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Old 09-24-2019, 09:55 PM   #1
adamdrives
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Default 78 242 overheats (?)

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.
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Old 09-24-2019, 10:10 PM   #2
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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...
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Old 10-08-2019, 02:06 AM   #3
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Damn! Hate when that happens. Was it the whole cluster, or just the temp gauge you replaced?
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Old 10-08-2019, 07:56 AM   #4
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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?
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Old 10-08-2019, 08:15 AM   #5
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Don't forget the gauge stabilizer. The old mechanical ones have a finite life.

https://www.brickboard.com/RWD/volvo...e_running.html
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Old 10-08-2019, 12:32 PM   #6
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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.
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Old 10-08-2019, 04:58 PM   #7
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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.
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Old 10-09-2019, 03:15 PM   #8
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Quote:
Originally Posted by cleanflametrap View Post
Don't forget the gauge stabilizer. The old mechanical ones have a finite life.

https://www.brickboard.com/RWD/volvo...e_running.html
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.
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Old 10-09-2019, 03:16 PM   #9
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Quote:
Originally Posted by rb92673 View Post
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.
It's a new, genuine sender, but if nothing else pans out that's an idea.
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Old 10-09-2019, 04:59 PM   #10
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Quote:
Originally Posted by adamdrives View Post
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.
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:


Last edited by cleanflametrap; 10-09-2019 at 05:19 PM..
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Old 10-09-2019, 06:49 PM   #11
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Yeah you are best off replacing the mechanical regulator with a 7810 MOSFET. It's pretty easy: https://forums.tbforums.com/showthread.php?t=321645
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Old 10-09-2019, 08:12 PM   #12
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Quote:
Originally Posted by Harlard View Post
Yeah you are best off replacing the mechanical regulator with a 7810 MOSFET. It's pretty easy: https://forums.tbforums.com/showthread.php?t=321645
Cool, I will look into that.

Edit: best place to get one? I see them on eBay sold by the ten, anyone on the board have an extra? RadioShack doesn't list them.

Last edited by adamdrives; 10-09-2019 at 08:22 PM..
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Old 10-09-2019, 08:26 PM   #13
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Wasn’t there a Volvo special tool for this problem? Maybe a resister with some spade terminal?
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Old 10-09-2019, 09:16 PM   #14
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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.
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Old 10-11-2019, 02:04 AM   #15
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Quote:
Originally Posted by 2turbotoys View Post
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.
Count me in, when they're ready. I am fairly certain it's not actually overheating so not urgent.
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Old 10-11-2019, 08:03 AM   #16
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Quote:
Originally Posted by 2turbotoys View Post
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.
Are you doing a connector to the regulator pins, or just soldering wires to it?
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Old 10-11-2019, 01:38 PM   #17
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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.
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Old 10-11-2019, 02:04 PM   #18
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Volvo (or maybe VDO) provided replacements using the solid state voltage regulator. They used the low-dropout automotive version.



Compare to the innards of the mechanical version



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Old 10-11-2019, 02:30 PM   #19
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VP and Skandix sell modified voltage stabilizers but they ain't cheap. They must be having trouble getting cores returned because the core charge is exorbitant.

http://www.vp-autoparts.com/us/main....971&searchfld=

http://www.skandix.de/en/spare-parts...ument/1021371/
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Old 10-11-2019, 02:36 PM   #20
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Wow! I'd better put them in the safe!
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Old 10-11-2019, 08:57 PM   #21
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I just took one apart to get a further look at, it really wouldn't be that hard to put the regulator in the case, but I can't see it being worth the effort. It would be a tight fit to get the regulator on the board at 9x10mm, and does anyone really care what the back of their gauge cluster looks like?
I was thinking more in the $20 shipped price range IF it goes as planned. Give or take a few dollars.

I see how this one failed, 2 of the buttons have a tiny wire soldered to them, one of the wires is broken. In theory it could be repaired..but my eyes and hands are not steady enough to solder wires thinner than a hair haha. And it would have to be soldered under the contact arm.

EDIT: After cutting all the bad stuff out, it might be pretty easy to update the original. If you guys interested would rather have that, let me know. I get the regulators on the 15th, so ya won't see anything till a day or 2 after that.

Last edited by 2turbotoys; 10-11-2019 at 09:37 PM..
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