Hello everybody! Have an inoperative tach on my 1991 MB 300D. Idle seems to me a little bit lower than it should be. A/C doesn't work as well but not sure they related. What has been done: 1) using manuals, checked signal from L3 engine speed sensor at both connectors X62 and X29/4 - everything withing specs. 2) bench tested OVP relay - it works fine. 3) pulled out instrument cluster and found blown resistor on amplifier circuit board. Found information somewhere on forum and replased it together with capacitor. Tach still inoperative.

So hard to find complete wiring diagrams to understand the route of signal from speed sensor to amplifier. If someone know what kind of signal should be at amplifier input, it will helps me a lot. I need to understand if I have bad amplifier or my signal lost or destorted on it's way from speed sensor. According information I found on forums, before reaching amplifier, speed signal passing thru Klima relay and EDS in some way.
Any ideas would be greately appreciated.

The EDS computer is the amplifier. It sends the tach signal out to the tachometer and the Klima relay. If the OVP relay is bad (which is the most common issue), the EDS doesn't work and you get a low idle, no A/C, no Tach. If you have an antilock light on the dash, the OVP is probably toast.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Thanks for quick reply. As I said before, I bench tested my OVP relay and it works fine. No ABS light on the dash. And, as far as I know, amplifier is in the cluster, EDS and Klima just using speed signal from sensor. My question was what kind of input signal should be on tachometer to make the needle move? I want to test it by simulating this signal.

The signal conditioning and amplification is in the ECU pin 20 is input from sensor pin 25 is the conditioned signal to the tac and diagnostic port.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Not sure I quite understanding you) Can you tell exactly, what kind of signal (voltage, a/c or d/c, waveform) I should have at tachometer unit? Thank you.

You seem to be the expert. Let us know when you fix it.

If you'd like to take it down a peg, pull up a copy of the FSM, it clearly shows the EDS computer *IS* the amplifer. Don't take my word for it, look it up yourself. Straight from Mercedes (you know, the people who designed it?). The Tach signal (used by the EDS for control of idle speed, EGR position, ARV operation, turbo waste gate operation [on cars with vacuum wastegate], and sent to the tach in the cluster and the Klima relay) is just an amplified version of the signal coming out of the crank sensor.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The lack of A/C and low idle suggest you have no Tach signal coming out of the EDS computer. Don't waste your time testing the Tach until you fix the root problem.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The pickup counts the teeth on the fly wheel 144 pulses for 360 degrees of rotation. A milivolt is generated which goes into the ecu the wiring is easily followed. Ecu cleans up that signal i.e. makes a clean square wave pulsing signal and the milivolts amplified to I think 12v peak to peak coming from ecu pin 25.

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

Sorry guys, no offences. I am not specialized in old diesel Mercedes but I have some knowledge in this field. There is very few informaton about how this system works. I've searched a lot but all I could find that's incomplete pieces of wiring diagrams and a lot of opposite opinions. And instead of getting answers I have more and more questions))
Why resistor in tach amplifier blown? what can cause it? What is the difference, when you testing A/C voltage at X63 and X29/4? Voltage is different, in my case 5.4V and 3.6V accordingly and it's match with specs. That means my speed sensor and wiring up to X63 is good.
I need to understand how system works.

What is your recomendation? To check output signal from ECU?

I wandering how it can be in mV if I measured it directly at X63 and it's about 5.4 a/c Volts? I am confused. They said X63 comes directly from speed sensor...

I'm not certain what or where x63 is but you are being misinformed the pickup is inductive and generates a milivolt pulsed signal. If you used an oscilliscope you can verify that. An AC meter won't do you much good as the signal is a frequency which one can easily calculate with the info I had provided.

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

Sorry, I made a mistake. At one pic connector called X62, not X63, but in manuals( I guess it's the same connector) it's called L3x. I will attach both files. And in manuals said when you checking speed sensor signal, you have to set up multimeter to a/c voltage. And as I can understand, you cheking speed sensor DIRECTLY, cause it also has a resistance test. I had 5.4 a/c voltage at idle, that's confirmed my sensor is good. Where I am wrong?

Attached Thumbnails

 

Attached Files

07.1-2006.pdf (654.0 KB, 57 views)

The time base for a ac meter is 60 hz in north america using a different time base shifts the ac voltage reading. I suppose someone came up with those values somewhere as being a valid test results. I've always used scopes to validate frequency generators.

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

I am not sure, but this attached pdf file for me look like original manuals. I have no problem with scope, I have it and I am using it pretty often. I just need to know what I have to see at tach amplifier input: waveform, voltage, whatever it shoud be. I wanna know what kind of signal I must have at working tachometer to compare it with mine.