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Help. I need an inside picture of an ECU D-Jetronic Computer 3CU
Hello, I require an inside picture of an early XJS V12 ECU 3CU D-Jetronic Computer . I have a 1976 V12. I needed my ECU to be calibrated for fuel mixture because of my mods on my engine. I had paid and sent my ECU back to AJ6 Engineering twice for adjustments, but, honestly, they need my car... (and I don't have 2 years for them to do it again, ) So, I am looking to do the adjustment myself. It's easy, but, I require the original values of the resistors that were changed in my ECU, so that I can put new adjustable resistors in accordingly.
In the picture, you can see some newer colored resistors that were added or maybe original? They are run in parallel with each other.
I am requiring just to adjust my bottom end mixture, WAY WAY too rich. It's eye watering... I can just increase my fuel pressure for the top end mixture. Everything else is good.
My ECU is 2 288300580.
I have just read up on the D Jetronic and it not as simple as replacing the resistors with values from another ECU, this will not work because these resistors are fine tuning adjustments and will be different between every ECU.
A little explanation - semicondictor tolerance can be as high as 20% and these parallel resistors (2% tolerance) are selected to tune each ECU individually. What you need to do is find out what values your ECU had and put them back.
I'll do a little more research on the ECU and see if I can identify which resistors do what, I can not see all the resistor numbers in the pics so can you post all the resistors numbers that were replaced.
It was AJ6 Engineering who changed and added things. I have no idea what the original resistor values were. The first time I sent it there, England, he was able to change a few things, including the the fuel trim. Because I added a longer duration cam, a stock ECU would normally have too much fuel at the bottom end. He was able to adjust for it. But, he made it WAY too lean. I sent it back. He then made it WAY too rich. So I know, he was able to change it with resistors. I need it somewhere in the middle. I would send it back, but, every time I do, it takes 1 year for him to change it. And I am worried he will pass away before I get the ECU back. He is getting on in years...
So, this is my story.
T701 with R702 and R703 might be a good start. Or even R202. The other areas he changed, I don't think effect the bottom end fueling ratio.
Can you post back the resistor numbers that were replaced, I can see a couple but not all of them, I've found a schematic of the D Jet so we can play with resistor values until you get it where it needs to be.
Other option is to put a MS2 PCB inside the D-Jet case (I have done this with a 6CU case), then you have full programability and can add ignition later, just a thought.
I think you should go back to an original ECU with a stock calibration and tune around it. Simply increasing fuel pressure from 30 PSI to 33 PSI would increase fueling very little at idle but offer more fuel as the RPM's increase where you need it. You can also upgrade to a pressure compensating fuel pressure regulator like modern cars started using in the 80's. These more common fuel pressure regulators slightly decrease fuel pressure at idle and raise fuel pressure 3 to 5 PSI under load. An aftermarket adjustable pressure compensating FPR is low cost and available quickly.
Performance cams often lower manifold pressure which will make the MPS think your car is under load and increase fuel at idle. You can mitigate this with a higher idle. Raising the idle 100 to 200 RPM may get back some manifold vacuum and offer up a better signal to the MPS. You may also want to try a MPS from an Porsche 914 which was calibrated to expect less vacuum at idle becasue the engine was more performance orientated. .
Are you using a WB02 to tune and measure AFR? Keep in mind that performance cams will always measure and smell rich at idle becasue the over lap sends unburnt and partially burnt fuel out exhaust.
If your serious about V12 performance and higher RPMs you need a different ignition system and while you're at it a different fuel delivery system. Asking a coil to provide strong spark to 12 cylinders at 4500 RPM and up is a bit much. The Jag v12 most likely suffers 5 to 15% misfires above 4500 RPM.
If I were in your place I'd run a pair a GM 6 cylinder engine controls adapted for v12 or a pair of microsquirts.
Last edited by icsamerica; 05-12-2019 at 10:34 AM.
Thank you. I sent a message to him. I tried buying another one on Ebay. I am always out bid at the last second, so I kind of gave up on Ebay. Maybe he will sell it to me off Ebay...
Thank you. I sent a message to him. I tried buying another one on Ebay. I am always out bid at the last second, so I kind of gave up on Ebay. Maybe he will sell it to me off Ebay...
You might try using a sniper service for ebay bidding. It puts in your maximum bid in the last few seconds. If you're outbid, then it went for more than you were willing to pay.
You might have more luck with it being in Canada too, few out of country bidders.
I think you should go back to an original ECU with a stock calibration and tune around it. Simply increasing fuel pressure from 30 PSI to 33 PSI would increase fueling very little at idle but offer more fuel as the RPM's increase where you need it. You can also upgrade to a pressure compensating fuel pressure regulator like modern cars started using in the 80's. These more common fuel pressure regulators slightly decrease fuel pressure at idle and raise fuel pressure 3 to 5 PSI under load. An aftermarket adjustable pressure compensating FPR is low cost and available quickly.
