tachometer input voltage? - PeachParts Mercedes-Benz Forum

nr5p · #1 04-01-2014, 01:12 AM

I have an 82 300sd with a non working tach amplifier. I've been wanting to practice with micro controllers for a while and figured I would make something useful with my arduino.

So what I am wanting to know is what type of signal drives the actual tach from the tach amp. Is it just a variable dc voltage and what is the range?

mach4 · #2 04-01-2014, 02:04 AM

Quote:

Originally Posted by nr5p

I have an 82 300sd with a non working tach amplifier. I've been wanting to practice with micro controllers for a while and figured I would make something useful with my arduino.

So what I am wanting to know is what type of signal drives the actual tach from the tach amp. Is it just a variable dc voltage and what is the range?

From what I've been able to determine the main sensor on the front pulley is a VR sensor that puts out an AC signal with variable voltage based primarily on speed, but also sensor gap. This signal is typically in the range of 10-11 volts. The tach amp changes the signal into a sine wave at 3.3 volts which is read by the tach module itself.

The later tach in the 85 for example, can be run with the VR signal driving the tach directly...not sure about the earlier ones.

Always interested to see what people are doing with Arduino. Here's mine - http://www.peachparts.com/shopforum/diesel-discussion/350205-engine-instrumentation-project.html

Make sure you keep us appraised on the projects progress.

funola · #3 04-01-2014, 11:46 AM

Quote:

Originally Posted by mach4

From what I've been able to determine the main sensor on the front pulley is a VR sensor that puts out an AC signal with variable voltage based primarily on speed, but also sensor gap. This signal is typically in the range of 10-11 volts. The tach amp changes the signal into a sine wave at 3.3 volts which is read by the tach module itself.

The later tach in the 85 for example, can be run with the VR signal driving the tach directly...not sure about the earlier ones.

Always interested to see what people are doing with Arduino. Here's mine - http://www.peachparts.com/shopforum/diesel-discussion/350205-engine-instrumentation-project.html

Make sure you keep us appraised on the projects progress.

Yhe VR sensor on the front cannot put out 10-11 volts. If the crank is spinning at 500,000 RPM, it may get near 10 v. As I recall, it is more like 10 milli-volts per my oscilloscope measurements

mach4 · #4 04-01-2014, 12:08 PM

Quote:

Originally Posted by funola

Yhe VR sensor on the front cannot put out 10-11 volts. If the crank is spinning at 500,000 RPM, it may get near 10 v. As I recall, it is more like 10 milli-volts per my oscilloscope measurements

I got the 10-11 volt number from your oscilloscope traces - I was going from memory (and I can't find the images right now) so I must have missed a few decimal places.

Sorry

JamesDean · #5 04-01-2014, 12:39 PM

I took these on my 82 300SD:

mach4 · #6 04-01-2014, 01:24 PM

Excellent.

Is that the output from the VR sensor or the output from the tach amp?

What are using for a scope?

JamesDean · #7 04-01-2014, 01:29 PM

That is the tach amp output. Measured at the back of the cluster just before the tachometer itself.

I picked up one of these scope on ebay a while back. Cheap enough and useful. Not as powerful/awesome as the Tektronics scopes we have here at work but then again they were $3K+

DS1052E 50 MHz Digital Oscilloscope | Rigol - Beyond Measure