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

squid1351 · #1 11-09-2003, 05:24 AM

im about reddy to buy a 83 240D with a stick and i would very much like to put a tach in there im wandering how that may be achieved. it has a leaky oil pressure gauge that need to be replaced so i wouldent be instaling a cluster just for a tach. but i also wanted to find out about an inductive pick-up tack and how thouse work as far as cost and instalation. thanks for you help.

leathermang · #2 11-09-2003, 08:51 AM

Lets include 1980 240d's in this question... since I am interested also....

R Leo · #3 11-09-2003, 09:01 AM

search this here forum on "oil, leak, gauge" You'll find out all about leaking oil pressure gauges.

ed by R Leo

#4 11-09-2003, 10:43 AM

Actrually, R Leo, I've never found a thread involving installing a tach in a 240D. I'll stand as third in line wanting to know how an OEM unit can be installed in place of the clock. Being a manual tranny, I like to be able to see where the engine is running,.

Stevo · #5 11-09-2003, 11:35 AM

"I'll stand as third in line"

Put me down as 4th in line. Actually when I got my 5 spd someone mentioned that I should install the clock/tach but they weren't sure about the pick up. Seems it was mentioned that "it may or may not be there" Where? I will be following this thread

#6 11-09-2003, 02:15 PM

Me too. I was on the sf bay bridge this morning pushing about 95 mph ( seems like there is still more room to go on my 83 240d ) . I am just wondering what is the red line on 4th. The speedometer shows 1st dot at 24, 2nd dots at 39 and 3rd dots at 64, but what's 4th. Thanks...

OhioMercedesBoy · #7 11-09-2003, 06:45 PM

Perhaps it is my being used to driving large diesels (I am a bus driver for the U of Toledo.. the best student job there is).. but wouldn't it just get to a point where it will not rev any more.. and still be safe? I know on the 8.2 Detroits and most of our 6V92s that they will sing along maxed out on their speed limiter, which is coincidentially also their rev limiter.
~D.J.~

#8 11-09-2003, 08:59 PM

Yup, MB diesels have rev limiters in them as well. The idea for the tach is to be able to see how hard the engine is working, and to be able to have a visual aid to keep that little engine in it's powerband. Personally, I still like tachs even in automatic tranny equipped cars. They can give you an early indication of tranny trouble that may not be immediately noticeable to the ear.

I once had an 86 Hyundai Excel. It had the ugly large analog clock in the cluster. I swapped it out for a tach (easy to do thanx to modular design), and it worked immediately. I wanted more guages, so I installed a digital set for oil pressure and battery voltage. It had a temp guage, so I modified one of the lines between the tranny and cooler and used the temp guage to monitor tranny temp. I like having a maximum number of guages in a car. It doesn't matter to me if the guage is necessary or not. More guages mean more info on the car's well being. Since I used to drive trucks, I got used to lots of info on the health of the truck, and inevitably look to get more than the designers thought of during the vehicles design. Being an MB, I won't put funky add-on's like those in, but a factory tach is icing on the cake for a manual tranny.

leathermang · #9 11-09-2003, 09:21 PM

"MB diesels have rev limiters " ..... to a certain extent.... but of course we have the example of Larry Bible's engine going wild and Kapow.
One thing that helps diesels is the fact that valve overlap in the form which gas engines use to make more power is not possible with a diesel... this limits the speed at which reving can happen and helps top out the power before the top of the rpm limit is reached in most cases...this is the basis of the efficiency of a diesel....

The Warden · #10 11-10-2003, 12:42 AM

Back to the tach question...I guess the big question here is what sort of signal does the tach on a 300D get? Is it a gear-driven signal generator, or a sensor, and what is the sensor reading (flywheel, injector pump, cam, etc)? Also, how does the signal get from the sensor to the tach?

IIRC, on pre '85 300D's, there's some sort of tach amp or something like that, but I really don't remember...

I'm guessing that a tach system from an OM617 could be adapted to fit the 616, but I'm unsure on what parts would be needed.

