How the Tach Amp works - Page 2 - PeachParts Mercedes-Benz Forum

techguy512 · #16 12-05-2007, 11:07 PM

Quote:

Originally Posted by OldPokey

Here is a capture from an oscilloscope.

Thanks John. Very helpful, again.

The input signal is too low to trigger an interrupt directly. Plan is to use the internal A/D and watch for X number of sequential samples above 200mv or so. There should be plenty of MIPS in the microcontroller I'm using to allow the A/D to be used in freerunning mode. I should be able to get about 100K samples per second...

OldPokey · #17 12-06-2007, 01:32 PM

I think you will still want to do some input signal amplification and conditioning before you feed it into your MCU ADC. You will need to block the DC voltage across the sensor, LPF out garbage and amplify the mV signal to something closer to your what your MCU ADCs are happiest with. Otherwise you won't get much dynamic range.

OldPokey · #18 12-07-2007, 12:42 AM

And in case you want to see if your tach is still working, here is a small circuit you can put together with parts from Radio Shack (if you can find one that doesn't only sell wireless junk and toys). Click on the following links for:

Large Version

Small Version

Connect the + pin on the Tach to +12V and the TD pin to the circuit's OUT pin. Play with the variable resistor and watch the needle swing back and forth...

pawoSD · #19 12-13-2007, 01:46 PM

I wish I knew more about circuitry.....are all the labeled items in your schematic items that a radio shack person could help me find there? Like....if I brought a printout of it would the person be capable of finding the parts for me? I could put it together, I just don't know if I'd be able to find the right components.

I'd like to try building one of those to test my tach. I have a new amp and a supposedly known working gauge, but I get nada....not so much as a blip. Once when my battery was dying (only a few volts left) it started showing 900rpm for a while, then nothing. So maybe the gauge works. I suspect my sensor pickup/cable. I may try my amp in my dads car to see if it works (his tach setup has always worked flawlessly) if my amp works in his car, then it has got to be my sensor.....I've replaced everything else, and cleaned all the wires/contacts with contact cleaner....and the gauge itself gets power...so thats all good.

patbob · #20 12-13-2007, 04:35 PM

Quote:

Originally Posted by pawoSD

I wish I knew more about circuitry.....are all the labeled items in your schematic items that a radio shack person could help me find there? Like....if I brought a printout of it would the person be capable of finding the parts for me? I could put it together, I just don't know if I'd be able to find the right components.

Depends on the clerk. I don't think knowing electronics is a job requirement. It can't hurt to try though, and the nice thing about them is that they usually sell all the other bits you need for making and prototyping circuits too. If the clerk you get knows about electronic prototyping, they'll be able to steer you at all the tools and pieces you'll need, explain why you want them and usually even how to use them. I've had nothing but good experiences buying parts from Radio Shack over the last 30 odd years. Their selection is very limited though.

For a bigger selection, and more knowledgable people (but perhaps not willing to help you at all), you can try one of your local NTE distributors (find one on www.nteinc.com). I go through my local one out here and they stock a wide range of parts. You just want to take a miss on anything suface mount as they aren't beginner-friendly.

For online orders, DigiKey is a pretty good outfit. Not the cheapest, but consistent and they carry an amazing variety.

Mouser is another good online source with an even bigger variety, and they are even willing to sell you $3 worth of parts at a time. But then you keep getting their phonebook catalogs every three months until you call and ask them to stop.

However, try and get your parts locally for your first project -- it seems to help beginners deal with the bewildering array of what's available if they can touch it. If you get too confused, then table the project until someone offers a kit or something (kits are really the best way for a beginner to get started).

BTW, Radio Shack, NTE, DigiKey and Mouser aren't the only suppliers out there, and the parts are pretty interchangeable between vendors, so feel free to find some other source. I've used all of them as my bewilderment level has declined over the years and been happy with all.

