Reader Question :

Dear sir,

I would like your honest opinion as in the past your suggestions turned valuable to repair my old Chevy. Yesterday I bought 2001 Mercedes-Benz ML320 and as soon as I drove it on the road I got the Yellow Engine Check light ON. Once it did that I could not accelerate the car faster because the rpm would not switch the gear Even though my gear was on D – Drive.

I took the car for the electronic diagnosis and the technician told me that he stored the transmission code but he said your car showed some thing know as Sporadically Rpm sensor n3y3/611 is faulty and advised me to replace them at a mechanic place.  The rmp sensor goes into a loop.

After this check the Engine Check Light does not show any more. I think the technician did update or reset the computer which is why the Engine check light is not coming up again. My question is shall I drive it until the Engine light comes ON again or Shall I take it to repair center and replace the rpm sensor.

The dealer who I bought this car from yesterday told me to just drive it until I get the check engine light comes ON again.

Regards, Habib

Hello there Habib,

My honest opinion would be to take this vehicle back and get a refund immediately.  I would never keep a vehicle that showed signs of trouble on the way home after the sale.

A 9 year old Mercedes……you are just asking to spend lots of money on regular maintenance let alone un-forseen problems like this.

How many 9-10 year old Mercedes do you see on the road on a daily basis vs 1-3 year old Mercedes?

Mercedes is a great vehicle, but once they start to show problems you are better off getting rid of them than dumping tons of cash into them.  I would suspect this vehicle was sold because of this strange problem no one would figure out.

You are going to need to see a Mercedes mechanic who knows these cars inside and out, I am not one of those mechanics.

It does sound like a problem with the VSS (vehicle speed sensor) but that is just a wild guess.

I would NOT guess with this, the parts are too expensive. So spend the extra money and go to the dealer or find a Mercedes specific repair shop.

Maybe, if the dealership wont take the vehicle back for a full refund, they will pay the repair bill.

You can probably get your money back via the lemon law since you had an issue immediately after the sale.

I would much rather see you get out of this vehicle and into a Lexus, which is a much more reliable and economically viable maintenance wise.

Sorry to have burst your bubble.

Austin Davis

Brake Booster and Master Cylinder — The brake booster and master cylinder is the heart of the brake system. Brake fluid is added to the cylinder and the booster is the vacuum source. When you step on the brake pedal inside the vehicle, vacuum from the brake booster forces the brake fluid inside the master cylinder to each wheel. The booster gets its vacuum from the engine via a rubber hose. The booster is what gives you the “power brakes”. Without the booster you will have to manually force the fluid to each wheel, this can take a lot of foot pressure.

Serpentine Fan Belts — Cars these days usually only have one or two very large belts that drive multiple accessories…i.e. power steering pump, a/c compressor etc. These belts are very long and very strong. The rule of thumb is if you can see two or more cracks within an inch of belt space you should replace the belt. When inspecting your fan belt you should do so on the back side of a pulley where the belt is being laid back. As you can see (or not see) there are tons of cracks in this belt when inspected from the back side of a pulley but you can hardly see any visible cracks on the same belt to the left of the picture when it is not bent backwards.

Fuel sending unit — This complete fuel sending unit is submerged inside the fuel tank. The sending unit and float is what gives the dash fuel gauge the proper information as to fuel level in the tank. The fuel pump is connected to the sending unit but can be replaced separately in most cases. To test the fuel pump and the sending unit the wires at the top of the fuel tank must be checked for power and ground.

Starter — On a rear wheel drive car or truck the starter is usually located under the car about the area of the driver’s feet. A front wheel drive usually locates the starter under the hood with the engine. The battery supplies electricity to the starter to engage and spin over the engine. Once the ignition key is turned to the start position the starter drive gear is pushed out of the starter housing and the gear makes contact with and turns the flywheel inside the engine.

Reader Question: I wanted to get a tune-up on my car and I called around to get some prices. The dealer quoted me 4 hours of labor to do the job. Another mechanic told me they charge a flat rate fee unlike the dealers. What is the difference? The mechanic that I chose only charged me for two hours, not four.”
Thank you,
Tom P.

Dear Tom,

Tom, you can easily see that when you call around for an estimate you can get some pretty dramatic price differences. Most larger shops and all dealers that I am aware of will charge an hourly price for each job that is listed in a shop hourly manual. For example your car tune up might have an estimated repair or replacement time of four hours in the shop manual. This time is calculated using the shop hourly rate {i.e., 4 x $60}. Pretty straight forward right? Not all the time.

So what do you get for the four hours of labor? If you call my shop and ask for a car tune up price we automatically figure in the price a fuel filter, spark plugs, air filter, PCV filter, and distributor ignition rotor plus all applicable labor charges. So my car tune up price would probably be much higher than a shop that only replaces spark plugs as their auto tune up package. You need to make sure you are comparing apples to apples when you are calling around for prices.

Some shops that I know in my area purposely give low ball telephone estimates just to get you in the door. Example: You call to get a price for a new radiator installed. Does the estimate include new antifreeze, a new radiator cap and taxes? I would automatically figure these items in to my estimate, not only because I want to do a nice turn key job, but also I don’t want to have to sell you parts later on that I already know you will need. For this reason, my phone estimates can be higher than some of my competitors, but your final bill might be lower.

There are also different brands of shop manuals on the market, so the GM dealer would be using a different labor estimating guide than my shop and most independent shops would be using. Not only would these guides possibly give different replacement times, but they could also classify an auto tune up differently like in my example above.

So the shop that Tom found only charged him for the time they actually spent working on the car? So is that a better deal? Maybe not, if all they did was replace spark plugs and call it a car tune-up. Just for grins Tom, call the dealer back and ask them how much they charge just for spark plugs. What if this shop has a slower mechanic, or deliberately takes longer to do the work just to rack up more time on the clock?

On a different note, as a shop owner I would want to know why you are requesting a car tune up in the first place. What I have found is that if the car runs poorly or differently than normal, the customer often automatically asks for a car tune up. Cars of today are very sophisticated and full of computers and sensors that can cause your car to act strangely. So a tune up might not fix the problem, and a proper diagnosis from the mechanic could be beneficial.