• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

78 242 overheats (?)

A

adamdrives

Active member
Joined
Aug 9, 2010
Location
San Jose
Hey y'all, I recently picked up a 78 k-jet 242 that had been sitting for 15 years. I've gone through the car and addressed most of the issues. The car now runs and drives just fine, but the temperature gauge always reads hot (as high in the white as it can) after a few minutes of driving, sometimes dropping down a little. I found the radiator was clogged (cool to the touch except at the top and bottom) so I replaced it, but the gauge acts the same. I have already replaced the water pump and thermostat and flushed the cooling system when I got the car. I tested the compensator by grounding the sender wire, gauge goes full hot. I replaced the old looking sender because I had a new one laying around, no change. When I drain the coolant it doesn't come out quite as fast as I'm used to, but it came out clean with no rust after a BG cooling system additive and flush. Prior to radiator replacement the t-stat outlet read as high as 200 or so via infrared thermometer.

I'm wondering if:

  1. the water jacket is clogged (like the radiator) from the car sitting for so long
  2. there is a problem with the temperature gauge itself
  3. its normal for these cars to have a gauge that doesn't read at 9 oclock all the time like the newer cars
  4. when I smogged the car the guy said it was running lean, maybe lean enough to make it run hot?

Any advice on what to look at next? My plan was to find a used temp gauge as I had heard those can be wonky. The fact that little to no rust came out during the flush using BG coolant flush makes me think the insides are ok, but the slow drain makes me think maybe there is some restriction in there that is just solidified.
 
I had the same issue, and fired a parts shotgun at it. New radiator, water pump, thermostat, temp sending unit, converted to electric fan, nothing worked. Cut the center out of my old thermostat even. I finally installed an Autometer temp gauge I had sitting around and it was the gauge cluster went bad...
 
Check the coolant for combustion by-products, could be the head gasket after 41 years. Does it overpressure the cooling system? Does it run cooler when you run the heat full blast?
 
I replaced just the gauge, but was to lazy to hook it back up yet, the Autometer is still in the car.

I probably have the voltage stabilizer problem as well, fuel gauge reads really high and then when it shows 1/2 tank left you can add about 9 gallons or so.
 
Measure the resistance of the gauge temperature sender and compare to the known resistance values for operating temperature ~180 degrees or whatever your thermostat is. On my 740 the gauge sender is towards the radiator.

If you don't know what the resistance should be, you can take the guage temperature sender out and measure the resistance in a pot of water at 180 degrees and 200 degrees. Then stick it back in and see what your reading is at operating temp.

Then you will know if you really are overheating, or if it is the sender or gauge.
 
rb92673 said:
Measure the resistance of the gauge temperature sender and compare to the known resistance values for operating temperature ~180 degrees or whatever your thermostat is. On my 740 the gauge sender is towards the radiator.

If you don't know what the resistance should be, you can take the guage temperature sender out and measure the resistance in a pot of water at 180 degrees and 200 degrees. Then stick it back in and see what your reading is at operating temp.

Then you will know if you really are overheating, or if it is the sender or gauge.
Click to expand...

It's a new, genuine sender, but if nothing else pans out that's an idea.
 
adamdrives said:
That is the three pronged relay looking copper bar that plugs in to the back of the gauge, no? I was calling that the compensator.
Click to expand...

Guess I did scratch my head on that one. There's a compensator of sorts used in the later clusters called the Temp Faker, and one might "test" it by grounding the sender wire. But seeing the gauge respond by pegging when you do this on your '78 might be normal regardless of whether the stabilizer is OK or not, so my bet is, as it was with Trev29, on the three-pronged thingie. They can fail open (dead gauge) or stuck fully on (high-reading gauge.)

Edit: The stabilizer on my cluster is right between the gauges:

cluster699.jpg
 
Last edited:
I'm going to make some of these if anyone wants one. I need 2 of them so no big deal to make more. I ordered 10 regulators to get them cheaper.
 
I'm just soldering wires to it, I don't know how to find the proper connector. The other end of the wire will be male spade terminals. I ordered them a couple days ago, shouldn't take too long to get here.
 
Volvo (or maybe VDO) provided replacements using the solid state voltage regulator. They used the low-dropout automotive version.

vstab3358.jpg


Compare to the innards of the mechanical version

vstab3349.jpg


vstab3351.jpg
 
You must log in or register to reply here.
Back
Top