adding a tach to a 240D (standard tranny) - Page 3 - PeachParts Mercedes-Benz Forum

jsp · #31 05-23-2005, 06:08 PM

Was anyone actually able to get a tach amplifier and gauge from a 300D and plug it in to an existing rpm sensor in their 240D? It seems there were several discussions, but no specific success stories.

Quote:

Originally Posted by Angel

Old navy: I think those dudes that go 100mph in thier 240s are ***seriously*** exceeding the redline

...and providing a testament to the solidity of the OM616 =) =)

Tach seekers- Thats the thing- the sensor on my 240 (looks exactly like the one on the 300 I owned for 3 months, not sure about the connector) and is mounted properly, same place and everything, but sends an RPM signal to the EGR computer (at certain rpms and engine temperatures the 240D's of this vintage (1982 and 83) have a computer that tells the egr valve when to open..., the black box is in the passenger side footwell IIRC.
The 300D tach signal goes from the pulley, through the sensor, to a wire to the tach amp (the cigarette fix thing), then from the tach amp to the tachometer.

If the sensors are the same part number (which I cant find out, not sure how to save going and looking for a p/n on both, and they are tough to see/get at in the first place on an OM617) then the amp and tachometer should bolt/power right up. Thats why I'm asking if someone can look up those part numbers for me.

This could be another 240D easter egg (appl to '82 and '83 models only =)

-John

Angel · #32 05-23-2005, 06:24 PM

and posted a thread about needing a voltage measured- I didnt put it in this thread b/c I need help from someone w/ a 300D - (whom this thread isnt really aimed at...)

I got the tach amp from the junkyard today (not sure if it works...) and I could just solder everything together, but I'd like to have an idea that things are **close** to working before I do it. If the 240D signal needs amplified, I might be able to make/cajole EE friends enough to make an amp for myself and post it up here.

Edit- the 240D egr sensor and the 300D tach sensor are **not** the same connector.

-John

#33 05-23-2005, 08:46 PM

3 months ago I sold a guy everything to put a tach in his 240d except the gauge I just emailed him asking if it worked and if he would share information on his installation. We shall see if he answers email he is in Germany though may not get a quick reply.
DHS

Angel · #34 05-27-2005, 08:40 AM

my 1982 240D has the EGR system which includes a crank sensor....

At the sensor, you get anything from a 0.1Vpp to 0.4Vpp signal proportional to engine rpm. (unplugged from the EGR computer)

If you go to pin 4 on the EGR computer (with the sensor plugged in), you get 0.5Vpp to 2.0Vpp riding on 2.3vdc.

Sounds like a capacitive pickup ? The computer gives it some DC bias that makes the signal larger (and easier to see ?) I know some electric theory but Cap./Ind. pickups escape me...

I'm guessing the OM617's have an inductive pickup, as they have two signal wires going to the crank sensor. If this is the case, then all I need to do is ground one OM617 sensor wire, and hook the other up to my cap. crank sender so the tach amp gets a voltage pulse.
I still need am OM617 sender signal to compare it to. I dont think that 0.5v will blow the tach amp, but I'd like to know if I'm in the ballpark.

-John