Just my thoughts on it

240Demon · #11 11-10-2003, 01:08 AM

My 1983 240D came with the tach pickup, but no tach. I guess all '83's and later 240D's had the pickup. It is above the harmonic balancer and is an inductive pickup. It even has a wire that runs into the car, but I have not traced the wire yet. Theoretically, I think you could get the pickup (if not already equipped) tach, and amp out of a 300D and it would work.

dieseldiehard · #12 11-10-2003, 01:18 AM

Pre-85 W123 turbodiesels use a small amplifier module, the dreaded $70 VDO "RPM SENSOR" that fails when the contact pins get weak, it is a cylindrical plug-in that lives on the left fenderwell, near the relay box. I installed one when I had a turbo engine put in my '79 300TD. The engine has to provide the pulses to this module thru a shielded cable, easy to get from a wrecking yard, unless it was cut like many I have seen (probably when the engine was being held in by a nusiance black cable!)
A wrecking yard is a good place to find the socket and a good module if you are lucky, then you need to mount it on a bracket, that might be hard to get off the fenderwell as I believe they are welded on so I made my own out of sheet aluminum. (addendum - I understand the bracket can be ordered from Benz! I've no p/n sorry. Ask a dealer to search for it before you make one, it is a PITA to fabricate.)
Oh, yes, you need to find the plug that fits the back of tach meter on the instrument cluster. Lookout because they come in two varieties, a 2-pin connection in some diesels, three pins in others. Maybe the W116 is different I am not sure but I have had both and found that the two pin units work with my module and the three pin did not.
Attaching the sender on the crank vibration damper area in the 240D is going to be a challenge. I have no reference of experience with the 240D to give any guidance. I suggest one look at the 300D turbo first and visualize this and take a picture maybe.

I am not even certain how it works actually, I thought a large "notch" in the damper near the timing marks were the source of the pulses. Another poster said there is a small metal pin that needs to be on the pulley/crank damper. Whichever is the case, the 240D might need this pin before any effort to install a tach is successful. Anyhow, basically the magnetic field that accompanies a piece of iron gets interrupted as it passes by the pick-up coil. It operates similar to modern magnetic ignitions in gas engines, and is called a reluctor. The amplifier integrates the pulses from the pickup (sensor) into an analog current for display on the instument meter.
I replaced the large clock in my instrument cluster with a tach/clock from a later model 300D. It now looks (and works) like a turbo car from the factory. I had to supply 12 volts to the module, I never figured where MB gets that source of current. so I tapped into 12 Volts with a 1 amp fuse and fed the module directly, grounding the other pin on it.
An electrical whiz that has time to spend on this "challenge" will figure it out in no time. Just wire it up and it should work right off the bat!
Dieseldiehard
1971 220 (gas) 4-spd manual 104041
1979 300TD w/ ’85 turbo engine 289560
1983 300D 237490
1985 300TD 206150
1975 300D (looking for an engine)
1987 300D 250K newest addition

P'bert · #13 11-10-2003, 01:26 AM

As has been mentioned earlier, the stock Benz tachs are driven by an inductive pickup. If this drive is not installed on an engine, however, and if an aftermarket tack is acceptable, one could intall an alternator driven tach. Every alternator is capable of driving a tach, although not all tachs have an easily accessible external terminal. A good alternator shop can make this modification.

Phil

'84 Euro 300D, no tach
'85 300D, working tach, suddenly dead clock

squid1351 · #14 11-10-2003, 01:31 AM

cool sounds like puting a tach in might not be such a hard task after all. but i have seen a inductive pick up digital tach. has any one given one a try? id like to know how those work out. on a diffrent note if all goes to plan i will have the car in front of my house by this thursday...

redbaronph123 · #15 11-10-2003, 02:37 AM

can i just tag along with this thread as i also want a tach.. but i have a 79 300TD (non-turbo)... i really dont know if it had one before or not.. and i dont know as of now if i have the tach amp needed.... i'll just sit in front of my pc for now. checking up on this thread to see how feasible and (cheap perhaps) this upgrade could be.. .tnx.