Cruise Control 300E - PeachParts Mercedes-Benz Forum

JohnM. · #1 05-03-2010, 03:20 AM

When I first bought the car, cruise worked, but never really used it. Then I didn't use it for a LONG time, and tried it. It was constantly shifting up/down and not maintaining speed. I replaced the cruise control amplifier with one from another 1991 300E. Now it works, but there is an ever so slight rpm fluctuation up and down (50rpm) that you can feel when cruise is engaged. It just feels wrong! It works great otherwise if you can get past the weird pulsing of RPMs.

Does it sound like possible solder contact issues with the used replacement too? Just not bad enough to totally fubar the cruise system....?

macdrone · #2 05-03-2010, 09:11 AM

Throttle actuator may be an issue. Hard to tell really. Take aprt your old one and try and repair the joints and see.

lkchris · #3 05-03-2010, 11:58 AM

If your speedo cable is stiff and causes the speedo to fluctuate the cruise control will simply follow this.

macdrone · #4 05-04-2010, 02:02 AM

Oh good point. Not a common issue but I have heard of that.

ps2cho · #5 05-04-2010, 02:18 AM

I have the same issue on my 260E. CC engages and works, it just "revs" slightly over and over repeating. I will be following this thread!

Assuming we both have the same issue, I have removed my speedocable dozens of times so I'm sure that is not the problem.

Where is the CC Amplifer John? I want to resolder mine. Is it easy to get at?

ps2cho · #6 05-06-2010, 02:07 AM

bump...Anybody know if the "revving" could be anything OTHER than the CC amplifier?

Is the Amplifier under that bottom dash cover?

I might be driving 300+ miles this weekend out of state so if I could get the CC functioning properly before then that would be a dream come true!

JohnM. · #7 05-06-2010, 02:15 AM

Quote:

Originally Posted by ps2cho

Is the Amplifier under that bottom dash cover?

Yes. Pull the headlight knob, plastic surround piece. Then pull the plastic piece around the ignition cover. There are a half dozen or so 8mm fasteners that need to be removed to drop the under dash bolster.

Ivanerrol · #8 05-06-2010, 04:36 AM

Make sure this little roller arrowed in the image is not missing or worn out.
Make sure your tail lights and brake lights have the right wattage bulbs and you have no problem globe alarm when you turn on the lights.

samiam44 · #9 05-06-2010, 12:02 PM

Did you clean and lubricate the throtle linkage on the motor?? Should be done at any 30k service, but always check the easiest things FIRST.

Michael

ps2cho · #10 05-07-2010, 11:56 AM

I got the amplifier out...I see zero signs of bad joints at all..

I am taking the one out of my 300TE and swapping it in as I know it works. You can see it seems there was a model update between 87 and 88:

Left is part #003 545 09 32 Date: 26/87
Right is part #003 545 05 32 Date: 20/88

Plug fits, so we'll see if it works. Will report back later with results.

ps2cho · #11 05-07-2010, 11:41 PM

Just drove 360mi with Cruise Control

Looks like the Amplifier was the issue!

JohnM. · #12 05-08-2010, 12:02 AM

So, no more rev fluctuation with a different CC amp?

I think the solder joints have to be inspected under some level of magnification to see the bad joints.

ps2cho · #13 05-08-2010, 01:47 PM

Quote:

Originally Posted by JohnM.

So, no more rev fluctuation with a different CC amp?

I think the solder joints have to be inspected under some level of magnification to see the bad joints.

Nope. Dead solid like it should be. My best description of my issue when looking at the economy gauge was the throttle was "panting real fast" when CC engaged.

No more!

I'll probably just grab one from the junkyard. I looked at the MB documents and it states that the earlier model shown had a lot of issues, so if you are going to replace make sure it is the later one.

JohnM. · #14 05-08-2010, 01:58 PM

Sweet! I'll keep an eye out for another CC module at the junkyards. I don't have an economy gauge in my car, but I could definitely feel the subtle jerk back and forth in speed.