'79 244 DL. Temperature gauge has been faulty since I got the car. It rises as I would expect once the car starts warming up, but never stops. Once the car is up to operating temperature, it is at the top of the gauge. I'm like 99% sure this is not an accurate reading, since the car runs fine and has a new radiator, thermostat, fan and fan clutch due to an unrelated issue (old metal fan broke, fell off and damaged the radiator) I initially assumed it was the temperature sensor and since it was an inexpensive part, I replaced it. No change. I would not expect this behavior if it was a short in the wiring, so it stands to reason that the issue is in the gauge cluster. According to the previous owner, the owner before him replaced the instrument panel. This one has a bad odometer, the fuel gauge tends to read pretty high and the speedometer needle likes to bounce around. Not sure if any of that is related beyond the parts being old. Any tips on what I should look at for the temp gauge?

Hi,

If the wire from the sender is frayed and if the core conductor inside the wire can touch a bare spot of metal it will read to the top.
If it can touch another wire in the harness as to get a grounding circuit or energized wire the gauge will read to the top.
Some cars up to around 1987 suffered engine harness issues mainly from either biodegradable insulation or spray on engine cleaners not getting rinsed out of wiring harnesses.
Both are valid excuses to cause your issue.

As far as the fuel gauge that could be a calibration error from the sensors arm inside the tank.
That sensor also uses resistance to limit the current to the fuel gauge.

Now, since both gauges are showing a high end readings, there is a voltage regulator on the instrument panel itself, that is supposed to limit voltage/current across the two gauges.
This is to be about Ten volts and it pluses on and off all the time the key is on.
You might look for that going out to the sender.
The newer instruments clusters with electronic speedometers, not cable driven, have solid state regulators and as far as I know they don’t pulse slowly enough to be read with a voltmeter.
In either case, you need a constant voltage that’s under the battery’s supply voltage. It needed to provide accurate readings on both gauges.

If both gauges are substantially off in that their readings are more than they should, be suspicious of the voltage regulator being faulty. They are mechanically prone to this but have been reliably used for many many years.
The gauges then receive or use a current from these regulators, through a sender unit to heat inside a bimetallic lever that moves the needle in both of these gauges.

As far as analog gauges are concerned, it’s the system is still used today in many things not so hi tech.
The early Volkswagen Bug vehicles use a light bulb to limit the current.
These cars were notable for being made in expensively and worked on a lot.
If the light bulb burned out the fuel gauge quit.
Luckily, Not too hard to get to though.
Bicycles and Harley Davidsons were career starters for a many mechanics of my day.
Or in most cases, by anyone needing more skills in something important.
Old machines are like textbooks from our ancestors that are not in libraries or downloads.
Working and studying on them teaches us more than just how to read but the reasoning behind them.

As far as temperature gauges have evolved you look out over the radiator to see a real mechanical gauge out there. They were using a Bourdon tube, that’s still in our pressure gauges today.
These Do Not require batteries.
I will not say that digital is an improvement in reliable technology, if its blank.
Fast Thumbs on phones, I’m sorry, but I don’t see the fruition of our minds coming from some games.
It’s More like dumb-ing down.

So If both didn’t work look there. An excessive draw on the regulator can affect the fuel gauge, one would think?

I hope I gave you some ideas of what you are “looking at working” and can see what might correct it.

Phil

I really appreciate the detailed information. I will pole around a little when I have some time and report back findings for anyone else experiencing this issue.

I think Phil probably has it nailed. If I recall correctly, the wiring harness with the temp sensor goes under the front of the engine below the timing belt cover. There's lots of heat and oil there to help disintegrate the troublesome "eco" wire insulation that Volvo used up until around your year. The damaged area that's shorting is possibly inside the harness under the engine and not worth slitting open to repair. As a fix, cut the wire close to the temp sensor, locate the other end at the main block connector at the firewall (driver side, a Volvo wiring diagram may help), cut the wire there and run a new wire from the temp sensor to the block connector, this time going over and down the right fender and crossing over on the firewall. I used bulk oven wire that's designed for hot, harsh environments. As I recall, the D+ alt wire is also in that harness and it wouldn't hurt to re-route that wire at the same time. That's what I did on our '85 240 and never had another problem associated with that harness.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Hi Dave,

Oh Yes, you have reminded me of what I have done on both my 1978 GT and my 1986 wagon.
My 1984 was already repaired by the PO’s previous shop workers. They put back in a split loom affair all the way around down there.
How many wires got replaced unknown to me, as I have a tendency to leave well enough alone.

The only difference in my way of fixing the problem was to use the old wire to pull in a new wire.
I soldered onto to the old frayed ones ends. I was able to do that because, I guess I was lucky in that they were not too twisted up or bound inside the harness.
I pulled it both ways before jumping in with the idea.

Your method will always work.
Dave Barton.com sells a replacement harness but between the effort working under the manifold and the unknown cost of his harness, all can leave open several options.
https://www.prancingmoose.com/volvoharnesses.html#79_240

With his, you remove all the bad wiring that goes to everything with completely new.
If you restoring or making an heirloom out of the car, it’s probably a better way to go.

I’m sure it was a lot of effort and shop prices help nurture the idea of selling the car. I new the PO and had worked on the car a wee bit so when it got hit in the left rear door and he insurance company wanted to total it I got a call. 112k for $700 about 20+years ago. It’s has about 190k now.
It may seem not to be much mileage but I have six 240s to swap trips in.
My inventory or investment equals about the normal price of a new midsized car today and I doubt they will be collectors of them 30 to 50 years out from their date of manufacture.


It’s really Nice to see your wonderful posts on the board.

Phil