tachometer frustration - Page 2 - PeachParts Mercedes-Benz Forum

Ya Mar · #1 05-10-2011, 11:46 AM

My Tach is a three prong model. Both cars are 1985's.

I did see a sensor on fastland for ~$65.00. I think I will order that since I can't get the old one off the Wagon.

We'll see how the wiring goes....I may have more questions!

But so far the tach is in the dash, the wiring is run, the amp is in place, and I just need to hook up the sensor.

Almost there!

toomany MBZ · #2 05-10-2011, 01:35 PM

You need a two pronged tach plug, as in the pic, for the sensor and amp to work. Otherwise, there is a wiring harness from the transmission, you may have that already.

Check behind the cluster for any unplugged ends.

The pic shows my CD where the amp would plug/screw into.

Ya Mar · #3 05-10-2011, 09:53 PM

Hey TooMany,

I think Phil from Fastlane has found the right sensor with the 2 prong.

That picture is exactly the same as what I have.

toomany MBZ · #4 05-11-2011, 04:33 AM

Sounds good.

Ya Mar · #5 05-13-2011, 11:01 PM

Hello Army and TooMany,

I got the new pickup sensor in the mail today from Fastlane Parts.

These are the parts I have:
3 prong style Tachometer from 1985 300TD wagon (has green and red wires coming out of the back)
Tach Amp from 1985 300TD wagon (has 2 wires going through the firewall to the dash, black and brown)
Tach Amp Bracket
Pickup Sensor (from fastlane)
24 gauge wire

Here is the problem:

I wired the black amp wire to the red tach wire and the brown amp wire to the green tach wire. This did not work. I tried it the other way around, still no success.

I'm concerned I may have taken off the head of the new sensor on the flywheel? I heard a clicking when I first started the engine and I think it may have been hitting but I'm not sure.

In any case, does this wiring seem correct?

It seems to me that it is an isolated wiring set up and that doesn't seem right.

From the tach amp: the black should be power, I can get this from the glow plugs. Brown should be ground.

But where is the green wire for the for the signal?

I feel like I'm missing something here.

I may end up opening up the dash on the 300TD wagon to get a good look at the wiring.

I've read about an ERG computer. Is this part of the 85 tach or is it a separate piece I need to remove from the wagon?

Hoping to get some ideas here because I'm getting frustrated.

Thanks!

Ya Mar · #6 05-13-2011, 11:02 PM

attached is a pic of the tach amp showing the black and brown wires.

Ya Mar · #7 05-13-2011, 11:03 PM

Obviously the way I've wired it doesn't make sense. *shrug*

Any thoughts?

Stretch · #8 05-14-2011, 02:41 AM

If think we need to bow to the superior knowledge of toomany - I don't know about the difference between a 1985 tachometer system and the ones that came before. So talking about coloured wires to me is a big old waste of time.

When I come to fit my tachometer system to my 1981 European 123.130 chassis I plan to compare the wiring diagram of my car with that of an American car with the tachometer option if I get stuck.

Did you check out this link posted in my "progress so far thread"?

http://www.superturbodiesel.com/std/thread-adding-a-tachometer

toomany MBZ · #9 05-14-2011, 09:24 AM

Sorry folks, but wiring isn't my thing.

But to use the set up you have, you MUST use a two pronged tach.

Ya Mar · #10 05-14-2011, 04:43 PM

Hello All,

So it is now my understanding (correct me if I'm wrong) that my w123 300TD wagon gets the tachometer signal from a sensor on the automatic transmission, runs it through the EGR computer, and then sends that signal to the tachometer on the dash. I do have a 3 prong tach.

The sensor on the front of the engine that connects to the "amplifer" is the diagnostic port. I don't think there is actually an amplifier plugged in here.

If you refer to the picture I posted earlier, I don't think that is the amplifer. When I unscrew the cap I find the 789 ports are from the sensor cable and the 1 and 6 ports have a ground and power wire running to them.

I could add another wire and run it directly to the tachometer but I'm not sure if it will work since I don't think there is actually an amplifer in the car.

Also, I'm wondering if my car has the tooth on the flywheel/crankshaft. Since it's a w123 euro import maybe it didn't have one? It does have the bracket already there but it sounds like that has a purpose beyond just holding the tachometer sensor.

It sounds like what I'll need to do is get a w123 2 prong tach, get an amplifier, and possibly add a magnetic tooth to the flywheel.

*shrug*

This has been a major PITA

Ya Mar · #11 05-14-2011, 04:45 PM

Note: while I have a three prong tack, when I cut the wire there is only a green and red wire.

DeliveryValve · #12 05-14-2011, 05:26 PM

Follow this thread.

Adding a tach to my euro 84 300TD, what do I need?

If you have a diagnostic port, then your 1/2 way there. The port gets RPM signal from the front balancer. All you need is a tach amp, wiring and a pre-'84 tachometer.

.

Ya Mar · #13 05-14-2011, 07:11 PM

Here are a couple more questions:

1) Can the ERG computer receive the signal from either sensor, the transmission sensor or the crankshaft/flywheel sensor?

2) Where does the ERG need power from?

I've removed the ERG from the wagon and have taken a look at the wiring plus a wiring diagram.

Basically, I just need to connect the ERG to power (i think?) and ground, and connect it to the sensor.

I think I'm almost there....Will keep everyone posted!

toomany MBZ · #14 05-14-2011, 08:14 PM

Depending on what tach you have in the dash, the 85's (three pronged) take the signal from the tranny via the egr computer.

You have, or are trying to set up an different unit that will take the signal from the sensor harness you recently purchased and will need a two pronged tach. A Hall effect unit.

http://en.wikipedia.org/wiki/Hall_effect_sensor

If you use a two pronged tach connected only to the amp, you should have a working tach.

The pic in post 21 shows you have a dust cap, just like the one in my CD.

Ya Mar · #15 05-14-2011, 09:17 PM

Quote:

Originally Posted by toomany MBZ

Depending on what tach you have in the dash, the 85's (three pronged) take the signal from the tranny via the egr computer.

You have, or are trying to set up an different unit that will take the signal from the sensor harness you recently purchased and will need a two pronged tach. A Hall effect unit.

http://en.wikipedia.org/wiki/Hall_effect_sensor

If you use a two pronged tach connected only to the amp, you should have a working tach.

The pic in post 21 shows you have a dust cap, just like the one in my CD.

Hey toomany.

You are right, in order to utilize my newly purchased tach sensor I would also need an amp (mine is just the dust cap) and a 2 prong tach.

Instead I have a 3 prong tach, ERG computer, and I'm wondering if the transmission sensor will fit either a 4spd standard or auto trans.

I would be removing it from an automatic trans. If those sensors don't work on the manual transmission, then I have no choice but to get an amp and a 2 prong tach.

Unless the ERG will accept the signal from the flywheel sensor?

Is the signal from the flywheel the same as the transmission?

That might be another option....

Thanks for all you help and ideas!

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