Help troubleshoot an intermittent tach. - Page 2 - PeachParts Mercedes-Benz Forum

NC Benz · #16 08-07-2007, 03:15 PM

Quote:

Originally Posted by funola

Take a DVM and set it to the A/C millivolt scale and measure the output of the tach amp (I do not have the pinouts at the moment but I'll post later). With the engine idling, you should read 100 to 200 millivolts at idle and it should increase as you rev the engine. If you get no reading, the tach amp or the mag pickup or the wiring in between may have problems, or it may not be getting power. Next, set the DVM to the ohms scale and read the pickup resistance which should be around 80 ohms (I'll post pinout later)

OOOOOO a good one! I will await those pin outs.....

funola · #17 08-07-2007, 04:14 PM

Pin #'s are clearly marked on the tach amo socket on mine. It is possible they are not labled on yours.

Resistance across pins 8&9 should be around 80 ohms. Take this reading on the socket with the tach amp removed. This is the mag pickup resistance. A mag pickup is nothing more than a long length fine insulated wire (aka magnet wire), wound around an iron core. If you do not get a reading, wiring to the pickup or the winding in the pickup is open.

2. With key on (glow light on). measure across pins 2&6 (2 is gnd). You should get 12V. If not, the tach amp is not getting power. Check fuse. Take this reading on the socket with tach amp removed.

3 With engine idling and tach amp plugged in, measure with DVM set on A/C miili-volt scale across pins 2&3 (2 is gnd). This is best done by wrapping 2 short lengths of solid wire to pins 2&3 of the tach amp then with tape to prevent shorts to adjacent pins, plug tach amp into the socket and clip DVM leads to the wires. You should get a reding of somewhere between 100 to 200 milli-volt. This could be higher or lower depending on the adjustment of the pickup (gap between pickup and tooth). If you rev the engine, the voltage should increase.

4. If the above 3 tests turn out good and the tach needle still does not move, try smacking the dash above the tach and see if it starts working. If so, the tach itself is "sticking" Mine "sticks" and I will take it out soon and see if I can repair it.

If you want to read about the gory details about my tach amp repair. See this thread:

Tach amp repair attempt

Skip the OT stuff about vacuum tubes and other BS unless you have time to waste.

sd300td · #18 08-07-2007, 06:02 PM

Quote:

Originally Posted by funola

Pin #'s are clearly marked on the tach amo socket on mine. It is possible they are not labled on yours.

Resistance across pins 8&9 should be around 80 ohms. Take this reading on the socket with the tach amp removed. This is the mag pickup resistance. A mag pickup is nothing more than a long length fine insulated wire (aka magnet wire), wound around an iron core. If you do not get a reading, wiring to the pickup or the winding in the pickup is open.

2. With key on (glow light on). measure across pins 2&6 (2 is gnd). You should get 12V. If not, the tach amp is not getting power. Check fuse. Take this reading on the socket with tach amp removed.

3 With engine idling and tach amp plugged in, measure with DVM set on A/C miili-volt scale across pins 2&3 (2 is gnd). This is best done by wrapping 2 short lengths of solid wire to pins 2&3 of the tach amp then with tape to prevent shorts to adjacent pins, plug tach amp into the socket and clip DVM leads to the wires. You should get a reding of somewhere between 100 to 200 milli-volt. This could be higher or lower depending on the adjustment of the pickup (gap between pickup and tooth). If you rev the engine, the voltage should increase.

4. If the above 3 tests turn out good and the tach needle still does not move, try smacking the dash above the tach and see if it starts working. If so, the tach itself is "sticking" Mine "sticks" and I will take it out soon and see if I can repair it.

If you want to read about the gory details about my tach amp repair. See this thread:

Tach amp repair attempt

Skip the OT stuff about vacuum tubes and other BS unless you have time to waste.

Hey thanks for the info. Do you think checking the ground first would be the way to go, since we know that the tach works perfectly with the air conditioning running?

