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  #1  
Old 07-03-2022, 02:20 AM
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Gauges reading max

Hi guys,
Hoping for some information.
1984 Mercedes w123 300d

I had to replace the ignition steering lock assemble as the keys would no longer turn in the ignition.

I removed the under dash plastic panels, removed the dash gauges noted down where what went, removed the ignition assembly, noted which vacuum tubes went where removed the key buzzer wires etc.
Got the new lock assembly swapped the ignition switch, and the buzzer switch and diesel switch and put everything back where it came from started up and everything I thought was fine.

But then I noticed the fuel gauge reads full, oil pressure is up at maximum and the temp gauge creeps up as you would expect but just keeps rising to max. (Removed radiator cap and could hold finger in water was warm but not hot) lol I know silly but I knew it was not hot by the lack of steam.

I know there is only 1/2 a tank of fuel,
Oil pressure gauge use to sit about halfway
Temp gauge again normal sits about 1/2 way up.
But as I said they are now all reading max.

Pulled the dash out again just in case there was a wire I forgot to reconnect but everything looks like it was correctly reconnected.

Hoping someone has an idea, I don’t have another instrument cluster or access to one to try

Thanks in advance.

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  #2  
Old 07-03-2022, 08:23 AM
t walgamuth's Avatar
dieselarchitect
 
Join Date: Apr 2005
Location: Lafayette Indiana
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Look for your ground on the cluster and clean it up or tighten it. When your gauges are acting squirrely look at your grounds.
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  #3  
Old 07-03-2022, 08:51 AM
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Good advice to check and clean the brown ground wires. You might consider installing a new ground wire from the back of the instrument cluster (there are several places to attach a ground wire) to the ground point on the steering column bracket. Worked for my 1985 300D when I had the same problem.
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  #4  
Old 07-03-2022, 01:25 PM
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Post Gauge Woes W123

Interesting as the oil pressure gauge is mechanical.....

I too would look at the grounds .
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  #5  
Old 07-03-2022, 04:01 PM
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Thanks guys,
The oil pressure gauge I thought strange as well,
Could I mount an earth from say the clock screws down to the earthing point or does it have to be from the PCB. I don’t really want to take the cluster apart.
Cheers
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  #6  
Old 07-03-2022, 04:09 PM
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A ground wire from a clock screw will work fine.
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Old 07-03-2022, 05:43 PM
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Oil pressure gauge is indeed mechanical. Nothing to do with grounding.

You sure it isn't at half (1.5 bar?) after a highway run? Short drives around town and it should be pegged to 3 or very close to it.
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Old 07-03-2022, 06:27 PM
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I haven’t as yet taken it for a drive, just idling in the driveway, I’ll go into town today and pick up bits to run another earth.
With the cost of fuel here at the moment Driving the diesel would be advantageous.
Here in Australia $2.25 a litre, that’s about $9.00 a gallon.
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  #9  
Old 07-03-2022, 07:06 PM
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Post Ground Wires

Be sure to use a BROWN wire to match the rest of the D.I.N. spec. ground wires .
It needn't be thicker than 18Ga. and be SURE to test the ground path on the body tub proper .

Use a TEST LIGHT not a gauge .
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1982 240D 407,XXX miles
Ignorance is the mother of suspicion and fear is the father

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  #10  
Old 07-06-2022, 08:27 AM
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Quote:
Originally Posted by vwnate1 View Post
Interesting as the oil pressure gauge is mechanical.....

I too would look at the grounds .
Yeah that is throwing me off too…
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  #11  
Old 07-06-2022, 11:09 AM
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Well, I know this, my 83 300D with 387K and change on the clock always pegs the oil pressure gauge at max on cold start. It usually stays at max during idle until the engine is warmed up, then it drops to about 1.75 to 2 bar at warm idle.

If you are really hung up about the gauge, you could always either swap in another mechanical type gauge as a test, or hook up an oil pressure gauge to the port on the engine and see what is going on.

It is possible that the gauge was sticking internally for some reason and would not go beyond halfway, and when it was removed it was jostled enough to free up the inner workings. A stretch yes, but still possible.
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  #12  
Old 07-06-2022, 10:12 PM
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When I bought a new steering Colum lock it came with a new ignition switch. And it needed a new ignition switch as I found the one of the contacts was actually burned off on one side and it was the arm itself conducting the electricity.
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Old 07-07-2022, 07:47 PM
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The oil pressure is just a nylon tube to a mechanical (Bourdon tube) gage in the dash. Worst it can do is leak oil. Normal (and good_ to max out after a cold start. Once warmed up and idling, mine read about half-scale and quickly go to max with a little rpm.

Temp gage should read ~82 C once warmed up, whether winter or summer though could creep up slightly under high load on a hot day. A T-stat doesn't regulate to an exact setpoint but rather requires a higher coolant temperature to open more. Engineers term that "proportional droop" in control system theory.

Your engine could actually be overheating and the coolant in the radiator feel cool if the flow is blocked by say a stuck T-stat. But your fuel gage is also reading falsely high, so likely an electrical thing. In 1984 cars, the "boot-up" current for the alternator field generation (google "Motorola alternator", in older GM cars too) is sourced not just from the "BAT warning" lamp, but from several other lamps. Check your wiring diagram. Perhaps if that bulb dies, the current takes another path which messes up the other gage readings. I've never tried that. Anyway, verify that bulb lights when you power up and engine off, then turns off when the engine is running. I know a little of it since I used an LED bulb there which doesn't provide sufficient boot-up current but if I rev the engine slightly the lamp goes out, either from residual magnetism in the alt rotor or current from the other lamps.

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