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  #1  
Old 05-15-2008, 10:10 PM
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87 300E Tach Question

Is this a valid test for an 87 300E?

"The flywheel sensor (L-3) is located on the driver's side of the "transmission to engine" bell. The wire snakes up around the oil filter and eventaually connects to the KLIMA relay (and MAY also be connected to the overvoltage protection relay - it is in 201s). The low voltage AC signal (small fraction of 1 V) can be monitored and the resistance of the sensor coil read at relay socket pins 1 & 2. Resistance should be ~1.9k ohm." Can it be read at other locations like the multi pin connector on the left fender well or even at the pins connecting the tach?
I'm supposing that each test along the line would eliminate a previous component from being bad.
Also, I read somewhere that an Ohmeter set at 1K will allow you to test the tach by making it jump. Is this correct also?

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Old 05-16-2008, 08:01 AM
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You have some incorrect info. The crank sensor plugs directly into the ignition module (EZL). The crank sensor should have 800-1000 ohms resistance & should generate about .5V a/c current at cranking speed.

The EZL processes that a/c signal into a d/c wave form & sends that info to 4 different computers that need that signal.

What is wrong with the car?
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Old 05-16-2008, 08:49 AM
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Tach doesn't work. Without just throwing new parts into the car I'm trying to find the best way to isolate the problem. I've been looking for ways to test the tach and/or measure output from the rpm sensor when I came across the above from previous posts.
Are we talking about 2 different things? I take it the crank sensor is different from the rpm sensor, or better yet, whatever the unit is that sends a signal to the tach.
The car runs fine other than the tach not working. After doing some research here I've found that usually there are other symptoms if it's the OVP. So touching an ohmeter's leads to the tach directly will not give me an indication of it working?
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Old 05-16-2008, 09:01 AM
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If you can measure dc voltage at the tach connection with the engine running you should find the problem.

OVP relay has nothing to do with that circuit.

Like I said, the EZL processes the crank signal & sends that dc wave to the fuel pump relay, klima relay, CIS control unit, & the tach.
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Old 05-16-2008, 09:36 AM
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How much voltage and what is acceptable range?
What about testing tach directly with ohmeter?
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Old 05-18-2008, 02:46 PM
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I've checked the green wire that goes to the crank pos. sensor (L5). It has 850 ohms. That comes from the bell housing and comes into the EZL at the lower right. The sensor from the front of the motor comes into the multi pin test connect right above the EZL. Does anyone know what the pins are to test from that point? Also, in the MB manual it indicates to reset the "KE control unit fault memory". Anyone know where that is and how to reset it?
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Old 05-18-2008, 03:21 PM
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You aren't listening. If the car runs you have confirmed two things. The first is that the crank sensor did its job. The second is that the ignition control module EZL did its job creating the DC pulse signal call TN (engine speed). This TN signal is what runs the tach.

There is NO memory and NO reset on an 87.
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Old 05-19-2008, 10:43 AM
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Ok. I pulled the inst. cluster and got 5 VDC at the connections to the tach. Does this confirm that the tach is dead or is there another test I can do on the tach?
If it is dead, is this enough to cause the A/C not to work and the cruise control to slow down and speed up?

Last edited by TAG; 05-19-2008 at 10:56 AM.
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Old 05-19-2008, 10:09 PM
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So, if I connect the 2 wires on the tach together and bypass the tach altogether, would that get my A/C working until I get a new one?
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Old 05-20-2008, 07:03 AM
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Wow..... is that a stretch..... but I can almost see it.

We could end this dance a lot sooner if you really described your issue. Is your problem that your compressor isn't coming on and you suspect the engine speed signal?
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Old 05-20-2008, 07:35 AM
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guys, please explain something for me.:
i need to read rpm to run some home made ecu box, if i tap into the signal going to the tach do i read a pulse or proper dc voltage?
i need d/c voltage so your help is very much needed
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  #12  
Old 05-20-2008, 08:47 AM
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A tach is a counter. Obviously the signal to it has to be something that can be counted. In this case it is a DC pulse with no particular duty cycle. Unless you understand what I just said you are going to have trouble with this.

IF you place a voltmeter on a wire containing a DC pulse of 12v the voltage that you will read will be determined by the duty cycle as the volt meter just averages. Thus a 12v DC pulse with a 50% duty cycle will read a constant 6v.
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Old 05-20-2008, 10:11 AM
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Quote:
Originally Posted by stevebfl View Post
Wow..... is that a stretch..... but I can almost see it.

We could end this dance a lot sooner if you really described your issue. Is your problem that your compressor isn't coming on and you suspect the engine speed signal?
stevebfl, yes that is the problem.
So I guess the question is...is what comes out of the tach the same as what goes in? Anyone got a tach on the bench who can let me know?


Last edited by TAG; 05-21-2008 at 10:09 AM.
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