300TDBioBenz · #19 08-07-2007, 07:11 PM

Could you guys recommend good gauges for these tests, as well as a good gauge for checking vacuum.

Thanks

NC Benz · #20 08-08-2007, 08:50 AM

Great stuff on both accounts!!

Thanks for putting in the time.

Quote:

Originally Posted by funola

Pin #'s are clearly marked on the tach amo socket on mine. It is possible they are not labled on yours.

Resistance across pins 8&9 should be around 80 ohms. Take this reading on the socket with the tach amp removed. This is the mag pickup resistance. A mag pickup is nothing more than a long length fine insulated wire (aka magnet wire), wound around an iron core. If you do not get a reading, wiring to the pickup or the winding in the pickup is open.

2. With key on (glow light on). measure across pins 2&6 (2 is gnd). You should get 12V. If not, the tach amp is not getting power. Check fuse. Take this reading on the socket with tach amp removed.

3 With engine idling and tach amp plugged in, measure with DVM set on A/C miili-volt scale across pins 2&3 (2 is gnd). This is best done by wrapping 2 short lengths of solid wire to pins 2&3 of the tach amp then with tape to prevent shorts to adjacent pins, plug tach amp into the socket and clip DVM leads to the wires. You should get a reding of somewhere between 100 to 200 milli-volt. This could be higher or lower depending on the adjustment of the pickup (gap between pickup and tooth). If you rev the engine, the voltage should increase.

4. If the above 3 tests turn out good and the tach needle still does not move, try smacking the dash above the tach and see if it starts working. If so, the tach itself is "sticking" Mine "sticks" and I will take it out soon and see if I can repair it.

If you want to read about the gory details about my tach amp repair. See this thread:

Tach amp repair attempt

Skip the OT stuff about vacuum tubes and other BS unless you have time to waste.

funola · #21 08-08-2007, 10:08 AM

Any cheapo DVM with AC and ohms scale that has autorange or manual setting will work. For vac get a Mity-Vac.

Quote:

Originally Posted by 300TDBioBenz

Could you guys recommend good gauges for these tests, as well as a good gauge for checking vacuum.

Thanks

funola · #22 08-08-2007, 10:15 AM

It never hurts to check the ground. Does the tach work when you turn on the AC and not work when you turn it off? Does it do that everytime without fail?

I do not have a system schematic that shows the A/C and tach so I cannot check whether they have anything in common. My guess is they don't. You have a very strange problem. Let us know what you find.

Quote:

Originally Posted by sd300td

Hey thanks for the info. Do you think checking the ground first would be the way to go, since we know that the tach works perfectly with the air conditioning running?

funola · #23 08-08-2007, 10:20 AM

You're welcome! I've learned a lot from this forum. Time to give back.

Quote:

Originally Posted by NC Benz

Great stuff on both accounts!!

Thanks for putting in the time.

sd300td · #24 08-08-2007, 11:53 AM

Quote:

Originally Posted by funola

It never hurts to check the ground. Does the tach work when you turn on the AC and not work when you turn it off? Does it do that everytime without fail?

I do not have a system schematic that shows the A/C and tach so I cannot check whether they have anything in common. My guess is they don't. You have a very strange problem. Let us know what you find.

Yes, without fail. Though sometimes, very very rarely, the tach will work momentarily with the A/C off. I think its happened for 10-20 seconds on two or three occassions during the past two months. It's pretty bouncy when this happens (a/C off).

Sounds like there may be at couple of things happening...

funola · #25 08-08-2007, 12:58 PM

With the A/C off and engine idling, have an assistant look at the tach while you jiggle the mag pickup with a long stick (be careful), then jiggle the tach amp, then bang on the dash above the tach and let me know what happens.

Also do the 3 tests with the DVM.

Quote:

Originally Posted by sd300td

Yes, without fail. Though sometimes, very very rarely, the tach will work momentarily with the A/C off. I think its happened for 10-20 seconds on two or three occassions during the past two months. It's pretty bouncy when this happens (a/C off).

Sounds like there may be at couple of things happening...