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Diagnostic: Coolant level sensor; must restart to test
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tencentlife
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PostPosted: Thu Jul 20, 2006 7:41 am    Post subject: Diagnostic: Coolant level sensor; must restart to test Reply with quote

The title is pretty self-explanatory, but to elaborate: On a number of threads I and others have advised folks who have a blinking engine temp light but plenty of coolant in their pressure bottles to jumper the connector for the coolant level sensor, to see if it makes the light stop blinking. I was perplexed by the number of respondents who tried this, but the light kept blinking, leading them off on what may be a fruitless search for some other problem. I did some tests on my own '87 to satisfy my curiosity.

All the following presume that you have a wbx that runs without overheating, temp gauge reading is normal, gauge LED is blinking while the engine is running, but coolant level in the pressure bottle (not the license plate overflow bottle) is within 1" of the top.

Here's the rub: if the light is blinking with the engine running and you pull the level sensor connector and jumper it, the light will continue blinking. You must turn off the ignition and restart the motor with the jumper in place to complete the test. If the light then stops blinking (after a few seconds), you have shown the coolant level sensor to be at fault.

If your light is still blinking, jumper the blue/green wire at the connector directly to ground, restart the motor and look again. If your light has stopped this time, the sensor ground wire (the other, brown one) is the fault, so trace it to ground and repair it.

If that doesn't stop the blinking light, try removing the level sensor itself and clean off the two contact pins that are immersed in the coolant. Reinstall, reconnect and test. Remember you must turn off and restart the motor every time you change something to retest. If you merely turn on the ignition without starting, the LED will blink for several seconds and stop, which is normal.

Testing with an ohmmeter across the contacts of the level sensor, you will not get a zero ohm reading with a good sensor immersed in liquid. There will be several hundred thousand ohms. An infinity reading, though, would indicate a faulty sensor if it is immersed in coolant. If you read continuity across an immersed sensor, but the previous tests failed to stop the blinking LED, there may be another electrical fault, e.g. a break in the blue/green sensor wire, or a problem in the coolant low level control unit, located on the main fuse panel.

Sorry if there was any confusion caused by our failure to explain the entire procedure. Now, back to your regularly scheduled hair-pulling.
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vwmaniaman
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PostPosted: Sun May 20, 2007 10:03 am    Post subject: Reply with quote

Hi tencent, There is a small maybe 2mfd electrolytic mounted on a small circuit board inside the gauge itself than can cause the blinking too. Only attempt to replace it after everything you suggested fails. It is surgery to replace it and replacing the whole gauge unit is less frustrating.
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tencentlife
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PostPosted: Sun May 20, 2007 11:32 am    Post subject: Reply with quote

Thanks for the addition, Mr. VMM.

The emphasis in my post was that you have to turn off and restart to verify results when testing the coolant level sensor circuit. I think what you're pointing out would fall under "another electrical fault".

But it is interesting to me that you located that, because it does fit with how I think the level warning controller causes the light to blink. The gauge itself has only one signal input connection. I think the gauge motor acts independently, but when the controller gets a low-level open circuit, it feeds brief pulses of higher voltage to the gauge input. The higher voltage pulses cause the LED to flash, but are too brief to cause the gauge needle to rise, since a gauge motor has a lot of damping built in. So the capacitor fits into that pretty well; a capacitor would store the warning controller's power pulses, and release them in even sharper pulses to make the LED flash. Or maybe the capacitor itself is breaking a power feed up into pulses by it's charge/discharge characteristics.

I'm not knowledgeable enough about electronic components to know for sure. Maybe you can tell me if I'm barking up the wrong tree. Or are you saying that if the cap fails, the LED will flash? Why?

So now you have me wondering if any of the folks who complain of their gauge reading too high might have a problem with that capacitor. I did also learn recently that there were at least two different gauge temp senders by year on late wbx's. I think that's a more likely cause.
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BigDog
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PostPosted: Tue May 22, 2007 5:52 pm    Post subject: Reply with quote

This is good information, but one may want to verify that in fact the coolant level is full before assuming the sensor is acting up.

