Help! My 760 Gauges are on the fritz

[RollinCole]

Great to hear from you and hope all is well! The only thing I can sort of say agitates the issue to happen, is having the heater on high temp (auto climate) but not selecting a fan speed. Once the gauges go, no amount of tapping or dash pounding restores them. Yes, only the fuel/temp gauges are affected. Engine harness fixed the rest. The only thing that seems to restore the gauges to working again is letting the car sit for a while while or, cool down? I thought maybe solder joints. I’ll steal a cluster off anders soon hopefully...👍🏻

[v8volvo]

I think you're probably on to something with it being related to temperature. The air ducting to the defroster outlets and some of the dash vents runs pretty close to the back of the cluster so the cluster could be affected by them. Maybe when the HVAC temperature is cranked up and/or the car has been running for a while and the inside of the dash gets good and warm, the heat is letting a solder joint open up and lose contact, or the heat is knocking out that little power supply unit for whatever reason.

Trying another cluster sounds like the simplest next step... if a different cluster has no issues then you know where the problem is. I think those early '84 clusters are somewhat unique, but any year 700 series diesel cluster would probably plug in and work at least to test with.

Good to hear that car is in good hands! Would be fun to see it again one of these days.

[v8volvo]

Turned out I did have a greenbook for instrument cluster and believe it or not, it describes the exact problem you are seeing and what can cause it. Images attached. It sounds like specifically for 1982 through 1984 clusters there is a little built-in voltage regulator unit that is meant to provide a stable 10 volts to the fuel and temp gauges. It says if both gauges are faulty, that is typically the reason for an 82-84 cluster. Guess it probably is intermittently failing, or maybe has a loose connection that you would be able to find.

Section "S22" describes how to test it and to replace it if there is a problem, could be tricky to test if it fails intermittently but given the symptoms maybe you already know enough to incriminate it. Don't know how you'd get a replacement for it now other than from another donor early cluster, pretty sure I have a spare cluster out of an early '84 gasser that might be able to provide one if you or Anders don't find one closer. Or maybe it is even a standard type of electronic part that a Radio Shack or similar could supply a close enough replacement for.

These are from manual TP 31166/2 published 11/88, pages 71-73.

[RollinCole]

Huge help man! Been off for a few days but here’s what’s been happening:

Swapped in a somewhat “good” cluster, but it was a 371,000 mile cluster and turns out the speedo was dead, and it had some short circuitry going on, so I plugged the original back in and hoped for the best. I originally thought an EGT gauge plugged in to the naked connector (I’m assuming for the gas application, four pin not used) was the culprit for shorting out the gauges in the dash, nonetheless, I have fully weighed it out, the same thing happened again after turning the heat on. Sounds like Tacoma speedometer will be my next move and have them inspect those diagrams you provided. A huge thanks for the brainstorming, I’ve been ripping my hair out over this one because they are two values I can’t live without being reliable. I think it’s been on the way out, though. The temperature gauge seems to peak on the high side at times with no real relation to the actual coolant temperature so think something is definitely overloaded. I’ll report back if I need a cluster, hold on to it! Hopefully I can swap the speedo...mine still has the nice 76,000 miles on it

[RollinCole]

Also, if you come by XXX on March 22nd you can see the fresh new engine and some other tasteful mods done do it! I also (somewhat secretly) have the US-Rare MK18 Ocean wheels on the way from Sweden topped with some coilovers for the extra-Swede look. Probably the coolest most fun Volvo I’ve ever owned. I also have all that paperwork you passed along with it; what a cool story, the previous owner had good taste, he just didn’t drive the thing enough! I’ve done most of the old-car suspension overhaul work so far and I’ve toasted a rebuilt engine and swapped in the fully-built new one with Anders and it’s been pretty good so far. Lots of good power and fairly dependable, but it keeps it interesting. Daily driver.