Performance cams often lower manifold pressure which will make the MPS think your car is under load and increase fuel at idle. You can mitigate this with a higher idle. Raising the idle 100 to 200 RPM may get back some manifold vacuum and offer up a better signal to the MPS. You may also want to try a MPS from an Porsche 914 which was calibrated to expect less vacuum at idle becasue the engine was more performance orientated. .
Are you using a WB02 to tune and measure AFR? Keep in mind that performance cams will always measure and smell rich at idle becasue the over lap sends unburnt and partially burnt fuel out exhaust.
If your serious about V12 performance and higher RPMs you need a different ignition system and while you're at it a different fuel delivery system. Asking a coil to provide strong spark to 12 cylinders at 4500 RPM and up is a bit much. The Jag v12 most likely suffers 5 to 15% misfires above 4500 RPM.
If I were in your place I'd run a pair a GM 6 cylinder engine controls adapted for v12 or a pair of microsquirts.
I understand everything you said. I have done most of this. I am using a new WB O2. When I sent my ECM to AJ6 the first time, he adjusted for the richness at the bottom end, due to my cam. He made it WAY too lean. So I sent it back. He then adjusted the fuel WAY too rich. But it took 2 years for him to do this. I can't wait another year. So, this is why I am researching this. I need it in the middle of what he changed. I will then install a resistor potentiometer in that circuit and fine tune it from there. Also, I have put WAY too much money into this car. That is why I don't want to put a different computer into my car. It is at that point where I give up and piece the car out for parts...
Can you post back the resistor numbers that were replaced, I can see a couple but not all of them, I've found a schematic of the D Jet so we can play with resistor values until you get it where it needs to be.
Other option is to put a MS2 PCB inside the D-Jet case (I have done this with a 6CU case), then you have full programability and can add ignition later, just a thought.
I don't know if these tan resistors have been changed (daughter board) (all the tan resistors are the same value) R107, R108 (tan), in parallel. R309, R310 (tan) in parallel. R137(tan), R134 in parallel.
These were definitely changed: (daughter board) This was a transistor that has the 2 resistors changed. T701, R702, R703
(daughter board) Major change here. R163, R164
(mother board) This resistor was added with 2 of the original resistors in parallel. R204, R202 (second last picture I posted)
I have a suspicion that it might be related to the R202, R204. Or the Transistor R702, 703.
And thank you very much!
*R163/164 - differential output of idle mixture and speed compensation.
*R107/108 - speed compensation
R702/703 - Idle mixture adjust and are listed at 830 and 860 ohms respectively, which are not standard resistor values. Leave these for the moment.
*R309/310 - Pulse width multiplier. This would adjust injector pulse width across the entire rpm range I suspect.
*R204/202 - manifold pressure
*R134/137 - speed compensation
* These resistors have no value listed as they are selected to trim each ECU individually.
This is not going to be an easy task as there are many interdependencies between the circuits. If you are going to attempt to adjust this yourself you MUST workout what resistors effect what part of the fuel map.
I would start by adjusting R163/164. there are 2 56K in series effectively making 2x 112K in parallel. I suspect these may adjust idle and light throttle cruising, but you will need to verify this.
Get yourself a 100k 10watt LINEAR pot. Linear is important as volume controls are LOG it must be LINEAR and a couple of alligator clips, cut one end off the clips and solder them to the pot, one to the wiper, middle pin and the other to one end. Parallel resistors you can remove both from the circuit and use 1 pot.
Remove ONE leg of both R163/164 (Leave one leg soldered in) set the pot to 56K and use alligator clips to add it to the circuit. Set up your WBO2 and start her up adjust the pot see what happens and write down your results. If you have success then remove the pot and measure the resistance and solder resistors in of that value. SEE NOTE
R202/204, as these are in the manifold pressure I suspect these will add fuel as MAP decreases. Do the same, and ensure you write down the results.
Document all the changes you make and the AFR at say 1000rpm increments and throttle position as I suspect you may need to go back and adjust previous changes. I do not know how interdependent these changes are.
NOTE: Parallel resistor formula 1/R1 + 1/R2 = 1/Rt, eg 100Ω and 120Ω in parallel "1/100 + 1/120 = 1/0.018333" = 54Ω.
Any resistors you replace I would use 1% tolerance 5W.
*R163/164 - differential output of idle mixture and speed compensation.
*R107/108 - speed compensation
R702/703 - Idle mixture adjust and are listed at 830 and 860 ohms respectively, which are not standard resistor values. Leave these for the moment.
*R309/310 - Pulse width multiplier. This would adjust injector pulse width across the entire rpm range I suspect.
*R204/202 - manifold pressure
*R134/137 - speed compensation
* These resistors have no value listed as they are selected to trim each ECU individually.
