83 300sd RPM's guage jumps occasionally - PeachParts Mercedes-Benz Forum

Greg300sd · #1 10-27-2004, 10:05 PM

First off, great message board with tons of knowledge. I've used several tips here already to get this 300sd running better. Lately I've noticed that the RPM guage will register a higher rpm than is actually being used. I know there is an rpm sensor under the hood somewhere. I also noticed that the problem can be repeated by....(are you ready for this?)...beeping the horn. It makes the rpm guage jump almost everytime! I don't know if it is related but the battery light glows a very dim red occasionally at idle. Don't know if the two are related. I'm thinking maybe a grounding problem? Anywho....thoughts or suggestions welcome. Thanks in advance.

boneheaddoctor · #2 10-27-2004, 10:32 PM

Low votage can cause a LOT of problems, check your grounds too. Fix that and I think your problem will go away. if Belts are tight check voltages at idle and above. you could have a regulator on its way out or the entire alternator. Check for corrosion on the grounds and battery cables and posts.

Try a Jumper cable between ground post and engine, check voltages, put the other between the motor and the chassis. If that fixes the problem you need to find what cables are broken or corroded, or its gonna be the alternator.

Greg300sd · #3 10-28-2004, 09:09 PM

Thanks...I'll try your suggestions and see how it goes.

#4 10-29-2004, 03:04 AM

Definitely check those things first
My tach used to bounce around/not show anything all the time
As it turns out, I was at the wrecking yard one day and saw the magical screw on tach thingy on the fender of a SD that caught on fire (inside)
I grabbed the part and threw it in the bag (no charge) just to have as a spare/sell
As it turns out, mine was bad...nice not having to buy that expensive part.
Screwed on the new one and works like a charm.

Sounds like a power problem though..
check the plugs on the back of the cluster too...

boneheaddoctor · #5 10-29-2004, 07:55 AM

THat is my second guess, if it still does it when he gets the charging system straight. It was showing a discharge at idle and indications it could be a grounding issue. Easiest thing for him was to disprove grounding issues using a set of jumper cables to ground battery to block and block to chassis.

If his voltages are dropping low due to a alternator/regulator problem, any sort of symptoms are possible.

If he had good voltages at idle and above, and grounding issues didn't exist then you are correct in assuming the black cylindrical item on the front fender about the size of a large pill bottle could be a problem. Or the pickup that runs from that to the front harmonic balancer area.

Greg300sd · #6 10-29-2004, 09:14 PM

I'm going to get to work on the suggestions posted tomorrow. I did forget to add ( if it helps sway this one way or another ) that the battery light glows very dim at idle and as soon as I increase the engine speed the light goes out. I also saw a post about a battery ground cable that runs from the battery to the chassis. I don't see that on my car

but I do see a wire that has been cut or broken only about an inch long extending out from underneath the negative battery post. Maybe the clues are starting to add up. Thanks.

boneheaddoctor · #7 10-29-2004, 09:36 PM

If its that my idea will prove it out. Black jumper to engine block and neg post of battery, take the red one and hook to chassis and engine. If its a broken or missing ground it will clear up. Use a DVM to see what the charging voltage is too. You may have a weak alternator, or just the regulator.