I found out the hard way that you have to check the coolant level while the engine is running. My tank was full to the top with the engine off and bone dry with it running. I needed to bleed the system and when I did, I added over a gallon of coolant.
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tencentlife
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PostPosted: Wed May 23, 2007 7:58 am    Post subject: Reply with quote

Quote:
All the following presume that you have a wbx that runs without overheating, temp gauge reading is normal, gauge LED is blinking while the engine is running, but coolant level in the pressure bottle (not the license plate overflow bottle) is within 1" of the top.


It's not a diagnostic for the whole cooling system. I saw your post about how much air you had in the top of your radiator (from http://www.thesamba.com/vw/forum/viewtopic.php?t=226321 ):
Quote:
I notice that whenever the coolant level sensor light starts blinking, the temperature gauge starts swinging to indicate that the engine is boiling over.


That's a whole 'nother problem. With a gallon of air in the radiator you're going to experience overheating as well. If the system is properly filled, with a working pressure cap and no leaks, the coolant level will stay high in the expansion tank whether running or not. Use this diagnostic once you've established those facts, but you still have a blinking light.
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BigDog
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PostPosted: Wed May 23, 2007 8:23 am    Post subject: Reply with quote

The condition I related in the other thread was that the light would suddenly start blinking and the gauge would swing fully to the hot position no matter what temperature the engine was. It did it when the engine was stone cold, or when I had been driving. As I was driving, the light would go off and the gauge would read a normal, often a low-normal temperature.

BigDog wrote:
Thankfully, the first time this happened I had driven about a block. The engine was stone cold and I had recently checked the coolant level. Everything went back to normal in less than a minute. So I knew the reading was bogus.


I understand that your thread is for those whose vans are not heating (mine was not) and whose pressure bottle was full. I was saying that for those who know less about these vehicles than you clearly do, they need to check the pressure bottle with the engine running, not off. Had I done that a month ago, I would have solved my problem a month ago.

BigDog wrote:
By bleeding according to Samba Collective Knowledge, I added almost a gallon to the system. The light had been doing its job all along.

Lesson learned: don't assume a faulty system when it could be a faulty owner.


If they have checked the bottle with the engine running, and it is in fact full, then they do not have a problem with air in their system. At that point, the rest of your thread is valuable.
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dbeierl
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PostPosted: Fri Apr 24, 2009 6:31 am    Post subject: Reply with quote

tencentlife wrote:
I think the gauge motor acts independently, but when the controller gets a low-level open circuit, it feeds brief pulses of higher voltage to the gauge input. The higher voltage pulses cause the LED to flash, but are too brief to cause the gauge needle to rise, since a gauge motor has a lot of damping built in.
Correct, although strictly speaking it's lower voltage -- the controller provides a low resistance to ground that simulates what the sender would do if the engine were overheating. The gauge is driven by a bimetal strip with a heating coil wrapped around it.
Quote:
So the capacitor fits into that pretty well; a capacitor would store the warning controller's power pulses, and release them in even sharper pulses to make the LED flash. Or maybe the capacitor itself is breaking a power feed up into pulses by it's charge/discharge characteristics.
Actually that cap controls how long the light flashes after it's been triggered. It is charged through a high-value resistor and when it charges to a certain level the timing circuit flips and shuts off the blinking. When the cap becomes leaky, it eventually discharges as fast as it's being charged, and the shutoff point is never reached. During the marginal stage it can be affected by humidity causing leakage across the board.
Quote:
So now you have me wondering if any of the folks who complain of their gauge reading too high might have a problem with that capacitor.
No, the entire blinker circuit has no effect on the gauge reading.
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240Gordy
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PostPosted: Fri Apr 24, 2009 7:41 am    Post subject: Re: Diagnostic: Coolant level sensor; must restart to test Reply with quote

tencentlife wrote:


. . . All the following presume that you have a wbx that runs without overheating, temp gauge reading is normal, gauge LED is blinking while the engine is running, but coolant level in the pressure bottle (not the license plate overflow bottle) is within 1" of the top . . .


pretty darn specific methinks.
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tencentlife
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PostPosted: Fri Apr 24, 2009 9:04 am    Post subject: Re: Diagnostic: Coolant level sensor; must restart to test Reply with quote

240Gordy wrote:
tencentlife wrote:


. . . All the following presume that you have a wbx that runs without overheating, temp gauge reading is normal, gauge LED is blinking while the engine is running, but coolant level in the pressure bottle (not the license plate overflow bottle) is within 1" of the top . . .


pretty darn specific methinks.


I thought so too, at the time, and I still do.

Dbeierl, thanks for the illumination. What, getting bored over at vanagon.com?
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PostPosted: Mon Sep 16, 2013 12:15 pm    Post subject: Coolant Level Sensor effected the Gauge Reply with quote

I had to disconnect the coolant level sensor when the plastic cracked and was leaking fluid from the expansion tank. I made a plug to seal the tank and left the electric connector hanging, but then the temp gauge started reading totally different.

In the end jumping it brought the temp gauge back to functioning normally.

http://www.thesamba.com/vw/forum/viewtopic.php?p=6853102#6853102

thanks for the in depth info
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climber2377
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PostPosted: Mon Mar 24, 2014 4:32 pm    Post subject: Reply with quote

i noticed that my light blinks when i start the vehicle and the gauge pins to full hot with the engine cold. thanks for this thread... i am going to have to check my coolant level in the bottle with the engine running.
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Ahwahnee
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PostPosted: Mon Mar 24, 2014 4:55 pm    Post subject: Reply with quote

A break in the wire from that sensor can also cause this though, yeah, the place to start is to eyeball the expansion reservoir.
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climber2377
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PostPosted: Mon Mar 24, 2014 5:23 pm    Post subject: Reply with quote

Which sensor is "that" one? In the bottle?
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Ahwahnee
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PostPosted: Mon Mar 24, 2014 6:29 pm    Post subject: Reply with quote

Yes, the one in the bottle. Can be tested with a jumper (or even a paper clip as I recall).
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Vanagator
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PostPosted: Tue Sep 02, 2014 1:56 pm    Post subject: Reply with quote

This may be a stupid question but hey...can you open the expansion tank cap when hot. (not the refill tank but the tank with the sensor in it)??
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Ahwahnee
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PostPosted: Tue Sep 02, 2014 2:36 pm    Post subject: Reply with quote

Vanagator wrote:
This may be a stupid question but hey...can you open the expansion tank cap when hot...


As my dear old English teacher would say "You can... but you may not".

If you open it while hot it is going to puke coolant and you will be dealing with a mess, replacing lost coolant and possibly first degree burns.

Open it when cold and it should be harmless.

You can even open it when the engine is running (and cold) if you keep the RPMs up and don't let the engine slow down until the cap is again tight but that is just a short cut to use in the final bleeding.
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PostPosted: Tue Sep 02, 2014 5:43 pm    Post subject: Reply with quote

Right on, thanks Ahwa!
I have that intermittent temp LED coming on & wanted to jump the sensor when it goes off down the road somewhere. Im usually miles from the homestead when it goes off.
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PostPosted: Tue Mar 24, 2015 8:55 am    Post subject: "Sticky"? tempreture gauge needle Reply with quote

Hi All,

Very informative, and my issue is similar (but different). -Aren't they all?

'87 Westy, 72K OG miles, engine never opened, proper maintenance, coolant full. Cruising down the highway last night at 65 in light traffic, outside temp is in the high 20's, look down at the needle as I do from time to time and to my surprise the thing is very near the bottom. Almost where it is when the van is off. (This is a first; normally it sits right smack in the middle.)

NO blinking LEDS's: not for the oil pressure, not on the temp gauge.

Van is purring along normally, I have two sleeping toddlers in the back, the highway is a bit remote, so I forge ahead with the needle super low for the next 1/2 hour. -I figure the cooler the better, right? Confused

We stop for dinner, get back in the van an hour later, and as the engine warms the needle slowly makes its way back up the gauge to the middle where it normally resides, and it stays there for the duration. And then, as if to re-assure me, there's a bit of traffic as we arrive home so it goes up ever so slightly and the cooling fan comes on as normal for about a minute.

What gives?
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T3 Pilot
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PostPosted: Tue Mar 24, 2015 9:31 am    Post subject: Reply with quote

inspect the wiring from the coolant gauge sender particularly where it connects at the sender in the Tstat housing.

You should consider doing a spring coolant system service/inspection
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PostPosted: Tue Mar 24, 2015 9:47 am    Post subject: Reply with quote

Thanks T3; will do. And if anyone else has additional suggestions, please do chime in.
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