my 1992 745T speedometer was getting flakey. sometimes it worked 100%, other times, it wouldn't read any speed higher than 44MPH.

did a bunch of reading, researching. I found this. Volvo 700/900 Speedometer Repair (along with articles on how to take the instrument panel apart)

thats exactly my speedometer. ... so I ordered all the caps, not just the likely 4, but the other 2 too. digikey has them on my step in a few days.

I take the instrument panel out, and remove the speedometer module and examine it closely. I can see capacitor puke on the board, just like this picture from that how-to...



YES! ... ok, seperate the speedo board, go over to my buds shop this afternoon, he has an electronics bench, he decapped it, I cleaned it up with a brush and PCBA cleaner til it looked like new, then we replaced all the caps with panasonic and rubycon and nichicon.

bring it home, reinstall speedo in instrument panel, reinstall instrument panel in car, static test, all functions look good, start car, tach works, turbo gauge works, all lights working correctly. phew.

go for test drive. ok, needle is off by about 2-3mph lower than it was before (which was about 1-2 mph low). and oops, if I get on freeway and go 65-70ish, it sticks at 42mph steady. grrrrrrrr.

then, 6-7 miles later, its reading 65-70, just under the Garmin GPS I'm using for reference.

so arrrrrrgh. all that work, and it didn't change anything.

 

"Some days you eat the bar, some days the bar eats you..."

 

hahaahah, I know. when I saw that black puke on the chip leads and under the cap, I thought for SURE that would fix it.

it may go back to my friend's shop, where we'll hook up a signal generator and power supply and scope it out. of course, it will work on the bench :-/

 

Rereading your post you say that at "65-70mph it's just under the Garmin..."
So, is it just incorrect by a few miles or does it do other crazy things?

 

when its working, its 100% fine except a few mile per hour offset to the low side. before disassembly, that error was just 1-2 mph, now its more like 3-4mph, that just means I got the needle in slightly the wrong place, and shoudl take it back apart and tweak the needle up a hair.

basically, its behaving EXACTLY the same as before. sometimes it works, other times it wont' read higher than 45-ish (44, 42).

I just found this paragraph I skimmed over before.
hmmmm. gonna check that ground down by the drivers door hinge.

 

update: the chassis ground connection behind the driver fender kick panel looked fine, but I removed/reinserted the spadelug a bunch of times. fyi, its the brown-white wire, not the bigger black one (which is for the ABS). if you have cruise, there's another wire there, too, which my diagrams say is brown-green or green-brown (actually my diagram says the speedo ground wire is brown-green too, but mine was brown-white).

 

The boards act up pretty regularly in these cars, the reflowing procedure is hit and miss for results. May be your speedo didn't even need the capacitors change YET, just the solder reflow... Usually the failure of the board causes the warning lights to stay out, which causes the alternator not to charge.

 

yeah. damnit, when I saw the capacitor puke on two of them, even tho it wasn't as bad as the one on that how-to, I thought for /sure/ that would fix it.

 

Logical assumption.
I'd drive the car for a while, see what happens, if it improves. From experience, these speedos sort of need to be driven (no idea why) to get going, especially after repairs...
One time I got a JY speedo that didn't work for several days, then "woke up" and has been fine for going on three years now ('92 940GL).