This is not going to be an easy task as there are many interdependencies between the circuits. If you are going to attempt to adjust this yourself you MUST workout what resistors effect what part of the fuel map.
I would start by adjusting R163/164. there are 2 56K in series effectively making 2x 112K in parallel. I suspect these may adjust idle and light throttle cruising, but you will need to verify this.
Get yourself a 100k 10watt LINEAR pot. Linear is important as volume controls are LOG it must be LINEAR and a couple of alligator clips, cut one end off the clips and solder them to the pot, one to the wiper, middle pin and the other to one end. Parallel resistors you can remove both from the circuit and use 1 pot.
Remove ONE leg of both R163/164 (Leave one leg soldered in) set the pot to 56K and use alligator clips to add it to the circuit. Set up your WBO2 and start her up adjust the pot see what happens and write down your results. If you have success then remove the pot and measure the resistance and solder resistors in of that value. SEE NOTE
R202/204, as these are in the manifold pressure I suspect these will add fuel as MAP decreases. Do the same, and ensure you write down the results.
Document all the changes you make and the AFR at say 1000rpm increments and throttle position as I suspect you may need to go back and adjust previous changes. I do not know how interdependent these changes are.
NOTE: Parallel resistor formula 1/R1 + 1/R2 = 1/Rt, eg 100Ω and 120Ω in parallel "1/100 + 1/120 = 1/0.018333" = 54Ω.
Any resistors you replace I would use 1% tolerance 5W.
Incredible thanks Warren. I believe it should be the manifold pressure resistors. It kind of makes sense. Because it seems from the letter given to me with my ECM, There is a starting point of fuel and everything after that gets manipulated as the rpm increases. My problem is the starting point. I can adjust things accordingly. Fuel pressure, T1, T2 and CW as seen in the following pictures.
Engines with more cam usually need more timing down low in the RPM range. As a rule of thumb... You should add 1/2 of your exhaust increase to base timing. So for example if you added 10 degrees more exhaust timing, add 5 degrees more ignition timing becasue you want the mixture to burn in the cylinder not on the way out the exhaust valve. You may want total timing to stay the same, so you may have to limit your centrifugal advance. You can run more initial advance in a cam'd engine becasue the dynamic compression is lower.
This is a super interesting thread. I'm fascinated buy these rudimentary fuel systems. This make me think of the 70's electromechanical analogue pinball machine I have. It has a organic quality of rythmic clicks, clacks and bells the digital machines just dont have. Getting this thing to run right analogue is just a matter of sport, no justifiable purpose otherwise. Good luck I hope you succeed.
If you know the spec of the cam, Please post as I can tell you if this is even possible. Also how much vacuum do you have at idle?
In the first picture I posted, T1 is connected to the diamond shaped, 4 blue resistor connection R163, R164.
And T2 is the bottom right wire R128, R129
.
Engines with more cam usually need more timing down low in the RPM range. As a rule of thumb... You should add 1/2 of your exhaust increase to base timing. So for example if you added 10 degrees more exhaust timing, add 5 degrees more ignition timing becasue you want the mixture to burn in the cylinder not on the way out the exhaust valve. You may want total timing to stay the same, so you may have to limit your centrifugal advance. You can run more initial advance in a cam'd engine becasue the dynamic compression is lower.
This is a super interesting thread. I'm fascinated buy these rudimentary fuel systems. This make me think of the 70's electromechanical analogue pinball machine I have. It has a organic quality of rythmic clicks, clacks and bells the digital machines just dont have. Getting this thing to run right analogue is just a matter of sport, no justifiable purpose otherwise. Good luck I hope you succeed.
If you know the spec of the cam, Please post as I can tell you if this is even possible. Also how much vacuum do you have at idle?
My cam is: JA-12-6000: This street grind provides increased torque and acceleration without being temperamental. The best power range is 2000-6000 rpm, and it will rev to 6600 RPM. INTAKE: 31/69 EXHAUST: 73/27 DURATION: 280 LIFT: .358"
Necessary mods: PAECO valve springs (a compression boost to 9:1 will give better acceleration, but is not mandatory). A Primapower exhaust system will add additional power.
I asked for a lift of .419 but I also asked for a larger lobe separation. I don't have anymore info on this cam.
My cam is: JA-12-6000: This street grind provides increased torque and acceleration without being temperamental. The best power range is 2000-6000 rpm, and it will rev to 6600 RPM. INTAKE: 31/69 EXHAUST: 73/27 DURATION: 280 LIFT: .358"
05-11-2019, 01:29 PM
Help. I need an inside picture of an ECU D-Jetronic Computer 3CU
) So, I am looking to do the adjustment myself. It's easy, but, I require the original values of the resistors that were changed in my ECU, so that I can put new adjustable resistors in accordingly.
... http://members.rennlist.com/pbanders/ecu.htm
