Background: After putting up with intermittently functioning cruise control for a couple of months after I bought my 300D, I purchased a remanufactured Programma amplifier, installed it and the next day enjoyed flawlessly functioning cruise control on the 500-mile drive from Houston to Tulsa. It engaged, disengaged, held perfect speed without any jerkiness, accelerated, decelerated, etc., all without any problems whatsoever. We stopped and turned the car off at least a couple of times for lunch, etc. and the cruise worked each time I restarted the car.

About an hour after I turned the car off in Tulsa I went outside, started it and took a short highway drive to a friend's house. Wanting to enjoy the pleasure that is fully functional cruise control, I tried to engage it and, to my utter chagrin, saw that it wouldn't hold steady speed, accelerated without my telling it to, disengaged on its own, etc. I didn't have time to mess with it then so I just tried to forget about it.

The day after that, I set off on another long trip - this time to Colorado. Well, I tried the cruise at various times throughout the three week trip and not once could I get it to engage AT ALL - not even intermittently as it did in Tulsa.

My question: I'm working through the test procedures from GDL and am having trouble understanding "Step 3" of the test, which they say is "CRITICAL":

Use a current-limited 13.8 Vdc Power Supply with a series DC Ammeter. Connect between pins 4 and 5 of the actuator plug.

What exactly is a "current-limited 13.8 Vdc Power Supply"? Does the car's battery qualify? I don't want to hook it up to something and make the problem even more expensive to fix.

Also, what is a "series DC Ammeter"? Will my little multimeter work for this test, or do I need to go buy some fancy device that I might never use again?

Finally, while I'm committed to working through the published test procedures, does anyone have a guess as to why the cruise would work perfectly one day with a brand new amplifier, and then stop working reliably later that day, and then stop working altogether the next day?

I would think you could use the battery as a current limited supply by inserting a small fuse in the circuit. See what is expected for current draw in their test and fuse the power wire just a little higher. You will probably have to get a fuseholder and a small glass type fuse. Or an auto spade fuse could be inserted or soldered into your power feed line for testing. The important thing is not to exceed the most current draw you will normally expect to see by too much of a margin with that fuses value. Or just get a fuse of the value they want their constant current supply set to.

There is absolutly nothing wrong with improvisation if thought out well. You do not really want to buy an adjustable current limited supply. I do not really blame you as you may never basically have another use for it. As you are aware if more current is drawn that your fuse allows the fuse will blow. So it is indeed current limiting.

You will get about 12.5 volts from a good resting car battery. 13.5 from a working battery with the engine running or close if the system is normal.

Most cheaper digital meters have a ten amp range so will do just fine in series if they want and expect you to see a less than 10 amp draw. If the expected draw is higher let me know and I will suggest a way to build a very cheap(free) meter shunt so your meter can read higher amperages.

If you test with twelve and a half volts your current may be slightly higher or lower than they indicate. Beware of this possible discrepancy. The change should be around ten percent if any. On the test with the engine running of course read the battery but it should be around the 13.5 volts they recommend for the test. Others will probably watch to see how you do.

This just circumvents more expensive options for that test. Should work well. If there is anything about what I mention that is not clear let us know.

Since your subject is very popular as there seem to be a smattering of defective cruise controls out there. Plus the rebuilder suggests the actuator can draw more current when defective. This damaging the cruise control module.

A preventative fuse or lets call it a current limiting fuse in that actuators circuit should have been suggested eons ago to protect the module. Another thing as I just reciently found out a very slightly erratic cruise amp may not be the amp. Same basically as is recommended for the heat control valve that can take out the ccu.

The cruise control module has no internal voltage stabilization or regulation from what I saw happening. Any electrical load that pulled the cruise supply voltage down a little was felt or evident in the very almost not noticeable speed change. If I put on my signals for example I slightly felt it. It could have been the actuator reacting but thought not as it is a very coarse device in comparison to the cruise amp.

Either the common grounds are questionable or there is some oxidation or marginal termination on a power feed in this car. I doubt if it is a design fault as it is too easy to rectify.

Just something people should be aware of if the erratic tendency shows in their cars. This possible defect will depend on the amount of real voltage swing present. It is not hunting but just a percievable change in speed my opinion. Let us know how things go.

As far as your last question of why the failure? Could be the actuator as you are presently testing that will be condemed or eliminated soon. Or most likely some component in your module gave up.

They do not change everything and remember these controls had a tendency of developing intermittent faults. They on that basis could have easily missed something. External wiring to the cruise and actator, Activation hand lever also could play a part. Almost any senario is possible.

I really like the possibility that if the actuator was drawing heavier than specifications it fried an output device in the module. This will become conclusive when they troubleshoot the module if that is the problem again.

Depending what they find will determine th cause by the way. They will make suggestions after the next repair if they feel they are relevant.