heat gauage 300sd

[bill threlkel]

I have been haveing a problem with the heat gauage in my 83 300sd. It will make lighting fast moves from normal to hot stick there for a second the drop down to normal. I replaced the sending unit still with no change. Also the gauage reads always hoter than normal. Can i put a mechanical gauage in this car? If so what is the thread size where the sending unit is installed is it a metric size?
Bill

I have the same issue - my aux fan does not ever activate - does yours. I can operate @ 70 mph for 2 mins in 20 F weather gauge shows 100+ deg F.

jeff

[jbaj007]

Could be intermittent short/open in sending wire.

Thead size where sending unit installs in oil filter housing is 12 mm. X 1.5 thread pitch. Metric adapters that fit this size are available for mechanical gauges. They are soft brass so don't over torque them. I broke one off in the housing yesterday while working on a home made NO/LOW oil pressure light/buzzer setup. I felt well blessed when I was able to "twirl" out the broken threads from the housing.

[kerry]

jbaj:
Why don't you start a thread to explain your no oil pressure buzzer set up. I'd like to put one on my vehicle.

[bill threlkel]

Hamgunner,Jbaj007
My aux fan does work i guess i will check the wire as Jbjoo7 said could be that if not then i will see if i can find those brass adaptors and put in a mechinal gauage. Also that low presser bizzer sounds like a good idea hows it installed?
Thanks guys
Bill

[ned2683]

Originally posted by kerry edwards
jbaj:
Why don't you start a thread to explain your no oil pressure buzzer set up. I'd like to put one on my vehicle.

yes please. my 116 died because of no oil pressure while i was on the high way. i'd like to know how to make a warning buzzer on my new car too.

[jbaj007]

NO/LOW oil pressure warning system on 300SD 1984. This is what I did, you may have another way.

Usual cleanliness, safety warnings apply.
If you do this wrong, your oil WILL run out of your engine and ruin your day!!!! Check all joints and connections on the bench FIRST!!!
Obviously, YOU need to verify ALL before proceeding. I hope this helps, but I can't be responsible. (yada, yada yada, lawyer, yada, atty., yada, yada).

Parts:
Radio Shack 12v. buzzer (cheap)
Radio Shack red LED (with built in resistor)
12mm X 1.5 male X 1/8" NPT female adapter (steel)
1/8" NPT street "T" (has a male with two females)(brass)
1/8" male X 1/4" female adapter(brass)
mid '80's Audi or VW (water cooled) oil pressure sender for idiot light) (~6 psi. ?) (cheap)
Tools:
1/8" NPT tap
1/8" NPT die
12mm.X1.5 tap

Try here for fittings http://www.fittingsandadapters.com/

If you have access to machining tools or can find other appropriate METRIC fittings (I looked everywhere for other metric fittigs and found nothing that fit this purpose) you can do this much more easily. A metric street "T" would make this nice, but it eluded me. Maybe someone has a source?

1. Remove the oil pressure sending unit from your car with a pan underneath. You will drip out ~ 1/2 pint of oil, then it will stop.

2. Braze (I soldered with a torch and plumber's solder) into the threads (small thickness of braze or solder) of the female side of the 1/8 male X 1/4 female adapter. Now retap that side with the 12mm. X 1.5 tap. You'll be surprised how close they were already. This is where your original Mercedes oil sender will eventually go, so be attentive.

3. Tap and die all 1/8" NPT threads in the "T" and adapters with the 1/8" NPT tap and/or die to clean them up and deepen the amount of "bite" for security. You'll see what I mean when you dry fit them.

4. The Audi/VW sender has a 10mm thread. Take your 1/8" die and run it over that. Yes,... rethread the 10mm. on the Audi sender and make it 1/8" NPT. They are SOOO close to start, after you run the 1/8" NPT die over it, they match quite well.

5. Assembly:
a. 12mm. X 1.5 male X 1/8 NPT female adapter goes into the oil filter housing (use an aluminum crush washer)
b. male of the street "T" goes into female of that adapter.
c. Audi/VW sender (rethreaded to 1/8" NPT) goes into the side branch of the "T".
d. 1/8" male X 1/4" female (brazed and retapped to 12mm. X 1.5 thread pitch) adapter goes into end of the "T"
e. Original Mercedes sender goes into the retapped end of that adapter (use an aluminum crush washer). Reconnect original wire.
f. Put a new wire (with spade connector) to the connector on the VW sender and run it to the buzzer. Connect other side of buzzer to a "start/on" hot lead. Connect the LED in parallel with the buzzer and place buzzer and LED where you want to.

I have all of the hardware done on my car and am running around to check and recheck stability and for any leaks. I have not hooked up the buzzer yet, since I have a few things to do at the same time to my instrument cluster. (my modified gas engine inst. panel with economy gauge as a boost gauge for the turbodiesel needs calibrating). The buzzer may get annoying since I expect it to buzz whenever the key is on without the engine running(oil pressure less than 6 psi). We will see if I go with only the light long term. Since 6psi is the same as zero, as far as I'm concerned, it should work to warn of a pesky ruptured oil cooler hose, broken filter canister bolt, road rage bullet through oil cooler (this is SoCal after all) or what not.

FWIW that's what I've done.

Edit: Hooked it all up with 0.3 bar VW switch and it works perfectly. LED right in between gauges and buzzer is unmistakeable.

[Aaron]

Generally, it's the actual gauge cluster which is at fault. It happens with both fuel and temp gauges in the 1980s cars. Make sure though, that the needle isn't shooting to 120 because of a stuck thermostat. Does it do this the first time you start the car or all the time?

[kerry]

On what vehicle did you add the low pressure warning circuit? I am under the impression that all the 616 and 617 engines had mechanical oil pressure gauges so that the process of adding one to those engines would be quite different.
Is it possible to T into the feeder line to the oil gauge and add a buzzer via that fitting? Does anyone know a source for a T that would work in that line?

[bill threlkel]

Aaron
The car will come up to normal tempture then slowly rise a little above normal tempture then spike to just below the hot red on the gauage, stick there for a milla second then drop to normal.
I saw this happen about ten times on a 650 mile trip last summer. At the start of this i stop and checked the car over and found nothing wrong. No boling of radiator to the touch radiator was normal in heat. No tell tale signs of an over heated engine.
Bill

[84300DT]

did you try the 'james bond' tap on the gauge trick? mine sometimes spikes a bit and tapping the left side of the dash moves the needle to the 'real' reading.