Another cruise control thread - 83SC cruise problems

dlshady · dlshady's Garage

Guys,

I know this has been discussed to death here but I've read through EVERY single thread that contains anything related to the cruise system and haven't found anything close to what I'm experiencing. Hopefully there's somebody out there that has been down this road before that can shed a little light on the problem for me.

Here's what I got: 83SC with a completely inoperative cruise system. I've been through this system from front to rear several times now, and this is what I come up with:

The schematic and the troubleshooting chart (Table C) in the Bentley manual don't jive. The second step in the chart shows that depressing the clutch pedal should result in a reading of 10 ohms between pin 3 and ground, but the schematic shows that depressing the clutch should merely ground the tempostat relay and interrupt the circuit on pin 3 between the cruise amp and the servo. In checking pin 3, I do NOT have 10 ohms to ground when pressing the clutch pedal, but I DO have an interruption in the circuit back to the servo. When checking resistance between pins 3 and 7 I have @15 ohms (this is the resistance of the servo and the wiring back to it), and it goes to open when the clutch is depressed. So by my logic, checking the circuit this way verifies that 1) the servo has the correct resistance, 2) the wiring back to it is indeed good, 3) the clutch switch and tempostat relay are working as they should.

So:

The clutch switch is good
The brake light switch applies @11.8 volts to pin 6 when the brakes are applied
Perfect continuity between terminal A on speedo and pin 11 on cruise amp, and the speedo reads correctly so I think it's safe to assume the speed sensor to be OK
12v to pin 5 on cruise amp with key on
12v to pin 8 of cruise amp with column switch in neutral position, goes to 0v when switch is pulled toward steering wheel. (The Bentley schematic shows this to be backwards but the troubleshooting chart shows it to be correct)
12v to pin 9 when the lever is moved up to the "set" position
12v to pin 10 when the lever is moved down to the "reset" position
12v (didn't have a good 9v battery) applied to pins 3 and 7 while the engine is running will cause the engine to rev up, so the servo seems to be working OK
Good vacuum verified to the servo with the engine running, and it will hold vacuum when checked with a handheld vacuum pump
Cruise amp was rebuilt by one of the recommended rebuilders here and was guaranteed to be good. I've also tried a spare amp that I have with the same results.

The only pin I didn't address above is pin 12 which is the ground circuit to the cruise amp and the gauges, and I'm convinced that this is where the problem lies. When checking pin 12 to ground at the cruise amp plug, I noticed that it would oscillate steadily between about .6 ohms and 1.5 ohms. I realize that's not a lot, but I've cleaned every ground on the car (and I mean CLEANED, as in with a wire brush on a Dremel tool) and it should be zero. The steady rhythmic oscillation was also a bit strange, so I looked further. Pulling the power wire to the clock caused the oscillation to stop, but the resistance of the ground circuit stayed at about 1 ohm.

What I found was that any load on the electrical system would increase the resistance on the ground circuit to pin 12. Just opening the door would increase the resistance by about 10 ohms because of the dome lamps. Turning on the parking lamps would increase the resistance of that circuit to about 180 ohms, while the headlights would only increase it by about 80 ohms. Just about anything on the car would cause the resistance of this circuit to go up. I can’t figure out what is causing the resistance in this ground circuit to climb, and I realize that an ohm or two isn’t generally something to be concerned about, but 60 ohms sure isn’t normal.

I decided that in order to rule out the ground being the problem, I’d just run a heavy gauge wire directly from the negative battery terminal to the ground lug on the speedometer. That way I could eliminate pretty much everything in between. With a dedicated 10 gauge ground wire installed, the same problem still exists, although the resistance of the ground circuit doesn’t increase as much as before. What I’m seeing now is that the resistance increases to about 60 ohms with the parking lamps on and about 30 ohms with the headlights on. Turning the key to the “on” position results in about 8 ohms on the ground circuit.

So at this point I’m just out of ideas. My brain hurts from analyzing this nonsense. I know that the Germans did a lot of stuff their own special way but dang, they don’t have their own set of rules for basic electricity… I'm at the end of my rope with this car and I'm literally about ready to push it outside with the keys in it, hoping that some poor sucker will steal it.

So what am I missing guys? Either the ground is the problem or I got screwed by the guy that rebuilt my cruise amp. At this point the ground seems to be the most likely culprit. Any suggestions, other than to wet it down with gasoline and dry it off with a match?

Thanks in advance,
Deron

ManniB · 10-30-2013, 04:45 AM

Deron,

I'm not completely sure about this, but do you disconnect the battery when measuring the resistance of your ground wires? If not, the voltage used by your DVM to measure the resistance may interfer with the battery voltage of the car. Try to repeat your measurement but disconnect either one of the battery poles before.

Manfred

kodioneill · kodioneill's Garage

I don't see a pulse check in you troubleshooting. You should have an on/off pulse at the connection to the speedo. Many times I've seen PO's remove the speedo and mix up the wires on the back.

wally509 · 12-19-2013, 10:32 AM

Does anyone know how sensitive the control units are to having 12V at pin 6 when the brake is applied? The Bentley manual says "battery voltage", but I am getting only 11.8V there like Deron when the battery is closer to 12.8V. Also and probably related, I am getting 1.3-1.5 ohms between terminal 6 and ground. Again the Bentley says 0 or infinite.

Obviously my system quit working. I was in the one of the tail light housings and replaced a brake light bulb for good measure. I can't say for sure that's when it stopped working but reading all these posts about the system grounding through the filaments causes me to ask...one guy's works with a bulb burnt out and another guys doesn't, etc.

I sent my unit out to be rebuilt since it's on sale as preventative maintenance and hoping it fixes the problem.

wwest · wwest's Garage

The brake switch output voltage WITHOUT the brakes applied MUST be ZERO. LED brake lights, for instance, will keep this voltage elevated to 1.2 volts of even ~3 volts.