another 300SD Tach issue

[automch]

When I bought my 83 300SD the tach just jumped constantly. I tried the normal fix of pressuring the unit tightly into the amp socket which made the jumping stop however the tach was totally inop. So I bought a new tach amp and the tach is still dead. Started checking things farther and fount the pickup missing completely. Installed a new pickup and tried another tach gauge and still no tach. The dampner has a small pin that is meant to excite the pickup and it is in place. There is 12v to the tach. My question is this: What exactly is supposed to be on the dampner to excite the pickup? This car has had the engine replaced proir to my buying it and the engine is a 952 instead of a 951. With the pickup disconnected from the amplifier and monitored with an oscilliscope ther is no signal generated so I feel it must be something about this dampner. Any advice?

[Maxbumpo]

The pickup may need to be mounted on the transmission/engine junction. I don't know what year the break-point was, but it moved at some point from the front of the engine (crank harmonic balancer) to the rear.

There is supposed to be a small magnet that provides the signal when its field moves past the pickup.

[Marshall Booth]

If the replacement engine was from an '84-'85 model year car (years that don't use the crank damper sensor to power the tach) then playing with the crank sensor or inner fender amplifier will not likely RESULT in the tach working. The later engines have a sensor on the flywheel bell housing and that signal drives a tach amplifier that's in the passenger cabin.

You need to know the model year of the engine to determine which sensor shoud work.

Marshall

[automch]

Thanks for the quick replies! Unfortunately I am unable to determine the year of the engine. However there is not a spot in the bellhousing to mount a pickup like the 85's have. The part on the dampner is just a pin about 1/8" in diameter and 5/16" high. I have adjusted the gap to no avail so I am of the opinion there needs to be some magnetic force to excite the pickup. Maybe if someone would post or email me a picture of their dampner I would know if this is the correct dampner or not.

[Maxbumpo]

The pin in the damper is the magnet, I'll bet. I think that what you are missing is the mount for the pick-up, which is a metal piece that replaces the crank position pointer (the pickup is attached to this).

You probably will need to visit a junkyard to analyze a complete car and figure out which pieces you are missing.

[automch]

Actually the bracket was there but no pickup was installed when I got the car. I have installed the pickup and the gap to the pin is close enough. Maybe the pin needs remagnetized? I have even thought of trying to attach a magnet somehow to see it that produces a signal, but I don't want to spike the amplifier with too much of a signal so I don't know how strong a magnet to try.

[Marshall Booth]

What is the measured resistance of the pickup?

Marshall

[automch]

Hi Marshall, I don't have the car here at work so will measure the resistance this evening when I get home. The pickup is brand new from MB. I will post what I find.

[automch]

Marshall, the pickup measures 76.2 ohms. It is a new OEM Mercedes part.

[denver123]

Gentlemen:

Does the 1985 300 TD (W123) tach operate in the same manner as that which you describe on the 300SD? I see what looks like the pickup just back of the dampener. I cannot find anything whatsoever in the Mercedes Service library about the tach. Mine reads accurately if I'm accelerating, accurately when my foot is off the accelerator, but reads a slightly shaky 1050 rpm's when I'm applying light power, as when maintaining speed. The result is that the needle jumps around a fair amount in the course of mormal driving. My indy mechanics don't have any idea what's up with it. I'd like to repair it if I can, but I do not know where to start. This all only started at about 212K. I have about 215K now.

Thanks,

Will

1985 300TD W123
1998 E320 Wagon W210 (wife)

[Maxbumpo]

No, on an '85 the pick-up will be at the junction between engine and transmission.

However, sounds like the tach. amplifier is flaky to me. If you've got the inclination, swap out your instrument cluster with a known good cluster and see if that tachometer behaves the same way. Only other component is the pick-up. You may want to check the ground connections and replace all your fuses with new.

1050 RPM should not be held routinely in normal driving; peak torque is at 2400 RPM and peak HP is at ~4800 RPM (can't remember exactly at the moment). If you are holding 1050 RPM, mileage is poor and engine will coke up and wear out more quickly. There is a reason why, at highway speed, engine RPM is 3000 and up. If you're driving around town at speeds of 45 mph or less, leaving the gear selector in '3' will provide better mileage and will keep the engine cleaner than using 'D'.

Notice that there is no red line on your tachometer. You cannot over-rev the engine with your right foot (but you can downshift too early if manually shifting gears and cause damage).

