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Tach amp repair attempt
For a challenge, I'm trying to repair a bad tach amp for a friend.
I picked away at the white silicone potting compound and found a badly burned out compoent that I cannot identify. Is there shematic of the tach amp somewhere? How about a wiring diagram of it in the car? I was not able to find a new one on Fastlane. Is it called something else? How much is a new one? Here are a few poor quality pics I took while taking the tach amp apart:
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85 300D turbo pristine w 157k when purchased 161K now 83 300 D turbo 297K runs great. SOLD! 83 240D 4 spd manual- parted out then junked Last edited by whunter; 02-03-2008 at 12:27 AM. Reason: attached pictures |
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enough to at least justify picking it apart .
I sent a well picked apart one off to another member several months ago who was going to dissect further. No word as of yet, but I will say with the amount of silicone you've removed there's still a bit to go (photos are of pretty poor quality too so it's really hard to see). Some components are marked, some of them are not (the green ones if I remember correctly). on any non-functioning tach the cigarette butt tricked should always be attempted first.
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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) |
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$$$ Ouch! Why is it called a RPM sensor? Technicaly, the Tach amp is not the RPM sensor. The magnetic pickup on the engine is the RPM sensor.
Anyone know whether the tach in the instrument cluster have any electronic components? Sorry for the poor quality pics from the phone cam. The phone is always on me, unlike my digi cam. Are poor pics better than no pics?
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85 300D turbo pristine w 157k when purchased 161K now 83 300 D turbo 297K runs great. SOLD! 83 240D 4 spd manual- parted out then junked |
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The tach gauge I believe works very similar to the fuel gauge. It receives a signal and places the needle based on the signal. Fuel gauge works on varying resistance. Not sure what type of signal the tach amp sends but the gauge does no 'processing' of it.
bad pics better than no pics? all depends on the context.
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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) |
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Would you consider getting yours back (if possible) and send it to me for repair? If yours is intact, along with the one I'm working on, and I can draw a schematic from them, and there are no proprietary components like an i.c. there is a good chance these things can be repaired or a new circuit built to replace it.
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85 300D turbo pristine w 157k when purchased 161K now 83 300 D turbo 297K runs great. SOLD! 83 240D 4 spd manual- parted out then junked |
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Quote:
obviously this is not the first time the subject has come up.
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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) |
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I'm sure it hasn't been the first time this subject has come up because this is my problem too! Mine works intermittently so I think I might be able to repair it. Now I've been told you could fill the top with pennies on top of the silicon and it will work for awhile. Something is making a count of the electromagnetic pulses and converting that to a consistent revolution monitor that can expand and contract the engine speed against the electromagnetic pulse. My guess would be a capacitor and a toroid with a voltage division network coupled to a threshold diode. I haven't taken mine apart yet, but when I do I'll post back and let everyone know what I found. I've got quite an extensive work bench with test equipment for restoring the circuitry on Antique tube type radios and this knowledge comes in handy! there is a schematic of the electronic ignition uniot on Chrysler vehicles from the 1970's and early 1980's in almost every auto repair manual and you might be able to work that backwards since values are given for the components.
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Quote:
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I have the silicone potting mostly picked out. Here's a pic
The i.c. is a LM1815N on mine and AFAIK is not proprietary. Yours must be different. Quote:
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85 300D turbo pristine w 157k when purchased 161K now 83 300 D turbo 297K runs great. SOLD! 83 240D 4 spd manual- parted out then junked Last edited by whunter; 02-03-2008 at 12:26 AM. Reason: attached picture |
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Quote:
Here's the internal workings of the LM1815N, see picture. Made by National Semiconductor, costs about $7 in an electronic supply store.
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Tomas, Sweden 1966 Mercedes Benz 230S with OM617.912, automatic. Disk brakes from W108 1983 Mercedes Benz 300TD grey, OM617.912 and 5-speed manual 1983 Mercedes Benz 300TD blue 7-seater, OM617.912 and 5-speed manual |
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Quote:
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I found the schematic for the RPM sensor! I'll have to start a new post on this topic because I cannot upload a bmp file to the post reply section of this forum. for those interested look in the headliner VIOLA!
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VIOLA! RPM sensor schematic found!
Now can anyone tell me how to upload a file bigger than 65kb?
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What type of file? Images, use http://imageshack.us , others, I can host it on my server if you want
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Ich liebe meine Autos! 1991 Mercedes-Benz 560SEL | Megasquirt MS3-Pro | 722.6 transmission w/ AMG paddles | Feind Motorsports Sway Bar | Stinger VIP Radar | AntiLaser Priority | PLX Wideband O2 | 150A Alternator | Cat Delete 1981 Mercedes-Benz 300SD | Blown engine, rebuilding someday... 1981 Mercedes-Benz 300SD | Rear ended, retired in garage. 2009 Yamaha AR230HO | Das Boot Excessive speeding? It ain't excessive till I redline! |
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