# Temp gauge is normal, then it just pegs out immediately!



## jetgl (Mar 31, 2007)

So, i just picked up a 89 carat jetta for my girls commuter car. the thing runs flawless. I was driving it, and the temp gauge was sitting right in the middle, then immediately pegs out, and the red light starts flashing. It stays like that until the car sits for a bit. its not overheating at all. It appears that the cts was replaced recently. What gives?


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## cuppie (May 4, 2005)

Which coolant temp sensor was replaced: the one for the engine management, or the one for the temp gauge? They're separate sensors.


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## jetgl (Mar 31, 2007)

he replaced the blue one


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## cuppie (May 4, 2005)

That one's for engine management. Has nothing to do with the gauge.

Most likely, your gauge sensor went janky, and needs new. Other possibilities include a gauge sender wire that's randomly grounding out. But, the failure pattern more decribes a bad sender.
The one you're looking for is located on the coolant flange (on the head), and has a single wire attached to it.
If you want to do a quick check of the sender (sorry, I don't have the resistance map handly), unplug it when the gauge freaks out, and measure the sender's resistance. If it's (near) shorted (~0 ohms), it's either failed, or the engine really is severely overheated. 
Directly measuring the enigne temp (read: with a thermometer or temp probe) will answer that question, though.


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## jetgl (Mar 31, 2007)

wow thanks for the detailed reply! Most don't go through the trouble! I had a 8v a few years ago and was extremely familiar with the electrical system. Now that I'm getting back into it I'm finding I'm extremely dusty. Ill go to the store and grab a new one. I know its not overheating. One second its perfect, and its like someone flipped a switch to peg the gauge out. Its not a linear movement towards hot.


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## cuppie (May 4, 2005)

You're welcome.
One other thing I meant to say (as a troubleshooting tip), but forgot:
when it freaks out, unplug the sender. If the sender was the cause of the freakout, the light will stop flashing, and the gauge will drop to full-cold.

Oh, and, the standard advice: get a Bentley for the car. Best tool you'll ever buy.


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## Fat Rabbit (Nov 18, 2001)

There are two temp sensors on the front radiator hose flange on the head. As mentioned, the blue one is for the ECU while the black one is for the gauge. The black sensor will have two wires. The black sensor is PN 251 919 501.

As suggested -- when it acts up -- unplug the black sensor and see what happens. It could be a bad sensor or a short in the wiring. 

One other thing -- I presume that you have checked the coolant level and it is correct. The coolant reservoir has a low level switch that sometimes can cause the coolant light to flash. The PN for that sensor is 251 919 372. However, topping up the coolant will usually fix the flashing. 

Some Jettas also had a low coolant level control unit on the fuse and relay panel. That relay would be in position 5 on the fuse and relay panel. 2nd relay row above the fuses -- fifth relay from the left. PN 191 919 376A Check the other stuff before the control unit. FR


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## jetgl (Mar 31, 2007)

Fatrabbit- You say that the gauge sending unit is the black sensor with the two wires, in front of the blue ECU sensor?


Cuppie- You say the gauge sending unit is the sensor with one wire, I'm assuming the one thats more towards the firewall, with a little rubber boot on it

Which one is it? Thanks again guys, I wanna get this thing reading right so my fiancee can drive with confidence!


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## cuppie (May 4, 2005)

"More toward the firewall" - on the transmission end of the head? That one's one of the oil pressure switches.
Coolant temp sender is (and always was) on the water outlet flange on the front side of the head.

FR is actually correct on the appearance of the temp sender - I had forgot that VW had gone to the 2-pin senders. All of my 'older' cars ('79 bunny, '82 'rocco, '87 Golf GT, and my '88 rocco) used the older single-spade-terminal sender.


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## Glegor (Mar 31, 2008)

cuppie said:


> You're welcome.
> One other thing I meant to say (as a troubleshooting tip), but forgot:
> when it freaks out, unplug the sender. If the sender was the cause of the freakout, the light will stop flashing, and the gauge will drop to full-cold.
> 
> Oh, and, the standard advice: get a Bentley for the car. Best tool you'll ever buy.


on the flip side of that, if its grounding out, it will peg the gauge, and make all the bells and whistles go off..

x99438320 on a bently, or ATLEAST A REALLY GOOD wiring diagram..


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## ps2375 (Aug 13, 2003)

I had an 85 Jetta that when the interior of car was more than 80*F, the water temp gage would read at 3/4 of gauge, with no alarm lite flashing. I went thru water pump and flushed/checked radiator and all sorts of checks. I finally parked it next to my scirocco and started them one morning together and took temp readings. They heated up the same, but the gauge in the Jetta kept climbing, seems something on the cluster didn't like to be warm. hen your car does this, is it actually overheating or just indicating that it is?