OldPokey · #21 12-13-2007, 06:43 PM

Here's your shopping list:

1 ea. TLC555 Timer
1 ea. 2N2222 NPN Transistor (2N3904, 2N4410 or any other small signal NPN part will do as well)
1 ea. 7805 5 Volt regulator
1 ea. 1N4001 rectifier
1 ea. 0.01 microfarad capacitor (any voltage)
1 ea. 0.1 microfarad capacitor (any voltage)
1 ea. 1 microfarad 16V capacitor
1 ea. 4.7 Kiloohm 1/4 watt resistor
2 ea. 10 Kiloohm 1/4 watt resistor
1 ea. 27 Kiloohm 1/4 watt resistor
1 ea. 100 Kiloohm variable resistor (potentiometer)

Notes:

The ground symbols mean those points should be connected together and then to the minus side of the power supply, which can be the car's 12V battery. The case of the tachometer should also be connected to ground.

If you can get a 10-turn 100K potentiometer, you will be happier than otherwise.

You can purchase a "solderless breadboard", which will allow you to put together the circuit without soldering, and will help you fix problems if you misconnect something. You will also need some solid hookup wire that is made specifically for solderless breadboards. It will make connections much easier - you can even buy a set of precut wires in a little case.

A packet or two of clipleads will be very convenient.

When you connect up the TLC555 make sure you know which pin is #1!

Pin 1 on the transistor is the Base
Pin 2 on the transistor is the Emitter
Pin 3 on the transistor is the Collector

The arrow on the 1N4001 points to the Cathode end, which on the component is the end with the painted band.

Make sure you connect the + side of the 1 microfarad capacitor to pins 2 and 6 of the TLC555, and the - to ground.

techguy512 · #22 12-13-2007, 07:51 PM

Quote:

Originally Posted by pawoSD

I suspect my sensor pickup/cable.

Not much can go wrong with the sensor. It's just a coil of wire. You can check for shorts or opens by using an ohmmeter, and measuring between pins 8 and 9. You should read between 50 and 80 ohms. Open will be much, much higher, and short will be much lower.

pawoSD · #23 12-13-2007, 10:44 PM

Thanks to both of you, I am going to begin testing and working towards getting my tach working again. Its never worked for the whole 4.5 years I've had the car.

Even with a new amp and gauge....I'll get to the bottom of it....

duchessvik · #24 03-21-2008, 08:04 PM

i hope someone like oldpokey is still checking this thread. my tach is a transplant into a 240D, bought a sensor, amp and tach head from different used sources. when i first hooked it up, it all worked fine. then the tack started dropping out, i tried the cigarette butt trick, that worked for a while. then it dropped out again, until i tightened all the pins in the connector. i have been battling my tach for a while, resoldering all the connections, replacing the wires (they were toast) with solid wire, etc. then things got really weird. my tach works, but only up to a certain RMP, depending on how much current i am drawing as a "whole" from the vehicles electrical system.
for example, no accessories running, my tach will run up to 4000 before flaking out. put the headlights on, that threshold drops to 3500. put the blower on I and the headlights on, 3000. put the blower on II and lights on, goes to 2500, and so on.
after re-re-re-reading this thread, i will try to see if my amp has any more loose connections. i checked voltage at the amp and tach, i get about 14 at idle or 3000 rpm with no accessories, about 12.5v at idle and 13.5v at 3000 rpm with lights and blower on III. so i dont think its a voltage in issue.
any insight would be appreciated.
i tried to build a tach tester, but i'm not sure i have all the correct capacitor values, and i dont know if you can hook them up in parallel or series like resistors to get the value you want, so i abandoned that project for now.
is anybody out there?
thanks,
Matt

pleiades · #25 03-22-2008, 01:10 AM

Looking just at the input and output values from the o-scope capture, it seems like a comparator, probably with an inverting amplifier for some gain, would do the trick. It would depend on how sensitive the tachometer gauge is to the width of the square-wave input.

