Coolant temperature gauge problem + electrical issues

[the vegenator]

Hey all - happy new year! Still chuggin along in my old Jetta. I've put 26,000 miles on it since rebuild - it's amazing how well it's done since I put the thing back together 2 and a half years ago!

Anyway, so I've had bizarre issues with my coolant temperature gauge that I couldn't even begin to understand, but after some research here I found a few threads that are helping me a little bit ( viewtopic.php?f=8&t=11876 , viewtopic.php?f=8&t=11707&p=82069 , viewtopic.php?f=8&t=12005 ), though they don't totally describe my problems:

The problem first came up last winter, then came back again this winter - when starting up in near-freezing temperatures or below, my coolant gauge will peg with the red light blinking. When the engine warms up it goes back to normal and shows an acurate reading. The weirdest part is, from time to time (more often than not now), the warning light will flash or stay on depending on what electrical accessories are on. It will flash with the turn signal or with each pulse of the windshield wiper relay, or will come on if I'm pressing the brake pedal, or may have a dim glow until I turn the climate control or radio off. It's only an issue in the winter. And none of which makes sense to me.

WOuldn't this suggest a bad reservoir temp sensor? As well as maybe corroded connections at the cluster? I'm flying blind so any other tips that would explain the electrical phenomenons would help me big time.

It's harder to tell what is just a faulty reading or if it's actually overheating... I just finished a 12 hour drive, and toward the end of the trip (night driving, outside temp around 45 degrees), despite the normal weird gauge issues i mentioned, the gauge would show hot and the light would come on in a way that made me worry. This engine has never come close to overheating before though (fan doesn't even work, HA) except in New York City in 100 degree rush hour traffic (heater on full blast ugh).

I'd ideally like to figure this out and get things back to normal. Barring that, I could also just install an aftermarket coolant temp sensor/gauge, though I wouldn't know how to begin http://www.auberins.com/index.php?main_ ... cts_id=134.

Y'all have never steared me wrong. I'm already in the right direction, I think I just need some extra help to get this right again. Thanks!

[82vdub]

Typically when there's weird electrical things going on when other electrical items are energized or de-energized (such as the blinker etc), this points to a bad ground connection. That's probably where I would start. Not sure why it would only be there for the cold temps.

Rigging up an external temp gauge is really simple. Mechanical gauges have a line that runs from the sensor to the gauge. Just find a port to screw it into that goes into the water jacket, or you could even rig up something in a rubber hose too. The electrical one's are pretty straight forward too. Just get 12v power to it (switched with the key) and you've got a gauge when you wire the gauge to the sensor. If you want the light in the gauge to work, then wire that to the instrument cluster light circuit.

[TylerDurden]

I was going to post early this morning, but I had to get rolling.

I plugged in the engine heater last night and expected an easy start and cozy cabin this am, despite the -12F temperature. Well, the engine started on two tries (usually takes one) and the heater was cold. Engine heater failed.

The kicker: my coolant LED was flashing constantly. Never seen that before. After the engine warmed up, a restart had no blinking LED.

I would also say a bad ground for the OP, but not sure what's happening with my buggy... no other signs of a bad ground like the OP.

[Fatmobile]

and make sure the temp sensor plug is good.

and make sure the coolant lever sensor in the resevoir is grounded well.