Faulty Tachometer?

[oscar]

Hi there,

On the recent trip to Sydney my tacho stopped working an hour into the trip.  Occasionally it would start bouncing erratically, sometimes hold the correct rpm, most of the time it lies on Zero.

I think I've read all the posts about tacho's but I don't know what to do.  I've had the instrument display out, pulled the 2 lead plug but don't know what to test.

I'm hoping this is an electrical issue but is it possible to rule out the tacho?  Can you tap live (low voltage) wires on the tacho's contacts to make the needle jump for example?

Also 116.025's post has this pic. Chad if you're watching, the right terminal are you talking about, is it on the coil or the bits on the wheel arch above it?

[116.025]

Also 116.025's post has this pic. Chad if you're watching, the right terminal are you talking about, is it on the coil or the bits on the wheel arch above it?

I'm always watching... haha, but back to being serious...

It's on the bits on the wheel arch above it, not the coil itself.

[oscar]

Ok, see if you can work this one out

For some reason my multimeter wont read rpm anymore.  It's a digital multimeter and as per instructions I put the positive lead on the (-)ve pole of the coil and the black negative lead on an earth point and get nothing?

Futhermore a test light shows pulsing at the plug to the tacho but it doesn't make the tacho register when attached to the terminals.  The plug can only attach to the tacho one way but if I attach the upper socket of the plug to the lower terminal of the tacho (which is wrong) and leave the top terminal unconnected, the tacho works.  This is weird!

[oscar]

Forget that last post.  Faulty multimeter.

I do need your help people.  I removed the instrument cluster and disassembled it to fix the tacho and clock, but I'm at a standstill.




Firstly, I won the Richard Cranium award today for stupidity.  I can't replicate the signal to the tacho whilst testing it, so obviously the best is to turn the engine on and connect the tacho.  To my horror, my car started to haemorrhage aorund the igniton, running down the zebrano, under dash panels and the mid console and floor carpets.  Of course it was oil coming from the oil pressure hose, but I had no idea this was going to happen, I had no idea how that guage worked.

My questions are how do you remove the needle of the tacho so I can remove the faceplate?
Can anyone point out which capacitors, resistors or other is the cause of the faulty tach?  Also how to test with a multimeter?
Lastly, the clock.  In the bottom pic, is it just the two blue bits and the small cap behind the left cap that need replacing?

[oscar]

For the clock and referring to this article http://articles.mbz.org/instrument/renew/ I took out too many things ie the small metal can behind the left cap.  So I'll put  that back.  Also, looking at my two blue caps, they're not gold like that one pictured in the article.  Styria has mentioned more than once before that a 73 350 would have a large clock not the tacho/clock combo.  I'm starting to believe him.  Especially with the blue caps and the 107 labelled speedo, mine might be a replacement.

It leads me to another question.  Does anybody know what the capacitors for the clock are. ie the old blue ones on mine were 16v 47microfarad.  The new ones I just bought are 50v 47microfarad.  What's the real caps supposed to be?

[oscar]

I know I'm rapid-firing posts off here but I'm impatient 

The clock is fixed!!  If anyone does know the correct rating of the two original "gold capacitors", please still tell.  I ended up using the new black 50volt 47microfarad caps and the clock has been keeping time for the last 4hrs and counting.   i also used a bit of Labelle 108 light oil which is plastic compatible, on the cogs to stop the noticeable but quiet noise of the gears.



Now back to the tacho.  This part which I cut off doesn't respond at all to the multimeter.  Firstly, what is it?  Is it a diode?  There's just the numbers and part number printed on it, no units.
There's a similarly coloured and marked but longer one around the other side of the tacho that has microfarads as a unit.  It too is gold coloured with a black band like the one below which I'd assume is a diode.  But diodes are rated in amps not farads.  I've read that there's a capacitor that causes the tacho to malfunction and I've run the multimeter all over the tacho to check for continuity in circuits and components including windings and I can't find anywhere that isn't intact. 
Is this the faulty part.  I'll replace it but got no idea what it is.

[Papalangi]

Oscar,
The 50V 47 microfarad caps will work fine.  The voltage spec is the max working voltage and as long as it's above the expected voltage in the circuit you're ok.  The originals were 16 volts because the battery is rated at 12 and the charging system can put out 16 volts.

The thingy in the picture is a polyester or metalized film capacitor.  It looks like it's rated at 22 picofarads at 10 volts but could be 22 microfarads.  Either way, it won't respond to a continuity check with a standard digital voltmeter.  Polyester caps rarely fail.  The reading you got on the other one was probably due to another component in parallel.

I too have oiled my carpet but the one that gets me is when I found a 12 volt source for my new car stereo and everything worked great till I closed the door.  And learned that '65 mustangs send power to the dome lamp rather than ground them when the door is open.

Michael

[oscar]

Yet another triumph.

Bad earth, that's all!!!!.

Another bonus, ever since I got the car the park brake light would illuminate when the car experienced G forces above a certain level.  Usually when I planted my foot.  I looked at the switch on the lever which didn't show any problems and didn't want to go digging any further under the dash.  So now by fixing this earth problem, the light stays extinguished unless I pull the park brake lever.  Has anyone else experienced this phenomenon?  My tacho is also smoother and more stable compared to when I thought it was working normally. 

So, the earth I think comes via the multipin connector on the PCB backing plate of the tripple combo gauge.  To ground the speedo and tacho backing plates relies on one screw forcing the speedo backing plate against a copper shim on the tripple guage.  That particular screw was threaded, hence the bad earth.
Here's a pic from the front showing the copper shim with a bit more solder added.



I soldered a wire to the top of that copper shim, then soldered it to the tacho backing plate. That's the fix!!! No need to pull it apart. 

That thin grey wire on the left which is now soldered to that shim, is the new gorund and I've effectively soldered the speedo backing plate to that shim as well.


But since it was pulled apart and inspired by dudu that posted pics of his refurbished cluster and repainted needles, I decided to do the same.  You'll agree that Blaze Orange looks more like Bloody Red!! I thought I got the right colour but now the Blaze Orange looks good.  Well I'm going to have to like it now, I went to the effort of using a OOO brush to dab out the orange markings on the speedo and don't fancy doing it again.




You probably noticed something else different. Whilst waiting impatiently for the paint to dry I got busy trying to come up with something to hide the black foam that's visible where a column shift indicator would go.  For my non optioned 116, as if it's not bad enough having spare indicator spots near the right hand turn signal and spare button spaces galore, I couldn't believe that this column shift indicator be factory filled with black painted foam.  So here's what I came up with.  I went the more conservative approach.  BTW, I downloaded a Mercedes font from this page, which has a bunch of other fonts.  I wanted it to look like the boot badge but couldn't find that font.  If you google Mercedes Font, there are a few around to download.  AMG69 if you're watching, this could possibly be one spot you missed to put your logo. I think an AMG one would look good.



I didn't go to a lot of effort taking pics but there is a couple more at http://gallery.w116.org/v/garage/oscars/cluster/