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S5 Turbo N370 ECU Capacitor Replacement

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Old 05-16-18, 11:54 AM
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S5 Turbo N370 ECU Capacitor Replacement

Folks,

the purpose of this thread is to start a discussion, and hopefully find some more information. I recently replaced every electrolytic capacitor in my car. This includes the N370 ECU in the car. Car was working before I replaced capacitors in the ECU. I had it tuned according to the FSM prior to the ECU removal and repair. The following issues were present when engine was operating pre-repair:

Throttle position sensors were difficult to set according to FSM with 2-light test method.
Timing was difficult to stabilize at designated idle of 750rpm. Would fluctuate +/-1 degree and jump with occasional errata.
Engine would run fairly smooth, but with a definite lack of power.
Engine hesitation and stumble between shifts.

I had chalked most of these symptoms up to old car problems, something I will need to spend a weekend on (I don't have vacuum leaks, I promise).

Anyways, in keeping with the other recent re-cap projects I have posted on here, I replaced the capacitors in the ECU with the following capacitors. (Digikey part #s included)
2x 35v 47uF 105C - 493-11789-1-ND
1x 35v 22uF 105C - 493-11394-1-ND
1x 50v 10uF 105C - 493-11389-1-ND
1x 10V 220uF 105C - 493-4632-1-ND

After replacing the 5 electrolytic capacitors, the ECU was reinstalled, and the car started. The following observations were made.
Throttle position sensors were out of adjustment, although they had not been adjusted in between
Timing was stable and not fluctuating, but retarded 5 degrees past the leading timing mark, around 1/3 of the distance to the trailing mark.

Slow and iterative adjustment to throttle sensors, engine timing, and BAC idle screw brought the engine to the stable state that was designated in the FSM. Idle was far more stable, timing more stable, and adjustments were easy to make compared to adjustments before the capacitor replacement. Throttle position sensor adjustment could consistently return to the set position. The previous set position was indicated at the extent of the adjustment screw, where the new position was closer to the center of the adjustment range. Adjustments held across multiple start, stop cycles. Car was taken for a drive and the power difference was considerable. No stumbling or hesitation during shifts, so operation was smoother, acceleration was smoother, power delivery was significantly greater.

Long story short, what I thought would be a minor upgrade seems to have had a significant influence on the performance of the car. It is much more pleasant, less obnoxious to drive. I'm glad performance has improved, but I would like to know more about this change of state. I would also like to know if it was an actual repair/improvement, or if I just arbitrarily changed the set position of the sensors. I haven't broken down the circuits in the ECU, but maybe if I get bored this next winter, I will. If some of the more knowledgeable folks on here could comment, I'd greatly appreciate it.

Matt
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Old 05-17-18, 08:34 AM
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That's awesome! You would generally think that "if the car runs, the ECU is good". That does make a lot of sense. The fact that the only thing changed was the caps and then the settings were out of adjustment makes me think you are on the right track.

Good job!

Dale
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Old 05-17-18, 11:07 AM
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Replacing caps in this car has been like a backwards country song. I have gotten so much functionality back, that it isn't funny. For less than $40 in parts too. Labor is a different matter because it's tedious, but I did it all myself. I don't want to tote this as a miracle cure, but I was about to give up on the car. It ran poorly and I wasn't enjoying driving it. Now it is like a long lost friend I have gotten back in touch with after 15 years.

There are a couple case scenarios I can think of that would explain some of the hesitation/jumpiness of the engine.
1 Filter caps not filtering out peak noise - replacement would allow them to dampen fluctuations from sensors.
2 Capacitors supplying surge power to sensors and integrated circuits so that they have the required amount of power at all times.
3 It may all be in my head.
Somebody would have to map the circuits to know.

The thing I haven't put my finger on is the 5 degree difference in timing. The timing sensor input is tied to the physical rotation of the engine, so there must have been some change in the delay period between timing input to ignition coil output. Perhaps the delay was shortened by decaying capacitors, which is possible given I had to advance the sensor, and gained considerable power and stability in doing so. The other possibility is that the change in the other sensors made the improvement, and adjusting the timing is just arbitrary to get a proper transient input to compensate for the delay, which I can't rule out either because I don't have that knowledge.

What do you think?

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Old 05-17-18, 02:40 PM
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While we're on the subject of "miracle cures", when my FC was still mostly stock, the best thing I ever did to the car was to replace the main ECU/engine wiring harness with a brand new Mazda OEM one. Doing *JUST* that one thing exorcised all the electrical gremlins associated with how the engine ran & idled; i.e., things like the TPS never staying adjusted for long, random OMP errors, check engine lights & limp home issues, weird idle fluctuations, timing fluctuations, etc. Car ran like it was factory fresh after the new wiring harness was installed.
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Old 05-19-18, 10:49 AM
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this is interesting. how hard was this to do? if resoldering the clock in an FC is a 1, and the wiper switch is a 10 (i find these a PITA, probably need a better tool), how hard is the ECU? 3-4?

i do not know why the timing moved, but the timing split can change based on the TPS, so i guess i could see that it would have done something. expected? no, but it is what it is. also a factor is that it was down on power, so something was not working right, vs not working as well as it could/should
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Old 05-19-18, 12:31 PM
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I'd say difficulty for me was about a 6 or 7, but I have large hands and an internet search's worth of "accurate" electronics education.

There are 2 boards in the ecu, the capacitors are on the board with the wiring harness pins. The boards are printed on both sides, so care needs to be taken to remove solder on both sides of the board to prevent damage from removal. There isn't much conformal coating on it compared to the CPU (burglar alarm/buzzer), but It was still kinda tricky. I went and got an adjustable heat soldering station because a cheapo $5 iron isn't going to cut it. I also used a high lead content solder. It was comparable to the repair that I posted here.

https://www.rx7club.com/2nd-generati...epair-1126102/

Here is a photo of the board, with the highlighted capacitors. The highlighted resistor was replaced as well.

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Old 05-19-18, 12:57 PM
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I have gotten a few more miles on the car after doing this. It still runs remarkably better. The timing split between leading and trailing didn't change, just the overall timing. I don't have any way of verifying this, but it was much easier to set the TPS after this. It also made the result repeatable after multiple power cycles and throttle returns. I had problems with hanging idle before when the sensors wouldn't return to their set position. It made the factory tuning procedures actually work like they are supposed to.

I didn't check the diodes on this board. I did that on all the other boards. 1 was bad on the fan amp. As well as it's running, I think I'll leave it for now. Did not replace transistors or voltage regulators.
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Old 05-19-18, 02:17 PM
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interesting... it makes me wonder if my T2's have always run so/so because they are out of capacitance...

actually my current one has the Fuel Pressure Solenoid on all the time, and i tested a lot of stuff, and it is looking like the ECU, i SEE nothing wrong in there, but that just means nothing melted...
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Old 05-19-18, 07:01 PM
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I would imagine that would be relegated to the banks of output transistors on the lefthand side, but you'd have to trace the pin to the voltage source. Should be pin 3M on the lefthand side of the picture. It seems the FSM accounts for a small amount of leakage, but the full voltage should only be engaged after 90 sec. It sounds like a switching transistor or something might be stuck in an closed circuit.

Source: FSM F2-79
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