240 Botched Small Tach Installation or Bad Gauge?

I just got around to installing a very nice looking small tach which I bought here on the board. Sadly, it doesn't work all the time. Sometimes it works just fine and then it drops in and out randomly. During my commute this morning, it came to life for about 5 seconds before dropping out again.

After pulling the cluster out to try diagnosis, I had been unable to find anything wrong. The light and tach have a common ground and the light works flawlessly. I sourced gauge power directly from fuse 13. When I check for power and ground with the car running I have a steady drop of 12V at the gauge terminals.

The only thing worth looking at now is my tach signal. Is it bad that I have the signal split to two devices? This car came with a remote start system, so I'm using the same white/red wire for that as well. Maybe my signal is getting pulled into that? When I get a chance I'll try to run a separate wire from the coil.

volvowagoon said:

The only thing worth looking at now is my tach signal. Is it bad that I have the signal split to two devices? This car came with a remote start system, so I'm using the same white/red wire for that as well. Maybe my signal is getting pulled into that? When I get a chance I'll try to run a separate wire from the coil.

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Yes, that is worth looking into. Instead of running a separate wire, which would do nothing to reduce a load presented by the remote starter, why not just disconnect the remote starter from the tach signal to prove whether it is the culprit. If it is, I'd think your ignition itself (waveform) would also be compromised even though you don't notice the change.

cleanflametrap said:

Yes, that is worth looking into. Instead of running a separate wire, which would do nothing to reduce a load presented by the remote starter, why not just disconnect the remote starter from the tach signal to prove whether it is the culprit. If it is, I'd think your ignition itself (waveform) would also be compromised even though you don't notice the change.

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Good idea. Thanks, Art. We don't really use the remote starter unless we're trying show off anyway. I should really just remove it. It's become a rat's nest.

I disconnected the remote start and ran the original tach signal directly to the gauge. No dice. I got rid of the wire completely and ran a long alligator from the negative side of the coil to the gauge terminal. Still nothing. Now I'm out of ideas.

I think those VDO tachs have a similar issue as the clocks. There are some electrolytic caps inside them that dry out and cause the device to no longer function.

Same issue with my big tach, comes and goes completely, accuracy is sometimes questionable also.
I think the caps idea is a good one, haven't seen fit to dig in to it just yet.

dl242gt said:

I think those VDO tachs have a similar issue as the clocks. There are some electrolytic caps inside them that dry out and cause the device to no longer function.

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Hmmm it kinda seems to operate that way too. Once it feels like working for more than about 5 seconds, it seems to work for the rest of the trip. Once you cut power, all bets are off.

Is anybody repairing these by chance? I'd sure hate to see it get scrapped because it's in such aesthetically nice shape. This is also the only one I've ever seen in person.

volvowagoon said:

Hmmm it kinda seems to operate that way too. Once it feels like working for more than about 5 seconds, it seems to work for the rest of the trip. Once you cut power, all bets are off.

Is anybody repairing these by chance? I'd sure hate to see it get scrapped because it's in such aesthetically nice shape. This is also the only one I've ever seen in person.

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You might ask RP80242GT+T

He fixed a couple of 52mm clocks for me a while ago.

See this thread also:
http://turbobricks.com/forums/showthread.php?t=281924

Paging Philski, I’m pretty sure he’s recapped a few of the VDO clocks and may have some information on that.

DD Ranch said:

You might ask RP80242GT+T

He fixed a couple of 52mm clocks for me a while ago.

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Will do. Thanks for the lead.

kyote said:

See this thread also:
http://turbobricks.com/forums/showthread.php?t=281924

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Interesting. Yeah, mine bounces a little when it's actually working. I just kinda chalked it up to being an old tach although I knew it wasn't necessarily "normal". I've certainly seen far worse though! I wonder if a VDO/Volvo gauge specialist could fix it. I also wonder what that bill would be like.

I'll do some more digging after my vacation. I don't want to go in there, but I'm actually an instrumentation tech. It would be a bit embarrassing to send this out before at least giving it the old college try. Lol

I forgot to mention cold solder joints. That happens on these being so old now. A cold solder joint where the spade connector is soldered can cause intermittent function.

To open those up is pretty easy. You gently pry the bezel off. I use a couple of small screwdrivers to get under the edge. Pry out the edge working your way around till it comes off. The bezel holds on the clear plastic to the gauge body so be careful. Once the bezel and glass are off you should be able to remove the circuit board. Been awhile so there may be a screw on the back to hold the board into the gauge.

I'm back, and I have not seen this gauge working all week. I'm pretty nervous to pry the bezel off. Hopefully I don't break anything.


"Carefully" is the name of the game. In fact, I have no idea how I'm going to get this thing back together. Bending the bezel back to its tight grip may be tough.


There's a lot more going on in here than I imagined. Where do I go from here? remove the needle and face plate? What about then? Re-solder joints and hope for the best?

Yep. That was the ticket. Now I'm lost again.






