'85 300SD Cruise works perfectly until speed exceeds ~70mph...w126 - PeachParts Mercedes-Benz Forum

eri69k · #1 09-14-2007, 11:23 PM

Hello everyone.. My 1985 w126 300sd has an interesting and often quite-frustrating

issue: new cruise control amplifier (http://www.adsitco.com/) recently installed, which remedied the completely non-operational cruise control system. I'm extremely pleased with the accuracy which this system maintains speed (+- 0.5mph verified by GPS) but once my speed exceeds approximately 70 mph the cruise fails intermittently. Very rarely does this occur on flat pavement; usually right at the bottom of a hill as I start to climb. The cruise control ALWAYS comes back on-line with a quick flick of the 'resume' on stalk but repeatedly cuts out (sometimes within 2 seconds, sometimes more than 30 sec) while climbing the hill. Never cuts out downhill, flat ground, or at less than 68-70mph.

I've verified proper operation (using cruise control testing procedure found here on MBshop.com) of cruise stalk, cruise actuator (operates smoothly with no rattling, grinding, internal gears in good shape and nicely lubricated and hold-in solenoid actuates effectively), speed sensor operation (measuring voltage, not with o-scope, up to approximately 45-50mph while on-jack stands in driveway) amplifier wiring/voltage, grounds, Osram taillights, linkage adjustment.... I'm not sure what to suspect: speed sensor is supposed to be pretty reliable and actuator seems to check out okay...I just hate to go spending mucho dinero for a new 'fill-in-the-blank(potentially broken part)' if it's not needed.

Has anyone else experienced symptoms I've described? Has anyone arrived at a resolution?

Please forward any and all info you may have.

thanks very much in advance,
Erik d.

dieseldiehard · #2 09-15-2007, 12:26 AM

From the description you provided (very accurate BTW) I suspect an electronic problem in the CC module. It sounds like the uphill load is causing the rpm to drop faster than the engine speed servo can be forced to correct, in other words an electrical limit is reached and there is a trip point or safeguard that tells it to drop the cruise to "off". I would first want to swap another module, than if that does not fix it I would swap another speedometer into the instrument cluster, that is where the road speed is generated by a rotating magnet, at least in the cruise situations I have worked on. I have never heard of this problem before. I send modules to General Development Labs for repair and they can, if you request, add an adjustable potentiometer to further fine tune the response and prevent surges or drifting. They might even test your module and advise. http://gdl-online.com/begin.html
The system you have is overall working so its not that anything is "dead" and you are better off than having a completely non functional cruise. I have used cruise at freeway speeds and I find it a great way to move your legs around, a welcome thing on long trips but I've never had any downhill dips at speeds like that and I usually kick it off manually when I am doing over about 75 MPH. When my cruise worked on the '87 300D I set it at 100 MPH one day and it held fine

I have seen CC drop out when I passed under high tension lines, the 150+ kV ones, and attribute that to poor shielding. MB had a note about using cruise in police cars (what, Mercedes police cars? not around here!) anyway it said something about rf getting into the module and it would kick off, at least the default is to off, it would have been a nightmare if they turned "on" with some passing police car making a radio transmission

eri69k · #3 09-15-2007, 07:51 PM

thank you for your prompt response!

I purchased the cruise unit with their standard warranty from Adsitco but I'm not entirely sure of how long it's for...likely 30-days. The first module I swapped in didn't work at all for some reason, I contacted Adsitco and they had a new module at my house the following day. Excellent customer service!

I will likely contact them again Monday to explain the

situation

and to see about any possible warranty

or if they've ever heard about this condition.

thanks again!
Erik d.

eri69k · #4 09-18-2007, 11:18 PM

has ANYONE else every heard of this issue? PPPPUUUUHHLLLEEEEZZZEEE!?!?!?!

Adsitco states it's not likely the amplifier, more likely the speed sensor. The only way to test this is with an oscilloscope, which just so happens to be absent from my list of desirable 'Tools to Purchase'...

I've tested the speed sensor with voltmeter but only up to approximately 30-35mph, while on jack-stands.

All appears well but I absolutely HATE 'throwing parts at a problem' without knowing its actually proven to be defective.

Jeremy5848 · #5 09-20-2007, 01:41 AM

In the actuator (the mechanical assembly, mounted on the engine, that moves the throttle) there is a form of rheostat or potentiometer. It is used to feed back to the electronic amplifier (the one you just bought) throttle position information. If the potentiometer gets dirty or damaged, the position signal is lost and the system turns off. The dirty part is often in a speed area that the PO never used -- if your car was owned by someone who never went over 65 MPH, for example, you might expect the symptoms you're seeing.

If this is your problem, you may be able to fix it by taking the actuator apart and carefully cleaning the contact surfaces. There's a DIY somewhere -- did Diesel Giant do it, maybe?

Jeremy

Brian Carlton · #6 09-20-2007, 11:27 AM

Quote:

Originally Posted by eri69k

has ANYONE else every heard of this issue? PPPPUUUUHHLLLEEEEZZZEEE!?!?!?!

Adsitco states it's not likely the amplifier, more likely the speed sensor. The only way to test this is with an oscilloscope, which just so happens to be absent from my list of desirable 'Tools to Purchase'...

