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Old 09-13-2004, 02:37 PM   #1
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Exclamation Low Coolant Sensor

Hi all,
I have an 84 310 Limited MH.

When the FO had it painted, the coolant overflow reservoir also got painted. This made it impossible to tell how much liquid was in it since it is mounted in a position where you can not look down into it. On our recent 7500 mile jaunt I checked it by using a "dip stick". Upon our return I decided to fix it. So I took it out and sanded the paint off of the reservoir. Yesterday when I reinstalled it, the low coolant light was on until I added coolant and then it went off. Today, the light stays on even though the reservoir has plenty of coolant in it.

I checked to see that both wires are still connected and that there are no obvious breaks in either. Any ideas?
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Old 09-13-2004, 05:25 PM   #2
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Quote:
Originally Posted by guy99
Hi all,
I have an 84 310 Limited MH.

Yesterday when I reinstalled it, the low coolant light was on until I added coolant and then it went off. Today, the light stays on even though the reservoir has plenty of coolant in it.

I checked to see that both wires are still connected and that there are no obvious breaks in either. Any ideas?
Is the sensor in the overflow tank, or the radiator?
It is possible the sensor is bad, and will need to be replaced
You could try cleaning the sensor, although most of them are cheap enough you may as well replace it when you take it out anyway.
Terry
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Old 09-13-2004, 06:40 PM   #3
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It's in the overflow tank

Quote:
Originally Posted by argosy20
Is the sensor in the overflow tank, or the radiator?
It is possible the sensor is bad, and will need to be replaced
You could try cleaning the sensor, although most of them are cheap enough you may as well replace it when you take it out anyway.
Terry
Actually, as far as I can tell there really isn't a "sensor". There appear to be two terminals that penetrate the tank. As far as I can tell, these are just terminals. One terminal connects to the dash, there other is connected to a wire which is grounded to the frame. I think when there is fluid in the tank, the circuit to ground is completed through the fluid.
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Old 09-13-2004, 06:45 PM   #4
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It's fixed

Quote:
Originally Posted by guy99
Hi all,

I checked to see that both wires are still connected and that there are no obvious breaks in either. Any ideas?
It turns out that the wire from one of the terminals to ground (frame) was mechanically intact but had very high resistance. It was kind of "crunchy" when flexed especially near the terminals. I replaced it and all is well.
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Old 09-13-2004, 06:55 PM   #5
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Quote:
Originally Posted by guy99
It turns out that the wire from one of the terminals to ground (frame) was mechanically intact but had very high resistance......
Great detective work! - I'm sure that your "find" will benefit many of us.
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Old 04-21-2006, 09:54 PM   #6
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Unhappy Continuing Low Coolant Light Woes

Hi all, I'm posting to this old thread so as to concentrate information on this topic for future searchers. Since my previous post on this subject I have had no end of frustrations with the low coolant indicator in my 1984 310 Limited MH. It comes on and goes off at apparently random times. I have had several different mechanics look at it as well as spending a good bit of time on it myself.

First a description: there is a wire that comes from the dashboard that is energized when the key is on. This wire connects to a terminal in the side of the coolant overflow bottle. There is another terminal in the coolant bottle which has another wire connected to it which is connected to ground. The things I have done include:

1. Replaced ground wire twice
2. Replaced section of 'hot' wire
3. Cleaned ground connection
4. Cleaned terminals on coolant bottle
5. Replaced nuts on terminals with new brass
6. Applied dielectric grease to all connections
7. Connected hot wire directly to ground (this is how it is now)

The light continues to come on and go off at apparently random times.

I have a new hypotheses, I mentioned in another thread (http://www.airforums.com/forum...rce-21067.html) that I have discovered that the hot wire going from the ignition switch to the volt gauge has much lower voltage than the voltage at the battery and than in order to get a correct reading at the volt gauge I had to "wire around" the ignition switch. My hypotheses is that the voltage in the low coolant circuit is low and that this circuit works by reading the resistance across the two terminals in the coolant bottle and is not working correctly because the input voltage is low.

So what do you think? Any other ideas?
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Old 04-22-2006, 12:04 AM   #7
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Hello Guy99,