Now what? Should I reflow some of the ugly old connections? I assume this is the capacitor I need to replace. Anybody know how to source this stuff? Sorry for the bad pic. The number on the top appears to say 47/16

The 47/16 is shorthand for 47uF (microfarads) and 16V (volts). Make sure you align the + symbol on the replacement part with the original + symbol (and usually there's a + mark on the PCB).

You can substitute a part with a higher voltage rating, such as 35V or 50V, but the parts get bigger so make sure they'll fit the available space.

Take a look through these parts and see if one of them has the same size and lead spacing:
https://www.jameco.com/shop/StoreCatalogDrillDownView?langId=-1&storeId=10001&catalogId=10001&position=1&refineType=2&sub_attr_name=Capacitance%20(uF)&refineValue=40%20%3C%3D%20%20%3C%2060&refine=1&history=qeulmytx%7CsubCategoryName~Passive%2BComponents%5Erdtvalue~Passive-Components%5Ecategory~20%5EcategoryName~category_root%5EprodPage~25%5Epage~SEARCH%252BNAV%40fpc5tkv5%7Ccategory~203060%5EcategoryName~cat_2030%5Eposition~1%5Erefine~1%5EsubCategoryName~Passive%2BComponents%2B%252F%2BCapacitors%2B%252F%2BElectrolytic%252C%2BRadial%5EprodPage~25%5Epage~SEARCH%252BNAV If this huge long link doesn't work, search for "47uF radial electrolytic" at jameco.com.

Once you see what you're looking for, you might be able to get one from a local phone/computer repair store, or find some scrap electronics boards and see if you can find one there. Paying shipping on a $0.25 part is a bit painful.

Wow, that brown cylinder really is an electrolytic cap. That's a strange looking one. I found a picture of the bottom end, and the plastic housing is sealed with epoxy. Who made those things?
It would be perfectly fine to use a normal, modern electrolytic cap there. Get a high quality one so you don't have to pull that gauge apart again for a while. Measure it like bobxyz said, but also look for phrases like "Japanese electrolytics". I don't know if there are still a lot of counterfeit capacitors out there, but they can go in a way that would ruin the board. Which probably still wouldn't be the end of the world, because all of those parts are relatively common off the shelf parts, even the IC. Lucky.

The narrow pastel colored boxes are film capacitors. I don't know if they're as failure prone as the electrolytics. Probably not, so it's up to you if you want to replace them while you're in there. If you provide all the inscriptions on each, I'll try to track them down.

Use lots of flux during both desoldering and soldering. It will significantly reduce the risk of damage to the board. That thing looks like it would be super easy to rip traces off of. You might as well reflow everything while you're in there. Get some fresh flux in that ancient solder. Just take a really good picture of the traces beforehand in case you bridge anything and can't remember how it's supposed to look.

Thanks guys! You all rule.

Again, sorry for the bad pics. I have a hard time getting my phone to focus on tiny stuff. I found a 47uF capacitor in my lab at work. It's a 35V but the smallest one we had.


As ancient as the original one is, it still shows which side is positive. I took note of that and desoldered.


It's hard to see, but the holes are cleared.


The stripe on new capacitors indicate the negative side. The long lead is positive. You have to make sure it goes in correctly!


Soldered


After clipping the ends, I started putting the gauge back together. The clear frame and gauge face can be installed in two ways. If the frame is backward, it will not slide into the metal case correctly. It's pretty easy to tell if its wrong, but something to remember before you break something. Also notice that my 35V cap is big enough that it needed to be bent slightly out of the way of the mechanism. You have to be careful not to drive the face plate screws home until you notice that everything clears! You can easily break the mechanism and then you're screwed. Once I got everything screwed down, I installed the needle and manually put it through it's full sweep. Everything clears without rubbing!


I'm really anxious to find out if this thing works. If I get some more time during the work day, I'll try hooking up a function generator and power supply.

Nice work. And if that doesn't do it, you can just go down the list of all the parts on that board. Which are all still common. This reminds me that I still have aging capacitors next to the irreplaceable Toshiba ICs on my speedometer. Has anyone figured out the signal for the Yazaki speedometer yet? If these cars survive long enough, we might have to come up with a digital replacement on an SBC.

Thanks! I forgot that I tied up the wires a little nicer so that I wouldn't have to pull the cluster out for reassembly. That meant I could test my work during lunch!

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It's alive! And no more bouncing like others have complained about either! Now if only my trim ring would show up. Oh well. I'm probably going to pull it back out to make an attempt at tightening the bezel anyway. It will never be perfect again, but the good part is that all the damage should be invisible once the trim is in place. I just need the lens to sit flush with no rattles. A thin line of electrical tape should do the trick.

Thanks for the help everyone! I'm almost tempted to go toy with my intermittent odometer again. lol

Interesting! My small tach appears to read fine, but drops from high to low readings much faster + has a little bounce/overshoot at the end if the needle moves fast enough...

Wonder if that's a design change or a similar worn-out cap.