Tach amp repair attempt - PeachParts Mercedes-Benz Forum

funola · #1 05-27-2007, 09:25 AM

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:

#2 05-27-2007, 09:31 AM

This is the 83 300D part:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ423G0KC1AE&year=1983&make=MB&model=300-DT-001&category=P&part=RPM+Sensor

jshadows · #3 05-27-2007, 09:50 AM

Quote:

Originally Posted by funola

How much is a new one?

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.

funola · #4 05-27-2007, 10:16 AM

$$$ 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?

jshadows · #5 05-27-2007, 10:57 AM

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.

funola · #6 05-27-2007, 11:30 AM

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.

jshadows · #7 05-27-2007, 01:36 PM

Quote:

Originally Posted by funola

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.

member PerkHouse has it and I've not heard from him in a while. Your best bet is to get with him and others from this thread to figure out the best approach.

obviously this is not the first time the subject has come up.

Knightrider966 · #8 05-27-2007, 02:08 PM

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.

Knightrider966 · #9 05-27-2007, 05:07 PM

Quote:

Originally Posted by funola

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.

Bad news! There is a proprietary IC chip in there and I can't identify it! It's not in my Phillips semiconductor catalog and I cannot reference it to any component link.

I think I now know why these things are so high priced! I know that there must be a way to find or make another type of broadcast sensor and I'll keep working on it and post back

funola · #10 05-27-2007, 06:08 PM

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:

Originally Posted by Knightrider966

Bad news! There is a proprietary IC chip in there and I can't identify it! It's not in my Phillips semiconductor catalog and I cannot reference it to any component link.

I think I now know why these things are so high priced! I know that there must be a way to find or make another type of broadcast sensor and I'll keep working on it and post back

tompaah7503 · #11 05-27-2007, 06:41 PM

Quote:

Originally Posted by funola

I have the silicone potting mostly picked out. Here's a pic

http://i31.photobucket.com/albums/c397/funola/Photo_052707_002.jpg

The i.c. is a LM1815N on mine and AFAIK is not proprietary. Yours must be different.

Here's the internal workings of the LM1815N, see picture.
Made by National Semiconductor, costs about $7 in an electronic supply store.

Knightrider966 · #12 05-27-2007, 06:59 PM

Quote:

Originally Posted by funola

I have the silicone potting mostly picked out. Here's a pic

http://i31.photobucket.com/albums/c397/funola/Photo_052707_002.jpg

The i.c. is a LM1815N on mine and AFAIK is not proprietary. Yours must be different.

No it's the same as yours, but mine had no visible lettering on it so I assumed it must be proprietary and it isn't! I found the circuitry to this system and I'll post it here shortly.

Knightrider966 · #13 05-27-2007, 07:21 PM

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!