The dash light should have 12v to it ,key on ,the wire down to the bottle should be a ground wire into the bottle as your past post says,the other wire from the bottle goes to ground .As the level of coolant goes low the ground circuit connects ,meaning the frame ground has full continuity with the other wire from the bottle up to the dash ,which provides a ground to the dash bulb socket to turn the light on .I dont see how as it is now the two terminals in the bottle complete the ground circuit ,but that is the objective ,maybe some part fell out when the bottle was removed ,turned upside down ,seems possible. resistance changing is the key ,your right ,no wire at that bottle should be 12volts ,nor should you rewire it to achieve that .It might be time to call the GM dealer and try to get a new coolant bottle as yours just may not work again ,the wires at the bottle are grounding when they shouldnt turning on the light.Or the wire going from the bottle to the light grounding somewhere as it goes up to the dash panel .If the wire you say is hot is now connected to ground ,it would be a direct short popping a fuse so be careful doing that kind of thing.All idiot lights ,dash lamps ,warning lamps are key on power at the light bulbs in the dash a ground circuit is employed to the sender unit ,oil pressure ,temp sender coolant bottle low level etc. Plus those 12v hot power to the lamps are fused at the fuse panel so if you ground any fused 12v source the fuse will blow saving your wiring harness from melt down .Now ,the volt gauge should read battery voltage (you already know this) so why it reads less than is strange .So Dont know what the deal is there ,possibly an old ignition switch being faulty .I would use caution in wiring around stuff .See if you can get a shop manual for a GM p30 chassis or larger so you can get good wiring schematics and info.Now all this info being said ,If the wire going from the dash to the bottle is 12v and your test light confirms this , somthing is wrong in the wiring there.

Good luck

Scott
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Old 04-22-2006, 05:44 AM   #8
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At the risk of sounding vague...

Quote:
Originally Posted by guy99
First a description: there is a wire that comes from the dashboard that is energized when the key is on. This wire connects to a terminal in the side of the coolant overflow bottle. There is another terminal in the coolant bottle which has another wire connected to it which is connected to ground. The things I have done include:

1. Replaced ground wire twice
2. Replaced section of 'hot' wire
3. Cleaned ground connection
4. Cleaned terminals on coolant bottle
5. Replaced nuts on terminals with new brass
6. Applied dielectric grease to all connections
7. Connected hot wire directly to ground (this is how it is now)

The light continues to come on and go off at apparently random times.

I have a new hypotheses, I mentioned in another thread (http://www.airforums.com/forum...rce-21067.html) that I have discovered that the hot wire going from the ignition switch to the volt gauge has much lower voltage than the voltage at the battery and than in order to get a correct reading at the volt gauge I had to "wire around" the ignition switch. My hypotheses is that the voltage in the low coolant circuit is low and that this circuit works by reading the resistance across the two terminals in the coolant bottle and is not working correctly because the input voltage is low.

So what do you think? Any other ideas?
I think you have a wiring problem not in your coolant tank. Many of these old coaches suffer from bad connections, or wires that have suffered metal fatigue from years of flexing and vibration. You DO have a voltage limiter on the backside of your instrument panel that could be bad, or a bad ground going to the instrument panel. The voltage limiter is an inexpensive part, and easy to install. Go there first, and if your troubles continue, start checking grounds from the panel, and voltages going in.
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Old 04-22-2006, 09:41 AM   #9
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Terry,
Thanks for the suggestion. What does a voltage limiter look like (sounds like its more than a simple resisitor) and where am I likely to find it?
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Old 04-23-2006, 06:08 PM   #10
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Guy, let me know when you solve this problem we're having... ...me too.
Have you tried scrubbing the contact points inside the reservoir? Often the coolant tank gets so gummed up that it can cover the contacts that need the coolant to be in contact with both to complete the circuit. I tried to scrub mine with a long brush but have still to look at the intermittant light.
I did paint my tank to make it dissappear behind the grill. I can look down into mine or see the coolant on the down tube.
I'll be curious to see what fixes this problem.
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Old 04-23-2006, 06:43 PM   #11
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The voltage limiter is attached to the rear of the instrument panel where all the printed circuits run. It should have three posts on it, and does function as a resistor. One that is bad can cause all kinds of mischief, like allowing you to think you have more gas than you do, a cooler engine, higher or lower oil pressure, almost anything that depends upon less than 12 volts to display the correct reading.
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Old 04-23-2006, 09:45 PM   #12
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I have the same problem with my 345. In fact when I replaced my overflow tank I kept my old one in case I needed those terminals. Interestingly, with the entire thing disconnected it still comes on occasionally. I know Dennis (87MH) has the same problem.

I've read previous posts which indicate that the low coolant sensor is in fact in the radiator. The only sensor I've found is the temp sensor.

Seems like it would need to be carrying some voltage or connect a ground through a relay (rather simply than make a ground) as its a low coolant light. So when the coolant drops below the contacts it should turn the light on - am I thinking the right way on this?
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Old 04-23-2006, 09:53 PM   #13
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Steven, it works kind of like that, when the voltage drops, the light is supposed to come on...in theory. That is why if the instrument panel is not getting enough voltage, it could cause the light to come on.
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Old 04-23-2006, 10:06 PM   #14
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Terry, as usual you are a wealth of useful knowledge. So a low voltage situation in the dash would turn the light on? Oh joy!

Hmm...my low coolant light comes on at random, fuel guage is inaccurate below 1/2 tank, sometimes I think the engine temp is reading too high (according to my brand new fan clutch). Interesting.

Are these voltage limiter things still available? Car part store? Radio Shack?

PS - This explains why the PO had a piece of black electrical tape over the low coolant light.
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