[Nutz 4 Benz]

Seriously I'm a gasser driver with a lead foot.

1984 300D

Where is the tach amp?
What else would cause it to not work?
How do I repair?

 

[Govert70227]

Blowing up the engine is difficult with a diesel and there are those little marks on the speedometer.

The tach amp is on the fender, driver's side. It is a diagnostic plug, inside the cap of the plug is the amplifyer.

 

[Rmac58]

I'll agree with you probably won't blow up your engine, but the 84 models went with a different set up, no tach amp.

It takes the signal from the transmission.

The usual culprit is the OVP located behind the dash on the passenger side. A tall silver relay with a red top that houses a spade fuse, check that.
It'll be easier to remove the relay first.

 

[::matthew]

I thought only 85s had the OVP (84 cali too?)

 

[300TD1984]

I thought our cars had a governor that limited engine speed?

 

[Rmac58]

Unknown for sure.

Here's a pic of my socket, yours is the same.

 

[I 26]

Blowing up the engine is difficult with a diesel and there are those little marks on the speedometer.

The tach amp is on the fender, driver's side. It is a diagnostic plug, inside the cap of the plug is the amplifyer.

Unrelated, what exactly ARE those marks on the speedometer?

 

[Govert70227]

Unrelated, what exactly ARE those marks on the speedometer?

Maximum speed for each gear: 1 dot for first gear, 2 dots for 2nd gear etc.
For the 240D the dots are at about 21 mph, 37 mph, 60 mph. For the 300d they are at slighty higher speeds.

 

[::matthew]

Maximum speed for each gear: 1 dot for first gear, 2 dots for 2nd gear etc.
For the 240D the dots are at about 21 mph, 37 mph, 60 mph. For the 300d they are at slighty higher speeds.

I thought those were maximum downshift speeds for each gear.

 

[MBeige]

I thought those were maximum downshift speeds for each gear.

The maximum down shift speeds are determined by vehicle load, throttle input, vehicle inclination, and current speed.

Kickdown can actually be done not just to one gear, but two (our diesels usually have this function at lower speeds). The max speed I can downshift with the kickdown switch is around 40mph on a light load. Never tried it with a heavy load, however.

But on my 190E when driving up a steep incline with some luggage and one passenger, the car downshifted at 50mph. It has the same differential ratio (3.07). The 300TE, when driving one time, was able to downshift using kickdown switch, at 60mph with only one passenger on board.

See below for speedo markings mentioned on manual.

 

[Govert70227]

When you have an automatic and you drive in D, you cannot really overspeed. If you drive to fast for a gear, even kickdown will not shift back to that gear. You can overspeed if you put the selector in S or L when driving to fast.

With a manual gearbox it is also possible to overspeed, certainly when downshifting. The governor will limit the speed when you stay in a gear, but that speed will be higher than the speed of maximum power. So if you are in second gear for instance and you press the accelerator, the car will go above 37 mph, until the engine reaches somewhere above 5000 rpm, while 37 mph is about 4400 rpm.

The older diesel, with a vacuum controlled governor, could not go above about 4200 rpm.

 

[ktmripper]

I roll a 84 300d. If you have a solid engine/tranny you can drive with your foot on the floor all day. If your car is auto, sounds like you may have a tranny issue. Is your final gear engaging?

 

[Scoot]

The older diesel, with a vacuum controlled governor, could not go above about 4200 rpm.

This is interesting. My 1985 (CA) car only goes up to either 4200 or 4800 parked with me pressing the pedal all the way down. So that's "normal"?

 

[Govert70227]

This is interesting. My 1985 (CA) car only goes up to either 4200 or 4800 parked with me pressing the pedal all the way down. So that's "normal"?

The maximum speed unloaded is between 4700-5400 rpm depending on the model. For a 1985 300D it should be between 4900-5200 rpm. Your engine speed is a bit low, but an accurate tachometer should be used, not the on-board one.

 

[Scoot]

The maximum speed unloaded is between 4700-5400 rpm depending on the model. For a 1985 300D it should be between 4900-5200 rpm. Your engine speed is a bit low, but an accurate tachometer should be used, not the on-board one.

How does one connect an "accurate tachometer" to a diesel engine?

 

[Govert70227]

How does one connect an "accurate tachometer" to a diesel engine?

Depends on the model. If it has a diagnostic plug, it can be attached there. Otherwise a crankshaft pickup can be installed. On even older models (pre-w123) they even used the oil pump drive.