240D Cruise Not Holding

[Redpeople]

On my new-to-me '78 240D, my cruise control doesn't work. No big surprise. I originally thought it didn't work at all, but discovered that it does come on, and when I hold it in the "accel" position, the cruise accelerates. However, as soon as I let go, the speed drops.

Any ideas? Any tests I should do?

Thanks.

[mach4]

Classic....

You can try the DIY fix of reflowing all the solder joints on the cruise control amp board and replacing the capacitors...or you can just send it to JamesDean for a very reasonable and quality repair

Cruise Control Amplifier Diagnostics & Amplifier Repair

[toomany MBZ]

I believe that is a servo cruise control unit, the amp number is 005 545 12 32.

[turkishcoffee]

Check your vacuum pressure first. I had the same symptoms from a slowly failing vacuum pump when I bought my car.

[JamesDean]

Hi All,

This is most likely a problem with the amplifier. A lot of the vacuum units I test have this failure mode.

001 545 21 32 is most likely the part number on this unit.

The 005 545 12 32 units didnt come out until the mid 80s so unless the unit was replaced its likely the 21 32.

The gas powered cars had 001 545 06 32, 001 545 20 32.

Manual transmission cars had 001 545 70 32.

Redpeople, if you are interested in having the amp tested/repaired, just drop me an email at JamesDean59@gmail.com. I'm happy to help.

-Kris

[Redpeople]

Thanks, Kris. Will do. Might be a bit, as I have a few more things to sort out first.

[Alastair]

The Vac system, and the cable from the Vac actuator as well as its adjustment are all pretty important on the vacuum operated type cruise system

Inside the Cruise ECU (Seems US terminology call it 'Amplifier') There's a couple of things Ive had faulty with these 001 545 21 32 units.

There's two 47uF 25V Electrolytics that can go--There's only two of these in there--Just replace 'em both.

Dry Joints of the soldering can cause troubles as well.

Another thing is an Electrically Leaky Reed-Relay. Now--This part can cause a weird fault, where you set the speed, and it'll very slowly decrease over some minutes....

Depending on how bad the electrical leak is, how fast the speed will fall from what it was set.
--The leak is actually many meg-ohms and difficult to read--as there's the set-speed cap connected to that point...

It appears (with the few Ive done) that the epoxy of the reed-relay is what develops the leakage resistance...

Problem is--Its a dual reed-relay using a single coil. At the time I repaired my own, I used two single reed-relays and strapped the coils in parallel.

For interest--Here's how this type works--Roughly.

The controller/ECU/Amplifier (whatever you wanna call it) is in fact a Comparator and Driver for the Vacuum solenoid device.

The controller system has a tacho-generator mounted on back of the speedo--(Coil and a magnetic pick-up) that generates say a 300Hz signal at we'll say, 50MPH.

This is compared to a similar signal thats generated Inside the controller by a Voltage-Controlled Oscillator, VCO.

This section has its reference Voltage Stored on a polypropylene low-leakage capacitor of 0.01uF

The amount of voltage on this cap sets the oscillator frequency and this is compared to that of the speedo tacho-gen signal.

When the Tacho-gen signal frequency is low in comparison to the VCO signal the comparator section of controller outputs a higher control-voltage to the vacuum-solenoid driver which then increases the amount of vacuum applied to the diaphragm--that pulls on the throttle-linkage to increase speed, that increases the Tacho gen frequency till its the same as the VCO signal--or thereabouts.

The set-speed capacitor is attached to a specialised (and now nearly unobtainium, and Very Easily damaged by Static) MOSFET, that buffers its voltage to the VCO stage.

--As that set-speed cap is so small in value--ANY external leakage of power will discharge it, and this Node, (Cap, MOSFET and Reed-Relay connection) is actually mounted in mid-air above the board to avoid any leakage paths, and hence Reed-Relays to supply Into that cap a charging-voltage, (Accel-Set) or discharge Voltage, (Decel-Set) via highish value resistors and the stalk-switch to set the speed via the voltage on the cap.


There's a fair bit more to the system, but that's the basics on how it works...

Correctly working the system will maintain a set-speed within 2mph, at around 70mph....

[mach4]

Originally Posted by Alastair

...Correctly working the system will maintain a set-speed within 2mph, at around 70mph....

Will the 240D do 70mph?

[Alastair]

Dunno!

But my 300TD n/a will do 90 odd!

[Redpeople]

Originally Posted by mach4

Will the 240D do 70mph?

Yes, downhill, drafting off a semi.

[Shern]

Had this issue recently.

I've got the vacuum actuated cruise control and a recently rebuilt CCU.
I set the speed at 70mph and over the course of 10 mins or so, it'll drift down to about 66.
I use the stalk to bring it back up to speed and same thing again.

Anyone else?
I realize the vacuum cruise units are sort of rare....