Hello folks!

This is my first post here, and I want to say thank you to the members of this forum as it has been an excellent resource for me up to this point!

I recently purchased a 1984 300SD after quite a search. The car is running reasonably well, but the paint is pretty ugly, it needs an hood, and has some issues that I’ve been slowly tracing. I’ll be chasing oil leaks for the foreseeable future, but I’m enjoying bringing the car back to its former glory. I hate to see these cars neglected. I’ll have about a thousand more questions as I fix it up, I’ve already found a few older modifications done to this car that have been making me scratch my head and I’ll need some help making them right.

Now that my long-winded intro is out of the way, here’s the issue I’m dealing with that I can’t seem to sort out. I am troubleshooting an inoperative tachometer, and after checking a few likely suspects and electrical connections that all look good, I decided to test the tach connector at the back of the gauge cluster with a multimeter. I seem to be getting power to the instrument, which led me to believe that I likely had a bad capacitor in the instrument itself. Once I pulled apart the cluster, I noticed that somebody had replaced a capacitor before, as well as the two capacitors for the clock. Through reading on this forum and others, I have surmised that the correct values for the clock capacitors should both be 100 μF. Whoever replaced these capacitors used one 100 μF and one 47 μF. This makes me wonder if they used the correct capacitance in the tachometer, or not. There is a 22 μF cap on the tachometer, just above a blue cylinder (another capacitor?) and that appears to be the only newer solder on the board. I can post photos if necessary. Is this the right value for that cap? If not, could it be the source of a totally dead tach, no movement of the needle at all? Does anybody know the correct value and anything else I might need to replace in that circuit? I don’t see any burnt resistors or bad solder joints.

Thanks in advance for your help! I’m a bit of an electrical novice, I know some basic safety and can solder pretty well but I’m an absolute hack when it comes to reading schematics.

Have you jumped the egr computer yet?? The tach signal is actually an AC signal and is easily seen with a cheap oscilloscope from Amazon. Doing this answers a lot of questions.

https://youtu.be/Z7eDX_2lMyg?si=HYFOtPYZbUYneSME

and welcome aboard. The cars WILL make you into an expert automotive electrician. Fortunately, 80s german tech is pretty simple. Remember they love to switch negatives and you'll figure it out.

Thank you! I sure have, no improvement after jumping the EGR computer. I have also checked the OVP fuse, the diagnostic port on the driver’s side fender, the two pin connector on the passenger side firewall (which I understand to be tach related) and all looks good with good connections. It seems somebody tried to fix this at some point in the past, because I have one of the green PCB tach amps attached to the diagnostic port and several capacitors replaced in the dash. I do also have several, maybe 5 or 6, various electrical connectors under the hood that have been disconnected by a previous owner, I believe during a shoddy attempt to delete/disable the EGR. I’m wondering if any of them are related to the tach.

I am going to replace the capacitors in the clock while I have the gauges out anyway, since they’re the wrong values. Any chance you know the correct value for the one in the tach, or do you know where I might find a diagram of the circuit?

ds:

Is a second W126 tach/clock package of interest to you?
Production date 8-82, 1 pulse/rev (6000 Imp.), model # 902/29/5.
PM me if you would like to know more.

Have you checked for a TD signal? As noted above, it may show as either a sine or square wave on an O-scope , but can also be read with a volt meter.

Frank,

I’d definitely be interested in a replacement if I can’t sort through the one I have, though I am leaning towards fixing what I have if possible. Is an 82 build date compatible with my car? I was under the impression that the tach system was changed for the 84 model year and is different from the 83 and earlier cars.

Sadly I do not have access to a scope, though I have an electrician buddy who may have one I can borrow. I have put a multimeter on the tach wire and it appears that I have voltage to the instrument.

ds:

What is the model # of yours? Stamped into the housing: 902/xx/x.
The style of the connector for the TD signal and 12V+ may be different between models.
The tachs for the gas engines have had two styles of connectors yet function interchangeably (for the same number of cylinders).

No. The 1984 is a 14,000 imp full scale gauge. The 6000 imp will not work with the 84.

If you want to use the 6000 imp gauge you need to read the signal from the sensor on the harmonic balancer up front on the engine as opposed to the sensor on the ring gear. You will also need a tach amp (which it sounds like you already have).

Likely not the path of least resistance.

So you’ve checked the connector on the EGR computer. And you’ve checked the red fuse on your OVP relay. And you’ve checked the connector on the back of the gauge. And you’ve checked the firewall plug. Huh sounds like you’ve watched my YouTube video.

