Figured out why my Tach won't work...
 

I was replacing the cracked brake fluid reservoir, and I noticed a loose cable. Looks like a flywheel sensor! Might explain why AC does not work either...

Checked around the bellhousing looking for a hole where I could see the starter gear, but there does not seem to be one on this engine/tranny. Judging by the parts the PO included in the deal, I think the engine/tranny came from something that used the TDC sensor on the harmonic balancer to feed the tach.

So now I have to figure out how to convert a one pulse per rotation to something that produces however many teeth there are on a ring gear per revolution.

Does anyone know how many teeth there are on a ring gear?

Is your '84 a California model?

My '85 has one of those where the engine and trans mate but it also has a pickup at the flywheel.

That is very likely. My 617.950 has the sensor on the harmonic balancer, but mine is non-op...I don't have that little tit sticking out of my sensor, perhaps it's been broken off.....hmmm...

It's a federal.

Where does the flywheel sensor go anyway?
 

Does anyone know wher on engines/trannys that use flywheel sensors the mounting point is supposed to be?

its near the back of the oil filter housing.... kind of hard to get at.

I could get you a picture.:rolleyes:

Please do!!! Thanks.

Weird. I thought only 85's and '84 Cali. models had the sensor on the bell housing. They also have them on the front, but is for diagnostic purposes, not for tach.

BTW, my tach has never worked..... :( :(

This one is definately a 1984 Model. Built August of 1983, actually!

Perusing the manual, this is an "Engine Speed Sensor" and it's supposed to be bolted on the left hand side of the engine near the edge. Of course, since the engine in my TD is an earlier model, the mounting points don't exist.

Here's a copy of the rotten picture that is in the manual:

sorry, this thread got by me... I will post a picture tonight when I get home.

Diagnostics
 

What kind of diagnostics? Is that what the connector on the driver's (left) side inner fender is for? See attached picture. Anyone ever seen what is supposed to plug in there?

Jimmy is correct that only the '85 and '84 California models only have that sensor for the tach which is controlled by a computer. The '84 Federals only the sensor up front at the balancer and runs through the tach amplifier.

Jeremy, You would hook up an external tach to that attachment (it is actually where the "old" tach amplifier supposed to be) so you can see what is the RPM while under the hood.

Right - but later federals still had the sensor on the flywheel. Looking at the schematic, the output of the sensor appears to go directly into the gauge cluster, where the gauge cluster on the CA model is connected to an output coming from the emissions computer. I attached a snippet.

so my 85 300sd California tach doesnt work - I am confused on what I have to check - and how? thanks for your help. Cary

Can anyone comment on the 85 federal tach - and its related equipment. I have the diagnostic port, but am unsure if I have a computer or whatnot...

I have to get my tach working, and Im only familiar with the old style as in my 83. The parts houses dont seem to differentiate much, and I cant find much info.

Thanks,

JMH

I will add into the question list, what to check on my 85 continental model. (euro) it too has the empty tach amp cover, so I figure it's got the bellhousing sensor. If I throw a older motor in, will I be able to swap over the parts to make the tach work?
John

What you need is the newer style (1984 +) flywheel and bellhousing plate. Then every thing should be able to hook up.


Here is more info and pics of the flywheel RPM pickup that needs to swapped over.

http://www.peachparts.com/shopforum/showthread.php?t=184219

I have the same set up on my '84 as in the photo with the red arrow. I think it is federal. I understood the '84s and '85s used that set up. My '83 has an amp there, with a sensor from balancer.

sorry for bringing this from the dead but my tach doesn't work and the sensor on top of the harmonic balancer has broke off but i also have one that i believe is supposed to go in the block like the one in the picture but it is just zip tied into the harness not bolted in. the harness has it wired in but not bolted into the motor has anyone ever run across this?

You have the same issue as me. Either drop the transmission and replace the flywheel and bellhousing plate or figure out some way to adapt the newer tack input to the sensor on the harmonic balancer. I'm planning on the later, but that's a project for after I get the car on the road!