Temperature gauge rising and falling

[EverettC]

My 1996 manual ford ranger 4x4 supercab is having a strange problem with the temp gauge. When I first start my car it takes a long time to get past C then when I start driving it will reach operating temp at 1/2 the temp gauge, but then the temp will suddenly drop to around 1/4-1/5 of the gauge and then it will rise and drop over and over. This happens when driving and when idling. I’ve done some research and it seems like I would just need to replace the sender and the sensor. Does anyone else have this issue? And what did you do to solve it?

 

[Shran]

I chased this problem on my '94 and it was ultimately a partially plugged radiator. I think what would happen is a big pocket of very hot coolant would make its way past the obstruction, spike the temp gauge, and then it would sit in the radiator for too long, causing it to cool much more than it normally would (thus the constant up/down.) Or it was getting hot enough to boil and there were pockets of steam getting caught... anyway I tried multiple thermostats, three water pumps, hoses, two fan clutches, flushed several times and the last thing was just to replace the radiator.

I saved that for last because it was new when I bought the truck but the engine had an incredible amount of dirt, rust scale and crap in the cooling system. Every time I would flush it, the bottom of the radiator would always be full of rusty mud and I'm sure it just got permanently plugged. It's fine now...but man...I have seen very few cooling systems that were that bad.

 

[RobWigglezz]

Their resistance changes as temp increases and the gauge reads that. The connection to the sensor could be loose or corroded which changes the resistance.

Or the flexible circuit board behind the cluster has s loose connection to the gauge itself. Smack the dash with your palm after it's warmed up and see if the gauge bounces.

 

[EverettC]

I chased this problem on my '94 and it was ultimately a partially plugged radiator. I think what would happen is a big pocket of very hot coolant would make its way past the obstruction, spike the temp gauge, and then it would sit in the radiator for too long, causing it to cool much more than it normally would (thus the constant up/down.) Or it was getting hot enough to boil and there were pockets of steam getting caught... anyway I tried multiple thermostats, three water pumps, hoses, two fan clutches, flushed several times and the last thing was just to replace the radiator.

I saved that for last because it was new when I bought the truck but the engine had an incredible amount of dirt, rust scale and crap in the cooling system. Every time I would flush it, the bottom of the radiator would always be full of rusty mud and I'm sure it just got permanently plugged. It's fine now...but man...I have seen very few cooling systems that were that bad.

Yeah when I flushed mine twice I got lots of crap out. I really didn’t want to replace the radiator just because that was the most expensive fix compared to replacing the thermostat and coolant temp sensor. I’m going to look into it a little bit more before I do get a new radiator.

 

[EverettC]

Their resistance changes as temp increases and the gauge reads that. The connection to the sensor could be loose or corroded which changes the resistance.

Or the flexible circuit board behind the cluster has s loose connection to the gauge itself. Smack the dash with your palm after it's warmed up and see if the gauge bounces.

I just replaced the coolant temp sensor and that didn’t seem to fix it but I’ll have to try smacking the dash after it warms.

 

[RobWigglezz]

I just replaced the coolant temp sensor and that didn’t seem to fix it but I’ll have to try smacking the dash after it warms.

My tach was bouncing and not reading sometimes and a tap on the dash would make it work. There's metal spring clips that hold the circuit board to the cluster as well as connect to the actual gages. I just bent them to make them hold tight again