Designing a Better Tach Amp - Page 4 - PeachParts Mercedes-Benz Forum

mplafleur · #46 04-07-2009, 08:05 AM

Quote:

Originally Posted by techguy512

If you ever want to diagnose the system, you can borrow the diagnostic tool. It gets loaned out periodically, but when it comes back I'd be happy to send it to you.....

I can't find the tach amp on the SD. Where'd they hide it?

techguy512 · #47 04-07-2009, 10:22 AM

Quote:

Originally Posted by mplafleur

Where'd they hide it?

Late in model year 1983 they removed it completely from the SD's, and replaced it with a system that uses the EGR computer to drive the tach. All they left was an empty plug with a cap over in on the left fender. If you have an empty plug with a screw on cap that is less than about 2" tall, you have the new system.

This system was also used in the 300d's but not implemented until later.

funola · #48 04-07-2009, 11:31 AM

Interesting! I wonder if the SD's EGR computer uses the same chip as the 83 240D's EGR computer? I know the 83 240D EGR computer uses the same chip as the tach amp in an 83 300D turbo, that's how I was able to install a tach in my 240D (in place of the clock) by taking the tach signal from the EGR computer to drive it.

Quote:

Originally Posted by techguy512

Late in model year 1983 they removed it completely from the SD's, and replaced it with a system that uses the EGR computer to drive the tach. All they left was an empty plug with a cap over in on the left fender. If you have an empty plug with a screw on cap that is less than about 2" tall, you have the new system.

This system was also used in the 300d's but not implemented until later.

funola · #49 04-07-2009, 11:36 AM

If a scope is not available, you can use a DVM in the AC volts setting to read the voltage. If you rev the engine and you see a proportional increase in voltage to RPM, it is likely the tach pickup coil is good. If you get no voltage then it is toast.

Quote:

Originally Posted by techguy512

Using a constant current source, put 30 to 50mA through the pickup coil (pins 7 and 9 in the amp socket). With the scope on AC coupling, you should get a zero volt signal when measuring from pin 7 (gnd) to pin 9. When the engine is started, you should see a small 'blip' once per crank rev.

mplafleur · #50 04-07-2009, 11:40 AM

Quote:

Originally Posted by techguy512

Late in model year 1983 they removed it completely from the SD's, and replaced it with a system that uses the EGR computer to drive the tach. All they left was an empty plug with a cap over in on the left fender. If you have an empty plug with a screw on cap that is less than about 2" tall, you have the new system.

This system was also used in the 300d's but not implemented until later.

Yup, I saw that. I thought it was a diagnostic connector.

techguy512 · #51 04-07-2009, 11:52 AM

Quote:

Originally Posted by mplafleur

Yup, I saw that. I thought it was a diagnostic connector.

It's called the diagnostic connector. If there's no electronics inside the cap that covers it, you have the new system.

scoodidabop · #52 06-22-2009, 07:08 AM

How do I get one?

math · #53 03-18-2019, 06:30 AM

Would love to get one of these if you still have them, or even just the schematic so i can piece one together for myself. I'm pretty handy with a soldering iron.

Marshall Welch · #54 04-24-2019, 12:19 AM

A little off topic, but I might need a new one, too. I’ve had one of these for probably 10 years and it’s worked great.

It might have been damaged when I hooked up a Noco Genius Boost GB40 to jumpstart my car. The tachometer was then pinned all the way to the right (>6,000 rpm) afterwards. I removed the unit from the plug after that.

97 SL320 · #55 04-24-2019, 09:45 AM

What is the output voltage of the booster? If sub 15 V it didn't cause damage.

What does cause damage is trying to crank with a depleted battery that causes the starter solenoid to chatter. This impresses high voltage spikes on the electrical system and will damage parts. Think about how an ignition coil works. The car owner then blames the booster / battery charger when the damage really occurred prior to the boost.

Marshall Welch · #56 04-27-2019, 06:49 PM

97SL320:
Voltage not listed in specifications in owners manual, but my guess is that it’s not >15 V. What you’re saying makes sense. Thanks for the input.

Marshall Welch · #57 11-28-2021, 06:31 PM

It turned out that the alternator was kaput, so I replaced it with a new old stock Bosch alternator. After driving the car around for awhile to ensure the battery was charged up, I plugged the Techguy tach amp back in, and it’s still working fine, 2-1/2 years later.

ykobayashi · #58 11-29-2021, 09:14 AM

Thanks for the update.

Do you have the red one or the green one?

My guess is that it was either picking up noise off the alternator and putting out false triggers, or it was in a diagnostic mode indicating a bad battery voltage level. The red version of the board had a lot of extra circuitry on it to do diagnostics.

Good to get it working. A person with the red board recently bought my replacement amp. He was having the same issue. I never heard back so I assumed my replacement fixed it.

oldsinner111 · #59 11-29-2021, 09:27 AM

with my w126 diesel, it was the socket, where pin's connect, that was the problem. as it aged you needed to push pins deeper, by a block of wood under the female plug. best fix would be do away with socket, or some how gold coat pin's and sockets.