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## jetgl (Mar 31, 2007)

:banghead:


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## jetgl (Mar 31, 2007)

Well, I replaced the black sending unit sensor, right next to the blue one in the front. Same thing. Even when I unplug the sensor, it acts the same. I wonder if this guy has the leads swapped or something.. I'm not hearing the fan kick on, so I dont think its overheating. No steam, runs perfect.


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## Fat Rabbit (Nov 18, 2001)

Good. At least we know that the coolant temp sending unit is not faulty. And you have checked the coolant level in the reservoir. Right???? 

You might also try to get the cooling fan to run. Sometimes it can take 15 minutes or more with the car sitting at idle to get the radiator cooling fan to run. Watch the fan and wait for the fan to come on. Have a friend observe the temperature warning system in the cluster. It is possible if you are driving the car around there is enough convection cooling from air passing through the radiator to cool the coolant for a time and the warning system is working appropriately but the fan switch is bad. Hopefully the fan system is functioning normally. However, The fan switch is not too expensive and is easy to replace. You could also have someone test the engine temp or do it yourself. 

Check the low level warning switch the same way you checked the temp gauge switch -- disconnect it and see if the issue goes away. That switch is located in the coolant reservoir -- 

Both the level switch and temp sending unit are wired into/through the coolant low level control unit which looks like a relay and is on the fuse and relay panel as mentioned in my post above. It should be in relay position 5 -- the PN is 191 919 376A or similar. If you have a relay with that or a similar PN -- remove that it and see if the issue goes away. Then report back. FR


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## jetgl (Mar 31, 2007)

Thanks will do. Coolant is topped off. I noticed the fan wasn't turning on also. When I click on the AC, the fan comes on. My temp is reading 196-204. But when it gets up to 204, the light starts blinking and it pegs out..I'll check out that relay.


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## jetgl (Mar 31, 2007)

Well, I think I found it. I think it was overheating. I pulled the connector off the coolant fan switch and jumpered it and the fan kicked on. The car has been sitting for a year, and the switch is filthy. Does anyone know what temp range I need for the switch? I cant read it cause its all corroded. Now I gotta address this bucking on/off throttle issue....


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## Fat Rabbit (Nov 18, 2001)

Turning on the AC automatically turns on the radiator fan. But that does not test the fan switch which operates based on temperature -- actually two different temperatures for the two speeds. 
I'd feel better if you could tell me that the rad fan kicks on and off on its own. It is just a matter of observing it over 15-20 minutes with the car running. 

Take the electrical connector off the coolant tank and see what that does. That is real easy to do. 

Then try to pull the control unit and see what happens there. The control unit is pricy new -- $30-40 or so. A junkyard part will work nicely. The way the wiring diagram shows -- the coolant temp gauge should work with that relay pulled. 

Some of the coolant level switches are replaceable and with some you have to replace the whole coolant reservoir. Once again try for JY parts if you need them. FR


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## jetgl (Mar 31, 2007)

Right now the car is idling, and the coolant res pug is disconnected. Nothing out of the ordinary is happening. Its warming up to temp. Do you know what temp these cars normally run at? Reading 165 on the gauge, fan still hasn't kicked on.


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## jetgl (Mar 31, 2007)

up to 196, fan still hasnt kicked on. When i pull the fan control relay (bright yellow relay) nothing happens. Its right above relay 53, top row fifth from the left. Im now up to 200, no flashing lights, not pegging yet, but about 5/8 across the temp scale


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## jetgl (Mar 31, 2007)

at 204, fan just kicked on. NOT overheating, no flashing red light. With the coolant level sensor unplugged seems if if its all fine and dandy. I plugged it in, and immediately the light started flashing, and the gauge pegged. Unplugged it, and it went back down.


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## Fat Rabbit (Nov 18, 2001)

So it is likely the level switch. It is a $3 part new -- on line. Problem is shipping $ -- unless you need other parts. A junkyard will probably ask $4-5 for a used one. I think I already quoted the PN above in a response. FR


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## in5ane (Dec 17, 2004)

*Did you ever figure it out?*

I'm having the exact same problem with my '89 except mine will sort of gradually climb to 3/4's on the temp gauge.

Just changed the CTS and the gauge sensor is next but wondering if that was your issue?

I fear I have radiator flow problems although my coolant is definitely circulating back to the overflow
:banghead:

I should mention my level sensor is also new as I just changed the overflow tank as well and checked all hoses


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