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Tach linked to oil pressure and/or a/c???

2K views 16 replies 5 participants last post by  mb1984123 
#1 ·
84 300TD with about 120k miles. tach hasn’t worked since I bought the car last month. I tried messing with the tach amp to no avail. No other remedies attempted.

Today while driving the car, as I came to a traffic light under breaking, I noticed the tach jump up to 20 and bounce around. As soon as I pulled away, the tach dropped to 0 and stayed there.

The repeatable conditions from traffic light to traffic light are:

1) a/c on, button to left of “ECO”, temp wheel on “min”
2) car in drive, under breaking
3) under braking, once oil pressure drops from “3”, tach starts bouncing between 20 and 10
4) with car at full stop, engine at idle, and oil between “2” and “1”, tach settles between 10 and 5
5) as soon as throttle is applied and oil goes back to “3”, tach drops to 0 and doesn’t move until braking begins and oil pressure drops below “3”
6) if car is in park, tach won’t move
7) if a/c is off, tach won’t move

After the car sitting off in the driveway for 20 min, I tried to replicate the condition (in drive and reverse) and the tach worked for about 2 minutes before settling back down to 0 with some slight bouncing between 0 and 5 regardless of oil pressure.

I have no idea why or how the oil pressure or the a/c is linked to the tach, but in this car it seems to be. Is there an incorrect connection or short that I should look for somewhere that I can fix?

Any other possible ideas so I can get the tach fully working?
 
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#2 ·
The soldered circuits on your tachometer amplifier are intermittently making connection by those variables you listed above, most likely through vibration of the engine.

There is a longterm fix for the amplifier: http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1652194-permanent-tachometer-amplifier-fix.html

or a short term fix(I'm sure this would fix it for a little while): W123 Mercedes Benz 300D Tachometer repair | Car Instructions This method allows the intermittent circuits to make connection with the aid of a wedge, sort of half assed but it could work for a while.
 
#3 ·
Thanks Smorrow. I tried the half-assed amp fix and got nowhere. I'm not ballsy enough to try the permanent fix (yet). I have no soldering skills or tools. For now I'm attacking the crank position sensor wire. There is a ton of oil down the front of the block that I think is coming from a leaky valve cover seal. The sensor is covered in crud which I'm trying with limited success to clean off.

Of course, this doesn't explain the intermittent nature of the tach, so at the end of the day, I'm sure I'll end up trying to lear how to solder.
 
#5 ·
Some follow up...
Got under and over the crank position sensor yesterday with degreaser and a toothbrush. The first thing I noticed is that the sensor pivots around the bolt that holds its bracket to the block. I'm reasonably sure that's not correct. The more gunk I cleaned off, the more the sensor and bracket just flopped around. So next step for me is to get down to the sensor and re-secure the bracket to the block if possible.

Driving to work this morning, no a/c, but headlights "on", tach behaved same way as initially described. Got to the parking garage and messed around with various scenarios.

-Lights on, in park, no tach, regardless of oil pressure or engine speed (which is different from yesterday's experience)
-Lights on, in reverse, tach bounces between 10-15, regardless of oil pressure or engine speed
-Lights on, in drive, tach bounces between 10-15, regardless of oil pressure or engine speed

-Lights off, in reverse or drive, tach drops down to bounce between 5-10

-The lights absolutely effect the tach needle position
-The selection of park, reverse, or drive absolutely effects the tach needle position

NONE of that makes sense to me, but I'll worry about it AFTER I clean and re-secure the sensor first. I am thinking the amp may not be the problem or at least not the proximate cause at this point.
 
#6 ·
You also appear to have an electrical ground problem. The biggest culprit is the ground behind the steering wheel. I would (and indeed I did at the time) disconnect the ground (my car has two, so I tackled one at a time), carefully sanded the terminals clean, reassembled and tightened thoroughly, and finally, after confirming the electrics worked as they should, sprayed some Metal Part Protector over the lot. The tach is also grounded behind the steering wheel, along with all the cabin equipment and instruments.
 
#7 ·
With just a quick search I can't find anything relevant on the grounds you're talking about, but I want to certainly confirm/deny the impact of the grounds on my problem.

To get to the ground behind the steering wheel, I'm assuming I need to remove the instrument cluster; do I need to remove the steering wheel as well? Where should I look for other ground you referenced (I have an '84 300TD)?

Thanks
 
#8 · (Edited)
You don't have to remove the instrument cluster or the steering wheel. Instead, you have to remove the driver's side underdash panel.