MB diesels are different and prefer to operate at much higher RPM than their American bretheren...

[denver123]

Thank you Meade. I bought the car 11 years ago with 95K on it. I always drive in "S" or 3rd gear unless I am actually on the highway or am on a fast boulevard or whatever.

When I say the tach reads 1050, that's what I mean. But that doesn't mean I'm saying the engine is turning at 1050, quite the contrary. It reads 1050 when you're only lightly pressing the accelerator pedal (to maintain speed) cruising along in 3rd gear at say, 45 - 50 mph, or really at any speed and in any gear when you're lightly pressing the accelerator pedal. The moment you let off the accelerator it reads what the engine is truly turning, like maybe 3000, 3500, or whatever it is. Similarly, if you press the accelerator pedal harder to increase your speed the tach will immediately start reading correctly again. The tach is acting funky, I'm not dirving any differently.

What is the thing just back of the dampener if it isn't associated with the tach? It has a cable running to it that splits off from the main cluster that comes from the firewall.

I don't think I'm experienced enough to remove my cluster, and also don't have another one.

I don't know how to check the ground connections, but I changed out all the fuses. Hmmm, might have been right around the time my tach started acting funny. The fuses all look good but I admit, I bought them at a place like Checker and they're probably not the highest quality in the world.

Thanks

Will

[michael557]

hi guys
well i got a unserviceable tach to,does anyone got a way of testing the rpm sensor (that is installed at the engine/transmition flange)
or a way of testing the tach gauge itself.
it driving me :mad::mad::mad:

by the way the car is a 1985 300SD with no amp in the fender,so no cigarette butt trick for me
good thing too,because i don't smoke :):):)

[Maxbumpo]

You can test the signal to the tachometer by pulling the instrument cluster and testing for AC signal at the wires that connect to the tachometer.

[automch]

BTW mine turned out to be a broken wire at the steering column support inside the harness. It was a bear to find but now it works like a charm.:D

[michael557]

thanks guys i will defenetly test if i get a signal from the rpm sensor
michael

[michael557]

hi Maxbumpo
what do you mean by testing for a/c at the plug
i have a 12v
my question is do i need a oscilloscope for that,because i don't see a way of doing this test with a multimetre
thanks

[Maxbumpo]

A very crude test (but simple) is to test for AC voltage between the two pins that bring the signal to the tachometer from the tach. amplifier or computer. Pull cluster, re-connect battery, start car, check for an AC voltage. Rev the engine to see if the signal changes (voltage increases). If all these are true, then your problem is almost certainly in the tachometer. If there is no AC voltage, then your problem is in wiring/amplifer/sensor end. You can test the sensor with same method.

[michael557]

hi maxbumpo

correct me if i get this straight
12v dc is send to the sensor and every time the crank turns the 12v dc is interrupted by the pin mounted on the flange,
this means that in reality you get a 12v with a frenquency of engine rotation in this case 17 hz at idle because of engine rotation .
17 hz because of 1000 rpm divided by 60 sec/min = 17 hz


thanks
michael

[Maxbumpo]

Close enough - you get the idea.

[michael557]

thanks maxbumpo

i hope i am right in this case because i do get a ac signal to the tach of 12v ac but no reading,i only can deduct that my tach is no good,time to find one that works now

thanks
michael

[Maxbumpo]

Tachometer needs that signal plus a ground connection to work. Make sure that you supply a ground if testing tachometer with cluster pulled from dash..

[michael557]

hi maxbumpo
i can get my hands on a 1988 300SE tach,do you know if they are interchangeable
are all the 126 chassie interchangeable
thanks

[Maxbumpo]

No, you can't put tachometer from gas car into a diesel car.

[michael557]

hi maxbumpo

do you know then if all the diesel tach are interchangeable like 1981-1991 because of the tach amp in the wheel fender in the early models,mine has no tach amp in the wheel fender (it is the rpm sensor on the bell housing).

thks maxbumpo you help is appriciated

michael

[Maxbumpo]

I'm not sure, but if you check the on-line EPC you'll be able to find out. Subscriptions are free, but registration is required.

[michael557]

hi maxbumpo
got my hand on a 1988 300 SE tach and installed it,no better result,of course i dont know if it is interchangeable or i got a sensor problem
something i need clarifications with is the signal or pulse comes from the front of the engine crank pulley? because i do see a sensor their
i am all confused,so what is the sensor at the eng/trans flange then?

thanks