Cruise Control, Ignition Coil, and Voltage

[mike123]

My previous owner installed a cruise control system which doesn't have any identifying branding on it but my best guess is it is the AudioVox CCS100. It never worked since I owned the car, so a while back I disconnected power to the little controller that sets speed, increases/decreases it, etc.

The mechanical piece of cruise control unit in the engine bay is grounded to the engine block and it has a wire connected to the left hand contact on the Bosch ignition coil.

My car has always measured low voltage on the instrument panel as well as via my cigarette lighter adapter - something around 11.5V when off and 12.5V when parked with the engine running. A multimeter on the battery terminals themselves showed 12.5V.

I noticed that after I removed the wire from the cruise control to the ignition coil, both the instrument panel and the cigarette lighter adapter show something like 12.5V when off and 13.3V when parked with the engine running. (In case relevant, my alternator is the 90 amp Motorola stock alternator).

Does this make sense to anyone? Does it indicate that the cruise control was somehow blocking voltage to the rest of the car, or was it maybe just causing the instrumentation to be wrong?

Not an urgent question at all, there's been no noticeable change to the car, but just figured I'd ask, in case there is something I can learn about the electronics of the car or in general. Thanks!

[Bitsyncmaster]

What causes most cars dash volt gauge to read low is the fuel pump current drives through the ground used by the instruments. The 6 amps of fuel pump current caused that ground to read about 0.3 volts and not 0 volts.

Did your gauge reading improve when you removed the logic box? It makes no sense that the servo for the cruse control used ignition coil voltage. It needs a MPH signal usually provided by a magnet and sensor on the wheel drive shaft.

[mike123]

Interesting about the fuel pump! My car has a Carter P60424 fuel pump from '81 Volkswagen Jetta, which was noted as looking small by a DeLorean tech but seems to be working OK since it was installed by PO in 2015 (I got the car in 2023).

I did not notice any voltage change when unplugging the logic box. For all I know though, the only draw it may have been making was the LEDs that backlit the buttons. The non-functioning of the unit may well have been an electrical connection issue.

The unit does have the magnet sensor, and seems to use the ignition coil connection for "over-rev protection". From the manual:

An integral part of the cruise is the 'over rev.' protection circuit. It's function is to monitor engine R.P.M. and disengage the cruise control when the clutch is applied or the transmission shift lever is moved in to neutral.When a sudden rise in R.P.M. is detected, the decision to disengage is made for 'over rev' protection, the blue wire must be attached to the negative side of the ignition coil. A noise suppressor 'in series' with the blue wire keeps excessive electrical noise from "fooling" the cruise into disengaging. For this reason make sure all cruise control wiring is at least 1 ft. away from all electromagnetic sources such as: ignition wires, alternator, regulator,and all high current carrying conductors.

Indeed it was that blue wire that I disconnected and then noticed the improved voltage readings. Maybe the suppressor was a resistor or filter of some kind, which was somehow drawing something?

[David T]

Removing that cruse control was the right thing to do, especially if your car is a 5 speed. Tapping power off the ignition coil was a bad place to power it, it should have come off a direct battery feed with it's own fuse. The cruise control may have been defective causing the voltage drop or it could have been because it was not hooked up right. In any case you fixed it.

[mike123]

Thanks! My desire to remove it is mostly due to it being connected to the throttle cable. Given it was malfunctioning, I worried it was a safety risk. But this voltage quirk was uncovered as I began pulling out wires.

I do think it was *powered* by the battery. From the manual, it sounds like it is taking pulses from the ignition coil to get a sense of how fast the engine is revving. And then comparing to speed sensor so that of it sees a spike in RPM but not an increase in speed, it assumes the clutch is pressed, or in my case the shifter got bumped to Neutral, and it stops pulling on the throttle until that discrepancy is resolved.

That is totally my guess though. Unless something was going very wrong, that doesn't seem to me like it should sap voltage... but maybe whatever caused the cruise control to not actually work, was also what was causing it to draw more from the coil than it maybe should have if it was working right.