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Author Topic: DDEC 1 Cruise Control  (Read 1461 times)
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« on: October 09, 2006, 03:22:26 PM »

Those few of us with '87 NJT MC9s equipped with DDEC 1, may benefit from my sucessful cruise control experiment.
Looking thru the DDEC 1 diagnostic manual, it was obvious that the cruise control for DDEC 1 equipped units amount to a series of relays or switches and about a mile of wiring. Appeared to be something to persue for convenience's sake and amusement.
The cruise control works in our DDEC 1s provided it is completed and wired correctly. Cheap to do.
The unit should probably be used only under conditions that won't "lug" the engine.
Unlike an auto cruise...when it's set, the throttle is disabled. You cannot "help" the unit over a hill by spooling up a little. So you gotta be moving on down the road at speeds where the engine is up near the limits, or on relatively flat ground. It maintains the speed spot on when operating at higher speeds.
All of the wiring is inside the harness, but in my bus, the wires were cut off just before exiting the harness in the front.
The VSS is functional on DDEC 1s (it is on all automatic NJTs...MUI included if someone wishes to install a Rostra or some such unit) in the system.
All you'll need is additional wire, (2) 12VDC relays (I used Harley Davidson aftermarket ignition start boost relays), and a 24VDC relay for the brake signal, (1) DPST on-off switch, and two small idiot lights (12V or 24V, doesn't matter).
The wiring that returns to the ECM is all in place and attached to the ECM. You'll want to verify that someone has not "scabbed" something onto the wires for other purposes, but there are several "700" wires that are also avail if the cruise wires have been hijacked.
Be sure to check the function of the system before plugging it into the ECM. If wired incorrectly, may let the smoke out to fhe ECM.
This concept may work on DDEC II and later, but I can offer info only on DDEC 1 units.
If you do this mod, and something smokes....I don't know nuthin about it! I'll fax a copy of the wiring to anyone that wants to see how it works. It's all you need to sort it out. Be sure that you understand how it works. Make sure that the brake disconnect is functional before using the cruise control unit.
The relays and pilot lights are easy to find at any electronic supply store. Keep in mind that the DDEC is 12VDC (set and resume relay) and the brake circuit is 24V (thus the need for one 24V relay). Pay attention to the relay circuitry...the relays make circuits in both the on and off positions. Both "on" and "off" must be seen by the ECM.
Drove down to T'ville from Charlotte and the cruise control was nice....!
Let me know if any questions.

JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.

Ayn Rand
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