Topic: Cruise control not keeping set speed

[ptashek]

The cruise control in my 450 has some existential issues. Whenever I set the speed and take my foot of the accelerator, cruise control takes over but is unable to maintain the set speed over any reasonable distance. Basically, the car will slow down something like 5mph every mile. What might be the cause here (vacuum leak?)?

[jbrasile]

ptashek,

A vacuum leak is a possibility but usually the control unit is what goes bad. Let me guess, it will accelerate fine when you move the lever up but will not hold speed correct? This is typical behavior of a defective ECU.

Tks,

Joe

[ptashek]

ptashek,

A vacuum leak is a possibility but usually the control unit is what goes bad. Let me guess, it will accelerate fine when you move the lever up but will not hold speed correct? This is typical behavior of a defective ECU.

Yes, that's exactly what happens.
I have one spare from my 350, will it work with a 450?

[jbrasile]

Yes the 350 ECU should work without any problems.

MT is right, but I just don't have the skills... hehehe, so for the 1980 450SEL I got an exchange unit many years ago from a supplier that used to advertise on Hemmings Motor News, can't remember their name but the system is still working perfectly. In the 78 car I may have a vacuum leak, just waiting for a replacement actuator to arrive from LA.

Tks,

Joe

[ptashek]

That's great news. I have no problems with electronics, and there probably isn't anything surface soldered inside those ECUs considering their age. That should make things easy.
I'll give that 350 unit a try, and if that doesn't help I'll rebuild both. Heck, while I'm at it I might ask some electronics-ninja if the unit could be emulated with something more modern.

[oversize]

And this is exactly why I won't buy a later MB unless it's under 10YO!!!  How much is a replacement ECU on a W140 anyway??  $1500...  $2K....  $3500??   

[ptashek]

Right, so I've hooked up the spare unit and it's an even bigger dud then the one I already have. It's proper dead.
I've taken a pic of each side of the PCB, layered the two in Photoshop with a few small tweaks and am working on reverse engineering the thing as far as the schematic goes.
I wonder if this whole pile of junk could be replaced with a MC14460 IC (and if it is still available). It's an almost complete cruise control unit in a single chip.  Looking at its datasheet (PDF), it seems doable at least in theory.

Here's the board from my 350SE:

[navigator]

Where does does this ECU live?

I have same problem with my 350

[ptashek]

Where does does this ECU live?

I have same problem with my 350

It lives just beside the steering column under the dash, at least on RHD models. I guess it's mirrored in LHD. Getting to it is not much of a problem - just unclip the panels. Getting it out is a bit trickier, as there's very little space to get your hands and a wrench in.

[navigator]

Thanks!!

[ptashek]

This is one of the best cruise control finds thus far...
The whole thing described properly: http://userweb.windwireless.net/~jimc/vdocc1/vdocc1.html
...and troubleshooting steps: http://userweb.windwireless.net/~jimc/mamerepairs.html#cruisetest

[ptashek]

OK, so I've finally replaced all the caps on my board, now it keeps the speed but surges a lot.
I guess next step is to re-solder all the joints?

One new find - the Motorola quad-comparator chip next to the Siemens TAA765A op-amp was replaced at some stage, between March 1977 and June 1979 (dates on my modules), with a LM2901N (datasheet [PDF]). While the Motorola is almost impossible to get, the LM2901N is readily available for a few cents.

[oversize]

It's it great that these things are repairable?  Love your work!   

[ptashek]

It's it great that these things are repairable?  Love your work!   

It is!

I haven't really done much, beside following existing advice on this and other sites, and sharing what I have found myself.
Compiling all the data, these are the things to check:

- cc amplifier - I'm quite sure mine is in good shape now
- throttle actuator - unlikely, as it does work (as in "pulls the throttle")
- throttle linkage - unlikely, it used to stick but has been well greased since and just works
- speedometer cable - if it is stretched or worn, the needle would wobble making the speed reading inaccurate (mine wobbles slightly at ~50mph)
- speed sensor - ties in directly to the above, as the sensor reads speed off of the cable and feeds it to the CC amp
- vacuum leaks - more than likely given that my central locking works for about 5 minutes after engine shutdown, then all the vacuum is gone

The surge is by about 5mph above set speed, e.g. if I set the CC to 60mph it'll go 60-65-60 all the time.

Are there any household methods to test for vacuum leaks?

[ptashek]

More updates from the CC repair front...

- throttle actuator - unlikely, as it does work (as in "pulls the throttle")
- throttle linkage - unlikely, it used to stick but has been well greased since and just works

I've taken the air filter box off today, and have cancelled almost all slack on the actuator linkage before running out of thread on the adjustment screw. The surging is still there, but far less, so a partial win on that front. I guess this is what I need to look more into. Does anybody know if the actuator cable is a replaceable part, or do I need to replace the whole unit? Other option is to cancel the slack by shortening the cable and re-terminating it.

- vacuum leaks - more than likely given that my central locking works for about 5 minutes after engine shutdown, then all the vacuum is gone

Need to get my hands on some vacuum testing equipment. This is still a likely cause.

I've also taken the car for a test drive on a motorway today, hit the CC stalk at ~60km/h and released it at 100km/h ("accel set"). The car kept accelerating up-to 120km/h, then just let go and did nothing until the speed dropped to ~90km/h, at which point it was trying to maintain that speed. Interesting. Why would it keep going after releasing the  CC stalk, and why would it store a speed ~10km/h less than actual? Could this point at invalid speed data supplied to the CC amp, and hence problems with the spedo cable or speed sensor?

I'm not giving up on this one

Below is a shot from my engine bay showing the location of the adjustment screw. Keep in mind this is a RHD model, so if you drive a LHD model element locations are likely mirrored. Click the image to see the full-size version.