OldPokey · #26 03-22-2008, 05:33 PM

You definitely have some electrical problems to iron out before you get back to work on the tach. It appears to me that your alternator regulator is on its way out and you are suffering from multiple lousy connections around your system.

As far as the tach amp goes, take it apart, clean out all the silicone RTV, resolder all the connections inside, and put it back together with new wire and it should work fine. If you plan on resealing it, don't use regular hardware store silicon caulk, you'll have to use electronics grade silicone you can buy from Mouser or Digikey.

Resistors add up in series, capacitors do that in parallel. Google "basic electronics" and spend a few hours browsing around - there are lots of sites available.

duchessvik · #27 03-23-2008, 07:56 PM

thanks for the input. and thanks to techguy512 for his offline help too. i have gone over the system again, added a fresh ground to the cluster, checked wire gages, reset the sensor gap. all with the same result.

i'll be on the lookout for some alternator diagnostics, for sure. ive had the car 2 years, PO didnt have any repair/maintenance records, so it could be an original 25yo alternator, who knows.

tyl604 · #28 03-23-2008, 10:25 PM

This is mind boggling but very interesting. Simple question. I tried adding a couple of pieces of cardboard to snug up the tach amp. Before that I got zero response on the tach (last 20+ years). Now I get readings (believe it or not) based upon the blinker or the fan from the AC. Turn the blinker off and the tach goes back to zero; turn it on and it pulses up and down with the blinker. Turn the AC fan on high and the tach reads strong. Tach does not seem to notice the engine.

I used WD 40 on the sockets and wire brushed the pins for better connection. So now some current is getting through.

Any idea what's happening? Should I try to melt out the silicone and look for something that needs to be resoldered? Could I have maybe one bad pin?

1981 300SD, 250K miles, owned 25 years. Drive every day and runs like a charm.

OldPokey · #29 03-24-2008, 12:25 AM

You probably want to resolder some of the connections on the board inside the tach amp. You'll never melt the goop there - it's silicone and it has a very high melting point. The best way to get it out is with some brushes, a dental pick set and lots of patience.

techguy512 · #30 03-24-2008, 10:46 AM

Quote:

Originally Posted by tyl604

Now I get readings (believe it or not) based upon the blinker or the fan from the AC.

Based on your description, you seem to have two of the most classic and most common "sick tach" symptoms.

For symptom #2, where electrical accessories cause flaky tach operation, I believe the cause for this is bad grounding at the cluster. I have no direct evidence for this, other than several instances where fixing the grounding also fixed the tach.

The cluster grounding is absolutely FUBAR. The ground is brought to the cluster through the circular connector on the back of the tri-gauge on the left. The ground is then brought to a small tab on the edge of the tri-gauge, where it connects to the backing plate on the back of the speedometer. The ground is then passed through the backing plate on the speedo to the backing plate on the tach. The screws that hold the gauges in the cluster housing are the only means of maintaining the connection. If the screws come loose, or if any of these connections develop corrosion over a 20 year period, the ground can go high-impedance, resulting in the ground level 'bouncing' up and down with accessory load.

I suggest you pull the cluster, and clean the points where ground is passed from component to component (see photo below). You also might want to consider running a short black wire with two ring terminals from the screw at the tri-gauge pad to one of the mounting screws at the tach. This eliminates the speedo as a series component in the ground path, and makes the ground connection parallel. for grounding, parallel is more better than series.

Regarding your #1 problem, I agree with the Pokster that the problem is likely cracked boards or joints inside the tach amp electronics module. When the joints crack, the connection becomes intermittant. The 'cigarette butt' trick works by jostling the broken connections until they 'make' again. It only works for short time because engine vibration soon makes them intermittant again. The cig butt worked for me for a few days, then it was back to no operation.

I elected not to screw with it. I designed my own tach amp (photo 2). Thanks to a bunch of you who helped me test the prototypes (you know who you are

)