure that one out). On the old 9c1.net forum, one guy cut the plastic so the needle could spin past H during the bulb test, eliminating the harsh bang. It solved the problem but looks ugly. I've fix my 9C1s by buying non-9C1 spare gauge clusters, and swapping temp gauges that still contains good dampening fluid so the bulb test no longer bangs it hard right. But it's getting harder to find a cluster that still contains the dampening fluid in the gauge motor after 25+ years.
 

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Storm9c1 Would you really take apart a 25 year old dashboard made of plastic and pull the needle off a gauge before pulling a plug off a sensor and doing a ohm meter test?

The time required to test the sensor is minimal compared to cluster removal.

Personally I would fire any mechanic that would charge me a hour or more and risk breaking my very old plastic when the factory service manual tells them to take a few minutes and check the sensor first.
 

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then we need to collectively come up with a solution to stop the darn thing from spinning on the shaft (I've yet to figure that one out).
If you looked at a circuit diagram you might see a simple fix. The FSM states that 55 ohms makes the gauge read 260 degF.

So if you cut the bulb test circuit and add a resistor larger than 55 ohms to the bulb test to coolant temperature gauge it will stop banging on the high needle stop. 100 ohms would as a guess get the needle to 3/4 sweep. The FSM says 1365 ohms is 100 degF a common 1000 ohm resistor would still demonstrate the gauge moves during bulb test.

Since the bulb test circuit actually does bulb test the brake warning light the best place to install a resistor would be on the cluster circuit board close to the coolant temperature gauge.

This is easy to do. Take a look at this picture of my early tests on calibrating the fuel gauge. The cooper board trace is cut and a resistor is used to complete the circuit path. The final product would have better soldering and conformal coating applied where the trace is cut.
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This is a rough outline of a modification. As always one should understand the circuit and the risks of damage before attempting any modification.
 

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Storm9c1 Would you really take apart a 25 year old dashboard made of plastic and pull the needle off a gauge before pulling a plug off a sensor and doing a ohm meter test?

The time required to test the sensor is minimal compared to cluster removal.

Personally I would fire any mechanic that would charge me a hour or more and risk breaking my very old plastic when the factory service manual tells them to take a few minutes and check the sensor first.
That's a little harsh. Maybe there's a misunderstanding here. The OP said he was going to ohm it out. So sure, I am not suggesting he shouldn't. There is a risk of breaking the connectors or wiring. It is what it is for these old cars. But if that testing comes up negative, I am offering the next steps.

Making the same argument.... Would I want a mechanic digging around on brittle wiring harnesses and connectors on the passenger side head that are heat stressed by the manifold? The heat shield alone protecting the sensor will probably turn to dust.

Getting to the gauge in the cluster is not hard. Remove the black bezel over the cluster (2 screws and it snaps out). Then remove the torx screws around the perimeter holding on the plexiglass cover of the cluster to access the gauge. I am not recommending removal of the entire cluster from the car to reclock the gauge needle. I am recommending reclocking it in the car with the power on, cold engine running. So basically it's just about 10 screws to access the whole shebang. Not much opportunity to break anything. Except the gauge needle if you aren't careful. Should amount to a 10 minute job. Whereas I think it's easier to break brittle underhood connectors, and break brittle wiring. Especially on a 9C1 that has likely been repeatedly heat stressed in service.

The OP also mentioned he has a spare cluster. That's another macro-level test. But this also assumes that temp sensor is good. There's a chance it's bad too. It's a common problem. Trust me.
 

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If you looked at a circuit diagram you might see a simple fix. The FSM states that 55 ohms makes the gauge read 260 degF.

So if you cut the bulb test circuit and add a resistor larger than 55 ohms to the bulb test to coolant temperature gauge it will stop banging on the high needle stop. 100 ohms would as a guess get the needle to 3/4 sweep. The FSM says 1365 ohms is 100 degF a common 1000 ohm resistor would still demonstrate the gauge moves during bulb test.

