Temp guage says "Hello!"

[Choking Hazard]

My temp guage is waving at me.  

First- I gotta overheatin' problem (apparently)
Next- I've replaced a good T-stat, and a good fan clutch.  I've fixed the inop electric fan, too.  And a little less than a year ago I pulled the cluster and regrounded the temp gauge.
But it started runnin hot.  Just a little bit for a couple of drives, then hitting 3/4 before I started all these replacement exercises.

Today the gauge started waving back and forth from just over middle to almost 3/4.  Back and fourth avery 1.5 or 2 seconds- dink dink dink.

Is it bad data?  Maybe the guage or grounding point has gone bad?  Before, I pulled the instrument cluster, the temp guage just went up and up, not all wavey.

Anybody seen this before?  I'm at a loss.  I don't wanna pull the cluster. but I also don't want to drain the cooling system again either.

[dino245]

My guage does the same thing on long drives, over 2 hours of so. I know it's not the guage or the wiring and I have theorized that it is the sending unit. I have been too lazy to change it since it still works on the daily comute but freaks out on longer trips. I will change it when the new motor goes in.

[2002maniac]

my car did this. I cleaned up the cluster grounds and it's fine now.

[CU Zcar]

My gauge was settling at about the 3/4 tick mark with occasional waving fits like you described until I pulled the cluster and cleaned up the ground pad.  After that I had a stable reading just under the halfway tick mark at operating temperature, but that was only a couple days ago.

[D. Clay]

One time I spent a lot of money until I found a bare spot in the wire to the sending unit. Put a thermometer on the thermostat housing. I thought my car was overheating until I did this. The gauge said hot but the thermometer said 190 degrees.

[Choking Hazard]

Put a thermometer on the thermostat housing. I thought my car was overheating until I did this. The gauge said hot but the thermometer said 190 degrees.

Okay!  Thats quantifyable, and a place to look.  I'll bet I can get some kinda stick on thermometer tape or sumthin.  Guess I should pull that instrument cluster again.  Anybody need a fan clutch?

I guess I would rather it be bad data instea of engine problems anyway, but I may flush the rad again.  I've bled it over and over, and I tried to be sure the T-stat was in with the arrow pointing up so air escapes.  Taking all that instrument crap off is a pain.  And I'll wonder if I should do the battereis "while I'm in there"....

[Choking Hazard]

I was drinking a beer listening to the tink-tink cooling off sounds so I started to prob about under the hood.  I noticed that the one on the right, going to the bottom of the radiator, was cool to the touch from the rad all the way up to the last 6 inches of hose before the t-stat housing.  The hose coming from the driver's side of the housing and going to the top of the radiator on the left side was quite warm.  I think the water pump p is OK becaue it produces a healthy stream back into the return bottle

Does that sound like blockage in the radiator or something else?  Which way does cooolant flow?  Nuts, this is annoying.

[D. Clay]

It moves from top to bottom in the radiator. In cross flow radiators it's side to side. M42's have top to bottom type. The upper hose is usually the hot one from the motor and the bottom one has a spring in it to keep it from collapsing under suction.

[Choking Hazard]

thanks.  I'm a guessin that means I have good radiator function.  Maybe water pump?

[Choking Hazard]

Finally got a thermometer.  I'm seeing 192 degrees when the guage is just shy of red (scares me to test it this way, but whatcha gonna do?).  That certainly sounds OK.  Is that pretty much operating temp?  I'm not certain but I think my thermostat is 90.

[D. Clay]

A 90 Celsius thermostat should run at about !92 degrees Fahrenheit. M42 thermostats are mostly 88, 89, and 90 if I remember correctly.