seeing that the cigarette but trick fixes this problem often it definitely points at ruptured metal traces or solder joints.

wouldn't hurt to reengineer the little bugger anyway.

__________________
1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)

Mine did exactly the same thing .... The cure was cleaning the pins and the inside of the .. receiving holes? with fine sandpaper and using "Dielectric grease" when reassembling. It has worked flawlessly since.
Having said that ... Now I am getting nervous that I may jinx myself

Alberta Luthier

That was my solution too. Well, I didn't use sandpaper. I used a pencil eraser on the pins and cotton soaked in contact cleaner for the sockets. Then I used the silicon dielectric grease, but not too much.

Unfortunately, it doesn't work for everyone.

__________________
Brian

_________________________________
Sometimes the only choices you have left are bad ones.
Agent Broyles (Fringe)

as a side issue when I cleaned up the fuse holders, changed to copper/brass fuses and used the Dielectric grease, that solved al kinds of iffy problems...possibly making a differance to the iffy tach among other issues.
Alberta Luthier

Where is the Tach Module located and what does it look like on the '85 300D? Mine is intermittant.

__________________
Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY

On my '83 300cd it is on the driver's side fender near the windshield washer bottle. It looks like a plastic toy garbage can over an inch in diameter and 2 inches high . I am not sure for your car.
Alberta Luthier

The only thing round in that location on my car looks like a diagnostic plug.

__________________
Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY

I think they stopped using the part we are talking about in '83. I think 84-85' models do not have them. 85's for sure do not have them.

So, are all of the electronics for the tack in an '85 in the tach itself?

__________________
Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY

Well I have been looking into putting a tach into my 240D for a few weeks and I picked up a amp and diagnostic port and gauge itself. I plugged in all the right connections but nothing. I thought well it must be the amp everyone says the go, so I took it apart and it looks like a new one as the electrical stuff is all of newer style. So I plugged it back into the car and read the measurements coming off the amp and well it was not the problem. The unit is a Lm1815N and all it is ment to do is up the output of the sender, so in other words more volts at the same frequency.

I then looked into the tach itself and it looks like the lm2917N-8 is bad, I have a new one on the way. Very simple unit, both the amp and tach.

I was looking at the wiring diagrams for a bunch of the w123D's and it does not look like there were many cars that have the Amp on the wheel well, most (later models) get the signal from the EGR system. That got me thinking about the amp in the EGR controller, so I opened mine up and look at that its the same lm1815N that in the tach amp. I did a little circuit tracing and it seems that pin 6 on my egr controller is an output from the LM1815N.

So I tested it and WOW same output as the tach amp, that should mean that I can put a tach in my car and not have to add the diagnostic port of tach amp. Nice and simple. I should be getting the parts I need to fix the clock and tach and will go do a little testing. Assuming I get over this cold.

I took sometime to map the Tach amp so that anyone who wants to make one can from cheap parts. I think the diagram is all correct but I have not made and tested it yet and I may not seeing as the EGR system might work for me. If it does I might have a nice tach amp that someone can have if they want to solder three of the leeds back on.

Attached Thumbnails

 

No. EGR computer, pass kickpanel.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

How did they get away with not using one on the 85 SD's? the 85 I have at the shop has a cap, but no amp...

__________________
RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

____________________________
FACEBOOK: PANZER450

Ok, I guess I have to pull the EGR computer. My tach reads about 500rpm low most of the time but intermittantly jumps to the correct rpm for a second or two every few minutes. I'll check the solder joints.

Since I have my EGR disconnected, is there any problem with driving the car with the EGR computer removed?

__________________
Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY

Doubtful, all the EGR comp does is control all those solenoids and check valves over there.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Hi Graminal95,

I was looking at your schematic. Could the two grounds that you show in the upper left actually go to +12V? I don't think the circuit will work as drawn.

Also could the diode in the upper right be a zener, probably 9.7V?

Thanks,

__________________
Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY