horrible

Everything works below 60 mph!

Resume,accelerate, all of them work up to 60 mph!

It used to be only up to 40mph but now it's 60 mph.

Any ideas??

SRHsl600

I have no clear idea, but....
Wheel sensor??? One of the safety systems involves the wheel rotation sensor that will shut the cruise control off if the wheel mismatch is out by several %. ( don't recall the value, but the tire rotation valves are just a few % )
In the past staggered wheels with wrong size tires could cause the error, maybe you are using up the rubber and the tires are getting closer to the matched set???
Hope this is not too misleading.
Welcome back to USA, but where?

horrible

Front tires are: 245/40ZR 18

Rear tires are: 245/45ZR 18

SRHsl600

That is the problem! the profile difference, 40 vs 45 with the same width. The back tires have a larger diameter. I have not done the math, but tire rack has all the numbers, my guess is if you want the staggered system ( did not work on my 500 1992 nor the 600 , because of front wheel suspension boot clearance I could not go bigger than 17" and 245/45, and I wanted a set that could rotate ) the back tire width would need to be 255/

SRHsl600

looked up tire rack specs for the 18' sport package the Front is 245/40 the back however is 275/35 The wheel sensor is doing its job.
good luck

horrible

I still haven't tried the tires. i just hate to buy new rear tires just to see if the cruise works.
I guess I could look for used ones.
It's funny though. I've called German independent shops and MB dealers.
Some say they never heard of tire size effecting the Cruise Control, others say it does cause
a problem so I guess I'll try the tires and see what happens.

Thanks for the reply's

cliffyk

According to the shop manual's functional description, the cruise control uses speed information supplied by the ABS module--for both the left front and left rear wheels--for "directional stability". Should the speed differential of the two exceed a specified value (3 kph at speeds below 150 kph, 5 kph at higher speeds for more than 0.5 seconds) the CC is disabled until the situation is corrected.



Putting this into English measure so that I can wrap my head around it those limits are 1.86 mph for speeds less than 93.2 mph, and 3.10 mph for higher speeds

Next let's look at the size difference of the OP's tires.

245/40-18 = 25.72 in dia or 80.79 in, 6.73 ft, circumference (the stock tire size)
245/45-18 = 26.68 in dia or 83.82 in, 6.99 ft circumference

Now as the acceptable differential is specified in units of velocity (kph/mph) we need to assume that the ABS unit is using some standard value to convert wheel rpm to velocity¹. I would put forth that is likely the dimensions of the stock tires; 6.73 ft circumference for the front (245/40-18) and 6.70 for the rear (275/35-18). I will use just the front tire circumference (6.73 ft) to simplify things a bit.

So with the OP's tires at 60 mph (5,280 fpm) the front tire will be spinning at 5280 / 6.73 = 784.5 rpm. The ABS unit senses this speed and converts it to a vehicle speed of 784.5 rpm * 6.73 ft (front tire circumference) = 5,279.7 fpm or 59.99 mph.

The larger circumference rear tire however is only spinning 5280 / 6.99 = 755.37 rpm, which the ABS interprets as 755.37 * 6.73 = 5083.6 fpm or 57.78 mph.

This makes the CC module logic believe there is a 2.21 mph difference in wheel speeds--just a bit more than the 1.86 mph limit, and close enough/well within what one could expect from automotive instrumentation. It disables the CC until the vehicle slows, making around 60 mph the practical CC limit for the OP's car with the different sized tires...

I needed something to wake me up this morning...

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¹ - It could be that the manual's specification of the differential limits in vehicle speed are just there for human consumption and that the CC module does not actually perform the conversions I used above.

horrible

I knew that! Duh!

Just joking. Thanks very much for the math. I was terrible in that subject.

It makes sense to me. I just wasn't aware the module was that sensitive.

Thanks again!

horrible

Cliffyk,
You were correct! I was skeptical! Especially when some technicians with over 25 years of
MB Experience told me they never heard of tire size causing a problem with the cruise control.

So, I installed the the same size tires on the rear as the front and eureka, I have a cruise control that works the way it should. Thanks again Cliffyk and all who said that was the problem.

cliffyk

Way cool! I like it when theory and reality come together...