This is my other car, now stripped and scrapped. The substructure behind the steering wheel is where you will find a cluster of wires with brown insulation. Those are the ground wires I'm talking about. They don't appear in this photo, because by this time I had already removed the main wiring harness from the car. If you really don't understand, or find any better pics, I'll take my car apart for a photo or two.




See here as well.
 
#9 ·
I had a similar problem in my 82 300D. The tach worked fine until the A/C compressor was engaged. When that happened, the tach died or started “jumping” between 0 and 6000 rpm for no apparent reason. So, I inspected the crank shaft position and discovered the wire insulation was broken (right on top the A/C compressor) and the connector had disintegrated. Replaced the sensor with a new one and the problem went away.

Also, do as PlaneCrazy told you. Electrical ground problems create all kind of weird issues.
 
#10 ·
PlaneCrazy, I finally understood what you meant about two grounds...the explanation and pics were perfect. I found the grounds no problem (although it was easier for me and my vision to remove the instrument cluster to get at the bolts). I took 'em apart, cleaned and lubed them. Everything continues to behave as before. At least I scratched that off the list of possibles. Frankly, everything behind and under my dash looks pristine, which makes the erratic electrical-type behavior all the more puzzling.

I'm still thinking its the sensor cable, but I can't get at that until Monday. luisg92, thanks for the heads up. Once I get the engine cleaned up and the cable off, I suspect I'll find something similar. I'll keep you posted.

In the "small victories" category, I did manage to free up the height adjustment on my seats. One day at a time...

Thanks for all your help.
 
#11 ·
Make a jumper wire from one of the four speedometer mounting screws to that ground group. That will help the dash lights, tach and clock operate consistently.
 
#12 ·
The clock and lights all work fine, the only issue being the tach and I'm more and more convinced that the crank position sensor cable that's cracked somewhere close to the a/c compressor. Every time I turn on the a/c that tach starts bouncing around. A/C off, and the tach goes to zero.

By creating a jumper between the speedo mounting screw and the ground (another ground cable), how will that improve the grounding of the instruments over the stock set up? I'm no electrician, which is why I ask. Everything else already on the stock grounding set up works correctly, so logically I'd assume the ground isn't the issue. That being an "assumption" is why I'm asking, what difference would the additional ground cable actually make to an otherwise apparently working grounding set up?
 
#14 ·
The clock and lights all work fine, the only issue being the tach
...
By creating a jumper between the speedo mounting screw and the ground (another ground cable), how will that improve the grounding of the instruments over the stock set up?
The tachometer is grounded through the instrument cluster chassis. The solder joints and circuit board traces are a weak point in the cluster and the indicator lights share that ground path. The jumper eliminates those potentially cracked solder joints and helps prevent any traces from burning.
The AC clutch, cluster and tach amp share the same ground point. G102, behind the instrument cluster.
 

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#15 ·
The tachometer is grounded through the instrument cluster chassis. The solder joints and circuit board traces are a weak point in the cluster and the indicator lights share that ground path. The jumper eliminates those potentially cracked solder joints and helps prevent any traces from burning.
Ahhh! Thanks for the explanation, that makes sense even to my electrically-challenged brain. I'll try to knock that out this afternoon and let you know what happens.
 
#16 ·
UPDATE: I made the jumper from one of the speedo mounting screws to the ground point behind the instrument cluster. No joy. Tach still bounces around.

I tried to get to the crankshaft position sensor (CPS) this weekend but was stopped by three of the four fan bolts. Bolts frozen, so I can't get the fan off to get down to the CPS. I have no idea how to get those bolts off. Ideally a box wrench might work, but I can only get an open-end on there and it's rounding the bolts. I tried a flare-end wrench (IAW Pelican's DIY), but couldn't get any one of two different flare-end wrenches on the bolts. No-go with small vise-grip pliers either. Looks like the CPS, cable, and tach will have to wait for a mechanic.

Regardless, thanks for all the help.
 
#17 ·
UPDATE: Replaced the CPS sensor and cable, the tach works great now.

Problem was none of the usual suspects. The 10mm nut on the CPS sensor mount had become loose and allowed the sensor to fall against the crankshaft pulley. Needless to say, that ground down the rubber casing on the sensor and into the copper coil inside. That caused all the erratic readings. The cable was in fine shape, no cracks or other damage. Once I replaced the sensor and cable, the tach resumed normal operation.

Thanks to all who helped.
 
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