I've tested the speed sensor with voltmeter but only up to approximately 30-35mph, while on jack-stands.

All appears well but I absolutely HATE 'throwing parts at a problem' without knowing its actually proven to be defective.

Adsitco has a reputation for poor quality products and virtually no warrantee. Dealing with them is difficult........at best.

My suggestion is to send the amplifier to GDL who will test it for you at minimal cost. Do it quickly or you'll never get a refund from Adsitco. In fact, you'll never get a refund........period. The best you'll get is another amplifier.

The actuator is not your issue.

eri69k · #7 09-20-2007, 08:45 PM

The cruise hasn't always not worked at 70MPH... It did work when I first installed the amplifier (for maybe 2 or 3 wk). While driving today I had it cut out at 69mph, resetting instantly (by 'resuming' with the selector arm), accelerates fine with the 'accel' function, holds fine on level ground, but right at the bottom of the hill during the transition from downhill (or level) road to uphill or just slightly up the hill it cuts out every time.

I personally don't think it's speed sensor related... Maybe the higher-current draw which occurs during the transition from light throttle to accelerating is causing this? How come not during uphill sections below this speed (which requires identical or greater amounts of throttle input, often being completely 'floored') without failure. This is the reason I'm stumped.

eri69k · #8 09-21-2007, 11:28 AM

Contacted Adsit Co. again and they're sending me another amplifier. The guy who answered my call was helpful but stated the obvious: "We don't have any mechanics here...we just sell the parts..." Hopefully this fixes my issue.

Will keep you guys (and gals?) updated.

thanks again!

eri69k · #9 10-01-2007, 09:58 PM

Alright.... replaced the amp with 'freshly re-manufactured amp' from Adsitco and symptoms exactly the same. Amp is not the issue here as this is my THIRD amplifier. Any other suggestions?

thank you VERY much!
Erik d.

300sdToronto · #10 10-02-2007, 03:40 PM

Well, I just replaced my cruise amp with a used one from a pick and pull, and have some similarly challenging but different symptoms.

I now have cruise that works - but not all the time! If I change mode on the climate control unit, cruise is disengaged about 50% of the time. I know this sounds absolutely crazy but there you go. I need a passenger with me to take notes, but it looks like a switch from heat to cool or vice versa is disengaging cruise for me.

I suspect I have a frayed wire or something similar behind the dash, but will need to spend time on the circuit diagrams to try and figure this one out.

Anyone had a similar mind bendingrpoblem?

barry123400 · #11 10-02-2007, 06:13 PM

I know very little about mercedes cruise contols. Virtually nothing in fact. It does appear that their cruise controls are not good at dealing with electrical noise from a couple of descriptions here.
How about disabling your alternator output temporarily by removing a wire. Then run car up over seventy and check cruise function.
When I was doing a little electrical work on a 123 I found the ground loops where loaded with alternator noise. It was affecting meter readings. There was even a square pulse efect at idle. I was on the verge of getting the scope out to have a look but did not. There was no point. That may have been the voltage regulator. Maybe this is normal. As rpms increase it is going to be worse with different frequencies generated as well. There is an external capactor on the alternator to deal with some of this. Since the car exhibited no bugs I never pursued it. This amount of noise would not hurt anything simply of a resistive load.
That cruise amplifier is a little more complex though and is an active electronic device. I would not spend any amount of time with this unless neutralising the alternator stopped the problem. Much of this depends on what the speed sensor is generating. A pulse or a variable voltage. I do not have a schematic to examine but if pulse excess alternator noise could get it. Talk about a long shot. I certainly would not take this to the bank. On the otherhand it is so easy to temporarily disable that alternator.

eri69k · #12 10-03-2007, 05:49 PM

excellent idea... Will try disabling the alternator as that of which you speak makes perfect sense.

Will update this thread with results but possibly not until this coming weekend when I have time. The closest ummmm... 'testing area' where speeding tickets will not be of concern is approximately 20-25mi away. Today while on I-40 I had the cruise working for a good while up,down, in-between hills at a GPS-verified exactly 80mph.... until some wonderfully 'nice' individual in a Volvo cut in front of me going approximately 27mph less that I was....

I got it to work again for a short time before traffic got the best of me. The electrical 'noise' theory seems to make sense in this instance as increased RPM possibly made it go away. I have not had the 'opportunity' to test it at speeds over the 80mph range but if an autobahn opens nearby.... let me know.

Brian Carlton · #13 10-03-2007, 09:20 PM

I've been all over the map with the cruise amps. Sometimes they work.........sometimes they don't...........sometimes they work when the climate control is on...........sometimes they don't.

Buy one from a junkyard.........or online..........and they work for six months.

I'm done.

$215. and you get one that will work for three years:

www.gdl-online.com

hangit · #14 10-03-2007, 09:32 PM

Ditto what Brian advised. Go with GDL. Peter may not have the fastest turn around time, but he is very knowledgeable and his repairs work. My 240D cruise works great now!

John

#15 10-03-2007, 10:30 PM

Quote:

Originally Posted by hangit

Ditto what Brian advised. Go with GDL. Peter may not have the fastest turn around time, but he is very knowledgeable and his repairs work. My 240D cruise works great now!

John

Same here, I just replaced both my actuator and amp from GDL. System works perfectly now.