Cruise Amp question

[MikeTangas]

Is there any way for the home mechanic to check the condition of a cruise amp?

I ask because today I decided to tackle the non-functioning cruise on the 560. When I say non-functioning I mean absolutely nothing happens when you try to activate the cruise. My sources tell me with that symptom the cause could be anything and trouble shooting begins.

I started by checking the actuator, it works - motor turns in both directions, nounusual noise and only a very slight whirring sounds when power is applied to pins 4 and 5. Current draw is ~137mAmps. Puting power to pins 6 and 7 activates the solenoid.

I pulled the cruise amp and swapped it for the best looking of the three I grabbed up at the salvage yard. Easy enough to get out, but a bugger to get back in. No one has mentioned about accidently knocking the bolt from it recess. What a PITA to get back in place, and hold there while mounting the amp. Anyways, got the amp in and went for a drive.

Now when I activate the cruise, the ccruise will engage and accelerate the car. Soon as I let go of the stalk, the cruise disengages. Symptoms point to a bad amp. Well, the old amp was shot, or was it? Maybe my dinking with the actuator cleared the problem and I swapped out a good amp. No, I didn't test the system further before changing the amp.

If there is a way to verify the condition of the amp, I can check the original out, as well as the remaining amps I have on hand. Maybe I could even have cruise by the time I head to Lost Wages at the end of the week.

[Deltacom]

Mike,

Check this URL out to find out.

http://mbca.org/MBCA_Troubleshooting_cruise_cont.htm

[greenacres2]

Before you go too far, replace the stop light bulbs with fresh Osram or equivalent bulbs. I read in another thread that the cruise system grounds through that circuit, and evev though domestic bulbs fit and light up when you hit the pedal, they could, according to that thread, cause malfunction and even damage to the amp.

Just replaced my cruise amp with a used one on Saturday, you're right about the bolt and full time acceleration went away, but cruise was still not right. Replaced the bulbs (which I meant to do before test drive but forgot), and the cruise now seems to work almost perfectly. Tail light bulbs were OEM, but there was corrosion at the filament so I did those also. Worth a shot.

[MikeTangas]

Thanks for that link to how-to repair the cruise amp. I'll probably give that a try on one of the amps. First, as much as I hate to crawl up into the dash any more than I have to, I think I'll put the original amp back in and give it a test.

When I started going through this car, I found the throttle linkage had been connected all wrong. The linkage to the cruise actuator went to the wrong bellcrank, which caused heavy resistance to the linkage where it drug across the actuator bracket. I'm thinking that possibly that resistance caused a problem with the actuator and my testing yesterday freed up the actuator.

I should have run a test before changing the amp, but in my haste to complete the job I was "sure" that the problem had to be the amp since the actuator checked out.

At least I know the switch and the actuator work , that's more than I had before. BTW, the brake bulbs are new Osrams.

[stevebfl]

If you are wishing to go inside the amp or if installing a used one is your plan, then the easiest way to do it is to remove the electronics from the aluminum shell and leave it in place. Substitute the guts if thats the plan.

Personally the unit is such a POS, that I would suggest a new one as the price keeps coming down (MB list is 260 currently for 023 545 21 32) and the number is changed again. I believe the newer units are doing a better job.

[MikeTangas]

Today upon returning home from work I immediately pulled the lower dash and swapped the cruise amp for the original. Test drive showed absolutely no cruise function - definitely a bad amp. Came home from the testd rive and put the semi-functioning amp in the vise (very gently) and pried back the tabs, then pulled the innerds.

No burnt chips or bits and everything "looked" ok. My problems, for all the tools I have, I don't have a small iron for electronic work. Great, I get to buy another tool . Before leaving, I decided to put in the next used amp in the pile. I mean, hell, I've got the process down to a five minute job. So in goes the next amp, and I head off to the tool store. I tested the cruise on the way and lo and behold everything works. Accel/decell, resume, shut-off at the stalk and by brake, speed holds steady, even climbing a 6% grade (and down the other side).

Now I have a small iron and will try to touch up the connections on the semi functioning unit, just to have a spare on hand. If that doesn't work (when the time comes), then I'll bite the bullet and get a new unit. For now though you can't beat the price of a free repair.