W126 Tach coil and sender issue - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 09-16-2012, 12:13 AM

My 1983 300SD has a non working Tach. I bought a new tach amp diagnostic module (under garbage can under the hood) from someone that was referred here. The green LED lights up but I get no reading from the red blinking LED. When you put the key in glow with this module, the tach powers up to 6000 and back down as a test. So I know the tach is working. I started looking around and that brings me to where I think the issue is? There is a pickup mounted above the harmonic balancer and I think its broken. I took a pic and posted it here. Is that pickup coil in the right spot. It looks like the bracket is broken. Where is it exactly suppose to sit. Does anyone have a pic of what a working one looks like? I don't even know if there is something missing. Thanks for helping me out.

vstech · #2 09-16-2012, 12:20 AM

I can take a pic for you, I've got several engines sitting around.
if you rotate the motor over slowly, you should see a pin sticking up from the balancer, it should PERFECTLY line up with the sender in the bracket.

turbozeke418 · #3 09-16-2012, 02:51 AM

A pic or two would be great. I think that the bracket is broken. I don't think that the sender and bracket are in the right spot. I'm not sure though. I haven't been able to find a pin on the balancer either.

toomany MBZ · #4 09-16-2012, 09:31 AM

Yes, check for a pin, it should have rubbed a spot on the pick up.

funola · #5 09-16-2012, 10:31 AM

Quote:

Originally Posted by toomany MBZ

Yes, check for a pin, it should have rubbed a spot on the pick up.

only if the gap was not adjusted iproperly. it should not have rubbed a spot on the pick up, if deep enough could destroy it.

oldsinner111 · #6 09-16-2012, 10:52 AM

Most tach probelms are in the connector where amp screw on,the female sockets slide down as the male is screw in.You have to find a peice of wood or metal and push up the female sockets as you screw the male tight.

turbozeke418 · #7 09-16-2012, 11:16 PM

Quote:

Originally Posted by oldsinner111

Most tach probelms are in the connector where amp screw on,the female sockets slide down as the male is screw in.You have to find a peice of wood or metal and push up the female sockets as you screw the male tight.

The socket where is screws in is perfect. That little silver pin thing on the pic is not on my car. There is a wear mark on the balancer from where the bracket was scraping it. I guess somehow the bracket broke and snapped off the pin??

toomany MBZ · #8 09-17-2012, 09:27 AM

Looks can be deceiving, but if the pin is not there, you won't get a signal to the amp.

turbozeke418 · #9 09-17-2012, 09:09 PM

OK I found it. I took a pic of it lined up and something isn't right here. Is the bracket broken? is it moveable?

funola · #10 09-17-2012, 09:52 PM

Quote:

Originally Posted by turbozeke418

OK I found it. I took a pic of it lined up and something isn't right here. Is the bracket broken? is it moveable?

The pin is there. Hard to tell if the gap is too great. Adjust it as close as possible w/o touching -FSM has a spec. Another thing you can check is continuity of the pickup coil. Should be around 80 ohms.

turbozeke418 · #11 09-17-2012, 11:11 PM

I think the issue is that the bracket is broken which holds the sender in place? The pickup does not come in contact anywhere near the pin right now. It is off by about 1/2 inch. I would think that the p/u should be right over the pin as the balancer moves? correct?

funola · #12 09-17-2012, 11:47 PM

Quote:

Originally Posted by turbozeke418

I think the issue is that the bracket is broken which holds the sender in place? The pickup does not come in contact anywhere near the pin right now. It is off by about 1/2 inch. I would think that the p/u should be right over the pin as the balancer moves? correct?

Broken? How? If it is bent it càn be straightened. Take the pickup off first before straightening or you will crack it.

turbozeke418 · #13 09-18-2012, 02:09 AM

Quote:

Originally Posted by funola

Broken? How? If it is bent it càn be straightened. Take the pickup off first before straightening or you will crack it.

I don't know. I posted another pictured of it with labels. It looks like the bracket that holds the sender on is cracked. The unit itself is pushed down and over so the pickup and the knob/tit thing don't line up.

turbozeke418 · #14 09-19-2012, 12:31 AM

I readjusted my pickup and adjusted it too close and the pin snapped off the end of the pickup. Where do I find a new one. Do they make them new? Does anyone have one available? Hopefully this was the problem all along and when I put the new one in everything will work?

I also read this in another post. Is this true? It was pertaining to a 1985... "The most common part to fail in regards to an inoperative tach is the OVP (overvoltage protection relay), mounted under the passenger hush panel. The only test I know of to check this relay is a "tap test". Check the fuse on the relay first, then give it a gentle tap while the engine is running. 99% of the time, the tach will start to work. It all works in conjuction with the Klima and EGR computer. The OVP protects compoments from voltage spikes. Alan"

techguy512 · #15 09-19-2012, 12:36 PM

Quote:

Originally Posted by turbozeke418

I readjusted my pickup and adjusted it too close and the pin snapped off the end of the pickup. Where do I find a new one. Do they make them new?

I also read this in another post. Is this true? It was pertaining to a 1985... "The most common part to fail in regards to an inoperative tach is the OVP (overvoltage protection relay)

Peachparts/Pelican has them:

Pelican Parts - Product Information: 001-153-84-28-M47

Note that the system in the '85 is completely different than the one you have. The switch happened sometime late in 1983.