Another Cooling Coolant question

[ORCoaster]

On my way home this weekend I was approaching the cool coast and thought I best bump that lever to the warm side a bit if I don't want to be as cool as a potato when I get home.  When I did that the temp gauge on the dash jumped like three lines.  Opps, gotta problem here I thought.  So I watched it climb a bit more as I was revving along in third up a fairly long grade.  Once over the top the temp came back down to the "normal" lines I run.  Then it jumped back up a bit.  I was plenty warm by then and about to pull it over and check for loss of coolant but when I moved the lever back to the cool side the gauge again went to the low side of the scale.

My question is this.  If I am not cycling water through those hoses does the temperature on the gauge really reflect the temperature of the coolant?  Or is it more of an indirect measure and I am really running hotter than the gauge indicates? 

By opening up the flow I think the sensor got the real temp on it and that is why it launched itself a few lines. 

Today I decided that the water returning to the overflow tank would be the hottest if I were not using the heater core flow.  So I took my second gauge and sender and put it in a tee and plumbed it in line to see what the measure is as I drive and play with the heat lever. 

Does measuring the coolant temp from the line into the overflow tank tend to be that much different than the sender on the flange?  I know most of us don't run two temperature gauges but for those that do let me know if you get somewhat differing temps as you drive.  I hope I am not looking for trouble as winter is coming and I need heat. 
 

[wdkingery]

i just had to wire my fan directly to a power source, thru a switch right.. damn do those wires get hot in that configuration! i think i am actually risking a car fire runnin the fan.. AND BOY DOES IT BLOW! i don't think it was designed to run at such a speed! must have had a resistor in line or something.
and what kinda flippin mountains do you go up that you need to climb them in third??? good god. i can climb anything in front of me, so long as i'm going at least 45 mph, in 5th. i can climb a decent hill @ 30 mph in 5th even.. but third.. and you get these 52 mpgs on 3rd gear hill climbs? i took yer advice and have been runnin the piss outta the little car; speeds as high as 70, definitely keepin with the flow.. not hittin 5th till 45 or so. matter of fact, i found she can run thru the gears much better if i wind it on out a bit. it's got like a sweet spot around some magical (higher) RPM. and i'm at 287 miles for my half tank (normally i hit half at 300 miles) so i think my mileage may stay the same!

on to the meat.
 my observation of the coolant and fuel gauges in relation to an added electrical load has always been a gain. i'm not sure if it's the gain you speak of, but i can tell you my fuel mileage will appear ALOT better with the headlights on. coolant temp certainly goes up too. definitely goes up with even so much as the parking lights runnin.

[ORCoaster]

Ah ha.  So the real problem here is not the temperature of the water at all then is it.  The voltage that runs those stinky VDO gauges is the trouble.  When I flipped the temp lever over I also hit the fan on low.  That is the voltage change and what you speak of a change in the reading I was seeing.

You a flatlander?  The coast of Oregon is generally not that much of an elevation change but when it does look out.  There are port towns that the highway dips down to and to get in and out of those major rivers, like the Umpqua in Reedsport there is a considerable elevation change.  So here is the problem.  30 miles per hour then the sign says 55 MPH.  Most drivers just tromp it and off they go right.  Well, if you have the fuel screw backed out and no real RPMs built up what do you do.  Drop it down a notch and accelerate up to speed.  My Rabbit does 58 in third up that hill and that is it.  Once at the top I can upshift straight to 5th as it takes a negative grade for a while around a lake.  And yeah I get good mileage doing this.  That is why I suggested to you to put some RPMs into your shifting.  It is more fun, runs better and the mileage is about the same or better. 

Like you say she can run thru the gears much better if I wind it on out a bit.  And That sweet spot is different for all cars, I can watch the rear view mirror and when the headlights of the cars behind me stop shaking I know I am in that spot.  Tough to hold as the road or wind will generally change but if the road is flat or fairly so then I just pull out the cruise wire and relax. 

So the change in location may make a small difference in the reading but I will watch the electric loading and see about the other gauges changing as well.  I never noticed the fuel gauge shifting drastically like the temp gauge.  I run an amp gauge as well and most times it is 3-5 amps at cruise.  I did notice the belt was loose on the alternator this weekend so that got a bit of a tighten as well.  I thought slippage was the trouble.  Maybe it was but on the alternator more than the water pump.

You still got me in the MPG department.  But now that you drive like more like a normal person we will see what happens over time.  I won't fill for about another 4-5 days.  I use the pickup in town here as the battery dies if I don't. 

[mystery3]

on to the meat.
 my observation of the coolant and fuel gauges in relation to an added electrical load has always been a gain. i'm not sure if it's the gain you speak of, but i can tell you my fuel mileage will appear ALOT better with the headlights on. coolant temp certainly goes up too. definitely goes up with even so much as the parking lights runnin.

The voltage regulator on your cluster is not functioning properly. I'm not sure if that's the op's prob though maybe.

[92EcoDiesel Jetta]

Temp gauge reading should not change with changes in electrical load. Check your grounds out for the sensor and the cluster. Coolant temps do not change that fast.

To diagnose, you can measure the resitance of the coolant temp sensor and determine what temperature your coolant is at.

[R.O.R-2.0]

well, on mk1 cars, there IS a flow control valve on one heater hose, and if you turn the heat off (close that valve) all the coolant is being routed thru the radiator.. turn the heat on (open the valve) and a substantial amount of flow can be going directly to the heater core. if your radiator is plugged at all, that wont help. or if your thermostat is starting to fail... liquids take the path of least resistance..

i dont generally trust VW gauges.. ive never seen 2 clusters that read at the same place, when reading the same temperature..

usually, i install an aftermarket temp gauge on all my cars..

[ORCoaster]

R.O.R.   i dont generally trust VW gauges.. ive never seen 2 clusters that read at the same place, when reading the same temperature..

usually, i install an aftermarket temp gauge on all my cars.

I think the same in that I don't trust the ones that give me L to H or C to H readings so I pulled a VDO gauge out of a Caddy and ran it next to the sweep one in the dash.  But there in might be the problem.  Same kind of sensor gives the readings but the position on the temp scale is different.  Not a problem if one is aware of which bar line equals 80 C  right?  But what I didn't like is that opening the valve to the heater core suddenly jumped both gauges about equally.  Up to 100 C.  That was my main concern.  But if both run off the voltage of the dash, which is how I wired them then both are subject to the same flux of the voltage regulator correct?  Time to check that out and the ground, I think I saw somewhere an added ground wire to the instrument panel was an easy to do.

I know the radiator and thermostat are new so I don't think that is a problem.  I am concentrating on voltage flux. 

thanks guys.