Since the bulb test circuit actually does bulb test the brake warning light the best place to install a resistor would be on the cluster circuit board close to the coolant temperature gauge.
Good suggestion. I think I investigated this years ago... and there was some road block, maybe something with how the bulb test branches off to the temp gauge. Or maybe I just ran out of time. I need to revisit. We need to make sure the resistor is inline with the gauge only and (1) won't affect normal operation and (2) only affects the bulb test function. The schematics in the FSM are oversimplified in this regard and has no detail on the board itself, so I'll trace it out next chance I get and refresh my memory. If I come up with something, I will post it here.
 

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Making the same argument.... Would I want a mechanic digging around on brittle wiring harnesses and connectors on the passenger side head that are heat stressed by the manifold? The heat shield alone protecting the sensor will probably turn to dust.
Post#1
Years ago I swapped my sender to a 3 wire style which feeds both the PCM and the dash gauge (when stock the dash gauge is fed by a sender in the passenger side head) I rerouted the wire & it's been working like that for years.
Post#3
Years ago I swapped my sender to a 3 wire style which feeds both the PCM and the dash gauge (when stock the dash gauge is fed by a sender in the passenger side head) I rerouted the wire & it had been working like that for years.
200624


The sensor is attached to the water pump and the OP seems comfortable handling his newer connector, wiring, and sensor.
 

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Nevermind. Sensor was relocated? Whatever. Don't listen to my gauge advice then. Good luck.
 

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The temp gauge gets it's signal from a sensor/sender in the passenger side head. Not the PCM sensor on the WP.

I've dealt with this many times. I may have an answer for you.

If you turn on the key into the run position, car cold (sitting for 12+ hours) and the needle sits below cold, the gauge itself is the problem. Even if the sensor is bad, open or shorted, the gauge should never sit below cold.

The fix is to disassemble the cluster and pop the needle off the pin/shaft, and reclock it to the "C" position. You need to do the needle reclock procedure with the cluster back in the car, the key on, the car cold (sitting for 12+ hours), engine not running.

Root cause:
If the needle bangs hard to H when you crank the car, it will slowly slip on the shaft again until it reads below cold sometime in the future.

I have yet to find a permanent solution to this problem. GM had a dampening oil in the gauge shaft that dries up after many years. If you can stop the gauge from pegging hard when you crank, it will help solve the problem. I considered installing a resistor on the crank circuit but it will affect other functions. Maybe just disabling the bulb test circuit altogether is the best option, but a big compromise. The bulb test circuit is what's damaging the gauge over time.

PS: this behavior is very prominent on 9C1 cars. Not so prominent on Impala SS or Civvy Caprices. I own several 94-96 9C1 cars and several more spare gauges and they all have done this at one point in life. I had a write up about this on 9c1.net 10 years ago. But that forum is long gone.
 

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If the temp guage get its signal from the sensor on passenger side head, could it be so that my fans are constant running due to a issue with that sensor instead of wp sensor ? I ask cause i have replaced just about everything but that . A.C. sensors, wp sensor,thermostat, im lost. A.C. Is charged corectly just wint turn on manualy but will str8t wired🤷‍♂️
 

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If the temp guage get its signal from the sensor on passenger side head, could it be so that my fans are constant running due to a issue with that sensor instead of wp sensor ? I ask cause i have replaced just about everything but that . A.C. sensors, wp sensor,thermostat, im lost. A.C. Is charged corectly just wint turn on manualy but will str8t wired🤷‍♂️
WP sensor feeds the PCM, which includes the fans.

Head sensor feeds the gauge only. The OP apparently rewired his car to use a single sensor on the WP. Odd modification considering having a sensor in the head is exactly where I would want to take my gauge readings. The heads overheat first if there is an issue. A total loss of coolant may not even be picked up from the WP sensor until it's too late.

As for your issue, perhaps it's better to start a new thread. Sounds like you may need a completely different set of troubleshooting steps, including telling us if the Check Engine light is on, and if you tried scanning for any codes. (hint: a CEL will run the fans constantly). I don't think we should hijack this thread. The OP hasn't even commented back yet after a week of us posting suggestions.
 
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