You’ve covered all the bases for an 84. I think your gauge is messed up. Best guess at least. I have an 85 300d gauge in my garage. Maybe I can read off the caps for you. The 2 caps you mentioned sound like the clock capacitors. Irrelevant.

The frequency to voltage converter is likely an LM2907 IIRC. Let me walk out and take a look. BRB

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Okay, thanks! And yes, the back of my tach is stamped 14,000 so presumably that is the tach that receives its signal from the bell housing connector. I’ll have to pull the cluster again to get the exact model number, I reassembled it temporarily today so I can keep driving it.

You’re correct, I did mention the clock capacitors, both should be 100 μF. Both were replaced, but one with 100μF and one with 47μF, and the clock is not working either. This makes me think that whoever screwed with it may have used the wrong cap in the tach as well, which has a replaced cap with 22μF capacitance. Trouble is, I have no idea if that’s the value that should be there or not!

Edit: it’s entirely possible I’ve watched your video!! I’ve consumed about every piece of content I can find on this issue, it’s all beginning to blur together lol. Thanks for the work you’ve done!

Edit 2: Tach model number is 902/74/1

Ok. Yep it’s a 22 microfarad (uF) electrolytic capacitor. It says 22/40 on it made by Frako. I assume that means 22uF/40V.

The chip on the board is an LM2917. I’d replace that too if you’re going to go out and order the capacitor at Mouser. It’s cheap. My feeling is the capacitor is likely fine if they replaced it. The chip more likely is blown. Might as well get both.

ETA as said above it would be wise to get a scope and track the waveform from the sensor to the gauge. You may have a disconnected wire. The proper way to do this is check the signal at the back of the gauge with the scope like the image shows. Then if it isn’t there work your way back to the fault. A cheap scope is really cheap now. I bought one for $60 last week Aliexpress. There are ones as cheap as $20.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Ok, I’ll see what I can do to get my hands on an oscilloscope and make sure I’m getting the correct signal to the gauge. In the meantime, I’m going to order two new 100μF caps and another 22μF cap for the tach. The previous repair used poor quality Chinese caps anyway, and I can solder new ones in in about 20 minutes, so I might as well make it right.

I hate to ask too much, but is there any way you can link me to the proper chip to order? I see a couple versions from Texas Instruments and, embarrassingly, I’m not even sure where the chip is located on the board to identify which one I need (like I said, I’m an electrical novice!) I do think once I see what the right one looks like, I can find it on the board and from there I can solder it in without much trouble.

Thank you all so much, this is a huge help!

No problem.

https://www.mouser.com/ProductDetail/Texas-Instruments/LM2917N-8-NOPB?qs=X1J7HmVL2ZHezbIGjS%252B%252BVQ%3D%3D

Big black rectangle thing with 8 bug legs. Make sure to get the orientation right. Take a photo before disassembly

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles

Just so ya know, you are looking for an AC signal. That one eluded me for awhile. Some where around 6-8volts I think. If ya want a cheap scope, this is the one I used in the video.

Search on amazon for FNIRSI DSO152 Oscilloscope - 2.8" TFT Handheld Digital 36bucks.

You guys are amazing. I’ll be ordering some bits for the tach tonight, even if it doesn’t solve the problem I want to take the time to make it right with good parts. I’m waiting to hear back from a buddy to see if he has a scope I can use, if not I’ll order that one probably tomorrow.

I likely never would have tested it as A/C, great tip. I’ll put my multimeter on there first thing tomorrow and see if I can learn anything in the meantime.

I’ll update with what I find, just in case anyone is curious!

Well, the plot thickens.

I retested the tach connector at the back of the gauge. I am showing 14 volts of DC power, but when tested AC, I get a momentary jump to 3 or 4 volts and then it drops off to zero.

I decided to check the only thing I hadn’t gotten around to looking at, which is the tach pickup on the bell housing. So far, unless I’m looking in the wrong spot, I have not found a terminal on the bell housing at all. I found one broken brown wire coming from the engine harness, with no connector on the end of it. If anyone has a clear photo of the pickup location, that would be immensely helpful. I haven’t found a replacement part for the pickup either, if anyone has a source.

This all made me wonder if this is the original engine and trans. I saw that there are a couple different castings for the bell housing on these cars, one smooth and one with cooling fins. My car has a smooth bell housing. That’s as far as I’ve made it. Any tips on identifying whether or not this engine/trans is original?