A tale of two Djets

[bengal taiga]

It was the best of times . . . ."

Swapping out ECUs (same color-coded part numbers) to determine why I might have relegated one to the bench a long time ago. My handwritten note is no longer legible except for the punctuation: "????". No other parts were changed - pressure sending unit and throttle position switch remained the same nor was anything readjusted. The swap produced little change except one thing: BoxA's dial potentiometer seems to have far more range than BoxB. I suppose another way of describing the difference might be using the term “sensitivity.”

As anyone familiar with these knows, each dial allows 21 - 22 clicks or settings. The dials on both boxes produce the same number of clicks and changing the settings on either box produces results that can be measured with a tach and overall engine vibrations. I was too lazy to set up a CO meter. Both potentiometers appear to be working. I have no idea if either box has been modified or repaired.

With BoxA, the idle can be enriched to the point where the tailpipe will spew rich "black" smoke and the engine will barely idle without some throttle action. The dial can be rotated in the opposite direction to the point where the engine want's to die - from being too lean. I am not complaining about this setup since the dial can be set to achieve an acceptably smooth and clean idle. BoxB does not seem to have the same range as BoxA. The mixture can be leaned or enriched, but not to the extremes offered by BoxA. Adjusting the dial will lean or enrich the idle mixture, but not to the point of causing the engine to barely run as with BoxA.

I have never noticed this difference before, probably because it has never been an issue. I am not aware of any published hardware upgrades for these ECUs, although there easily could have been. It sure would be interesting if they could be reprogrammed using software, but then there would be the issue of locating the 5" floppy drives . . . “

I am curious, anyone else notice differences between the two seemingly identical ECUs?

The shop manual and other resources advise against mixing and matching color coordinated PSUs and ECUs. Anyone disregard the advice with positive results? For that matter, what - if any - negative results were encountered?

Thanks.

 

[coupeking]

Peder-
The DJet ECU's were color coded initially with either a green, yellow or red dot. There were differenced in calibration between the systems and I was told years ago--when mechanics still were using DJet--that interchanging a 'red dot' pressure feeler with a 'green dot' ECU would cause poor running. Perhaps this is the problem?
FWIW-
ps/ck

 

[D-jetronic]

I confirm Coupeking remark on interchanging different colored components.

 

[bengal taiga]

I confirm Coupeking remark on interchanging different colored components.

Thanks for the responses.

Per my original musings, I am aware of the need to color match ECUs with PSUs. In fact, the shop manual addresses this in a very "quaint" fashion:

"Random combinations of these components are not possible and result in excessive fuel consumption and inadequate engine behaviour."

Permissible combinations:
PSU without paint dot - control unit without paint dot.
PSU Blue dot - control unit with Blue dot.
PSU Red dot - control unit with Red dot."

The manual also indicates that the red dot = optimal engine behavior." 13-63/1

One can only guess what exactly is meant by inadequate engine behavior. (Inadequate for the likes of Piquet or Lauda?) And if a red dot is for optimal engine behavior, doesn't that permit the inference that a blue dot or no dot are less than optimal? Guess I am glad I am a red (closer to orange) dot'r!

BTW, both of the boxes mentioned in the original post bear the same color dots and - as noted, there really does not seem to be any operational differences except for the potentiometers.

Danke.