'84 W123, no tach amp? - PeachParts Mercedes-Benz Forum

Ceristimo · #1 03-28-2016, 12:14 PM

Hey guys,

The tachometer on my '84 300D has not worked since I've owned the car. I never paid much attention since so many other things were broken, but now that a lot of the other things have been fixed it's time to take a peek at the lower priority issues, right?

I went to look at my tech amp yesterday, and realized my car does not seem to have one.

Where the tach amp should be is just an empty plug with a screw cap. It looks like this:

There is nothing underneath the cap, although some previous owner did stuff the cap full of paper?...

Looking at the plug only pin 2 and 6 are active, and looking up a schematic pin 3 should be the tach signal, so I'm pretty convinced this '84 never had a tech amp.

I did a little bit of research and found that the cars that don't have a tach amp should have an Overvoltage Protection relay under the dash board that controls the tach and A/C.

So, I then went hunting for the OVP, which should be under the dashboard, behind the kick panel on the passenger side, right up where the passenger air vent is. It should look like this:

However, I don't have that either.

So, I guess I am either missing my tach amp or the OVP or does anyone have any idea what else could be going on?

mach4 · #2 03-28-2016, 12:46 PM

It's possible that you've got the tach signal going through the EGR controller. I thought this was only on the '85s but it's possible that the late '84s also had this setup. If so you'll have the tach pickup on the flywheel.

Sorry for the bad picture, it's before I got my engine cleaned up.

Ceristimo · #3 03-28-2016, 01:01 PM

I think I have an early '84? Well, it's listed as a 1984 on the title, but when I look up the VIN, the car was made late '83. I believe October, '83, but I would assume that was sold as an '84.

Boy, that tach pickup looks hard to get to when everything is on the car...

So, just to get the facts straight then, and please correct me when I am wrong:

1) Most W123's have a tach amp. All models up to 1984 have a tach amp
2) 1984 and 1985 have the OVP underneath the dash or a tach pickup on the flywheel?

So as far as I understand three different systems have been used for the tach?

mach4 · #4 03-28-2016, 01:44 PM

Only 2 systems - Tach Amp or EGR Controller-based. The Tach Amp has the pickup on the front pulley (the other version has the same pickup, just used for diagnostics) while the EGR-based system has the pickup on the flywheel. The OVP is only used to protect the EGR controller.

Here's how the latter system is configured

A quick way to check which one you've got is to see if you've got a wire coming off the side of the IP between the pump and engine- that is the rack position sensor.

Rick76 · #5 03-28-2016, 02:22 PM

No tach amp for MY84 and 85.
Federal version had the speed sensor as shown in post 2 feeding directly into the instrument cluster.
California version had the same sensor feeding into the EGR control module. The EGR module then feeds the signal to the cluster.

BillGrissom · #6 03-28-2016, 03:04 PM

My 1984 300D has no tach amp. That plug was then termed "diagnostic port". Supposedly, dealer techs had a test box that plugged to it, but wonder how many ever did. Otherwise, that front pickup is not needed. Indeed, I tried to remove it since just a gunk trap, but they hard-wired the cable into the engine harness and I wasn't going to snip wires.

Mine is a CA model, so supposedly routes the rear tach signal thru the EGR controller box. I think those engines also have the "rack position sensor" on the injection pump (engine side). Since 1984 fed models route the signals straight to the dash, the cluster might be a different PN from CA, or there is another box nobody mentioned, or the CA cars use the same cluster but don't use some components. Finally, perhaps all 1984 models condition the signal on the cluster, and CA models send it on to the EGR box. Perusing schematics would inform.

Both front and rear pickups are 2-wire sensors. They are a simple Variable Reluctance (VR) type, which consists of a magnet and a co-axial pickup coil. The same are used as crankshaft sensors in many gas engines (see Megasquirt site).

Ceristimo · #7 03-28-2016, 03:18 PM

I guess my question is this:

If the car does not have a tach amp, it should have the OVP under the dash, right?

Since I don't have that either, does that mean I'm missing that OVP relay?

I'll crawl underneath the car tonight (or tomorrow, might not have time tonight) to see if I can locate the tach pickup. Thanks guys, I'll post back with what I can find.

mach4 · #8 03-28-2016, 03:38 PM

Some potentially informative reading for you - Quick Fix: 1985 w123 Tach & A/C after EGR Failure

Rick76 · #9 03-28-2016, 04:18 PM

Quote:

Originally Posted by Ceristimo

I guess my question is this:

If the car does not have a tach amp, it should have the OVP under the dash, right?

Since I don't have that either, does that mean I'm missing that OVP relay?

I'll crawl underneath the car tonight (or tomorrow, might not have time tonight) to see if I can locate the tach pickup. Thanks guys, I'll post back with what I can find.

You will not have the OVP relay in a MY84 Federal

Ceristimo · #10 03-28-2016, 10:05 PM

All right, just looked to see if I could locate the EGR controller, but my car does not have that.

So, to recap:
My '84 has no tach amp, no EGR controller, no OVP. I'm confused as to how the tach is supposed to work...

funola · #11 03-28-2016, 10:30 PM

Quote:

Originally Posted by Ceristimo

All right, just looked to see if I could locate the EGR controller, but my car does not have that.

So, to recap:
My '84 has no tach amp, no EGR controller, no OVP. I'm confused as to how the tach is supposed to work...

You pulled the passenger plastic kick panel by the fender and there was no EGR controller behind there? Was there a harness with the large multi-pin EGR controller connector? Is yours a Euro model? If so, it probably never came with a tach. If not a Euro, check if there is a tach pickup on the engine block above the harmonic balancer. If you have a pickup by the harmonic balancer (and no pickup by the tranny bell housing), then someone probably removed the tach amp and put a cap in it's place and that may be the reason your tach is not working.

Ceristimo · #12 03-28-2016, 11:09 PM

All right, I just crawled underneath the car, and I do have the pickup on the bell housing. It's hard to see, because everything down there is covered in a thick half-inch crust of old engine oil.

So, I'm correct in assuming that that wire goes straight into the cluster?

So my non-working tach in that case can only be caused by either the pick-up failing, a break in the wire or a faulty cluster, right?

funola · #13 03-28-2016, 11:30 PM

Tach pickup on the transmission housing does not go directly to the cluster. It goes in to the EGR controller then out to the cluster. But you said you do not have a EGR controller so this is turning out to be a mystery.

Ceristimo · #14 03-28-2016, 11:43 PM

All right, I will have to get my car up on ramps so I have more space and trace that pickup on the bellhousing then.

I do indeed not have an EGR controller, nor do I have the OVP, nor is there a tach amp. The plug where the tach amp normally would be is not even wired up for a tach (only pin 2 and 6 are there) so I'm pretty convinced it has never had a tach amp.

Rick76 ( a few posts up) said the following:

Quote:

No tach amp for MY84 and 85.
Federal version had the speed sensor as shown in post 2 feeding directly into the instrument cluster.

If he's right, than that means there are three different setups:
1) A tach amp on the drivers side fender with pickup on the engine
2) Pickup on the transmission that apparently feeds directly into the cluster
3) Pickup on transmission that feeds into EGR controller

My best bet is that I have number 2, but boy: this is all confusing. Reading up on it a lot now, and the more I read, the more confused I get...

mach4 · #15 03-28-2016, 11:58 PM

Quote:

Originally Posted by Ceristimo

All right, I will have to get my car up on ramps so I have more space and trace that pickup on the bellhousing then.

Might I suggest starting your trace at the cluster...less sludge, easier to get to and you might just figure out what's going on without even having to get under the car.