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It was designed some 50 years ago. ;-)

Reads 0.58v (Pin11) on the faulty one. 0.006v on the spare unit which shows 5v Vcc.

First of all that doesn't make sense. Assuming your measuring wrt GND.

In essence the current through Q1 is determined by a diode drop across R1, so for 1.2R that's going to be 0.5A and for 1.8R 0.33A

But for that I would expect 0.6V on the base of Q2 to supply, through R4, the bias current to flow in D1 and D2 and lower Q1 base wrt 12V by 2 diode drops. It's a PNP transistor. Are you sure you haven't got the faulty/good measurements reversed?

Next I would look at the chain ZD2 / R8 / R9 and compare voltages. They look as if they are designed to shutdown the 5V through an over-voltage on the 12V input.

Volts dropped across R1 should be ~0.6V

Can you check again at pin11, or just in case the diagram is wrong, at the base of Q2?

After some research Q1 (17344) is a big beast as I expected, a TO66

Do you have a datasheet for the RCA 17344 ?

It might be from, like, 1978.

Now, a theory would be, the 17344 is a PNP Darlington pair.

The other thing I was thinking, is Pin11 could be a disable input, causing the 1041AE to stop driving the injectors, so the injectors don't get currents from a load dump. But then the readings don't make sense.

Paul

No, I've only gone along with forums regarding this ECU.

Yes, likely of this vintage.

If it was a Darlington then it would have an indeterminate base current from being biased with two diodes so I feel it is unlikely. BICBW

Yes, those were my thoughts. But I felt that was too much information and was thinking of a more piecemeal approach especially with the discrepancy with readings.

Either way, as you agree, the readings don't make any sense.

Here is someone's beermat version of the circuit, which is at least drawn in a more sensible organisation:

That picture has ZD2 marked '24'. According to:

I'm guessing it's not in the BZX... series since a BZX..24 would be a 24V zener, which doesn't sound right. The existing zener being a 1N5341, suggests it's either the 2.8 or the 4.7V. Would either of those make sense?

Now you mention it, Dave's readings do slightly make sense. Because pin 11 is at the base of Q2, the base-emitter junction means it won't rise above

Hmm, the network at the base of Q3 divides the voltage at the anode of ZD2 by 15.3. Vbe(on) for the BC184 is 0.55-0.7v, which multiplied by 15.3 gives us 8.4 to 10.7. I wonder if this is an overvoltage detection network.

Let's say the zener is 4.7v. If the battery voltage were to go above

There will likely be sufficient tolerance of the resistors and transistor that might mean this would need to be more accurately tuned via component selection (eg transistor grading).

But, if this is so, I don't see how the 5V regulation works...

Theo

The +5V regulation is with ZD1. It's used as an awful shunt regulator. Q1 is a constant current source from 12V supplying power to the 5V rail with ZD1 dissipating the extra current.

The above hand drawn schematic link is confusing for some as it shows Q1, a PNP, essentially upside down from a current flow perspective.

Everything measured reference to ground unless otherwise stated.

Voltage across R1 is 0.15v However, on both boards it is 1R2 rather than

I believe the design voltage would be be nearer 1.2V if the circuit was working as intentioned.

The voltage is consistent with base voltage.

If this voltage is at zero then that can explain why Q1 is not conducting sufficient current. The voltages around Q2 and R4 will probably explain what's going on and may suggest Q1 is faulty.

But ZD1 is a 6V2 zener, not 5V. The circuit is clearly wrong IMO.

I think ZD1 is a clamp: if the rail goes above 6v2 then it will accept the extra current. It'll also clamp any negative spikes on the rail to -0.7v. If the rail is at 5V then I wouldn't expect much current to flow through ZD1 and it would be essentially open circuit.

But then I'm not clear how the voltage *is* regulated. Unless it's some strange current mirror from ZD2?

Theo

Most stuff can handle 5.5V without issue, absolute maximum rating of logic is generally 7V. As you say it may not be correct.

From a design POV I don't see the issue with a proper regulator, ie in discretes or something like a 7805 which I'm sure existed in the vintage era of this circuit.

ZD1 is a 5W device, consistent with being a shunt regulator. I can't see any active control of the 5V, unless there is something strange happening in U101. I can't find a datasheet of a LFC1041AE and assume that Pin 11 is a logic port.

I'm pretty sure the sole purpose of ZD2 is protection.

At a raised voltage of the 12V rail Q3 will turn on, turning Q2 off and with it any current through Q1 making the +5V fall.

The LFC1041 is a Ferranti part, so I suspect it's a custom logic chip made for Lucas. The PDF I linked upthread has a block diagram. It looks a bit like the Ferranti ULAs in 8-bit computers like the BBC Micro, but that might only extend as far as the packaging. Ferranti did make transistors and analogue chips too. So I suppose we can't rule out some analogue stuff in there.

If Dave is measuring 5V on the rail, I don't see how ZD1 is regulating it. If the rail was 6.2v, fine. But at 5v it's well into the non-conductive region, eg on the OnSemi datasheet for the 1N5341:

I agree that tolerances can allow 5v logic to run at 6v, but that should show when Dave measures it?

Theo

Connectional logic with the SD1 lot is that the 4CU is entirely analogue. It can't be remapped in a conventional way. Those 4CUs used for racing etc were modified mainly by frigging the signal from the AFM. And of course larger injectors. The next Lucas generation - the 14CU - is digital and can be re-mapped using software.

UK (mapped) versions of the 4CU are plentiful used.

All I've basically done was to make a test rig based on a MegaSquirt Stim. This, with an external PS and via an adaptor lead, provides power, tach pulses (variable from 0 - 6500 RPM in practice) and pots giving 0-5v to each of the ECU inputs - TPS, AFM, CTS, IAT, etc. LEDs across the injector outputs. By looking at the injector pulse width on a scope, you can check that reacts to a sensor voltage change. Not an exhaustive check, but does usually show if there is an actual fault. Actually quite rare. Most of the problems are with peripherals, rather than the ECU itself.

The one in question isn't UK spec. And was locking on one bank of injectors at random - intermittently. Not good for the engine.

But the first thing I noticed was the low Vcc. Every other one I've looked at has been pretty well spot on 5v.

I did manage to see the locked on injector output once at 0 RPM (no tach signal) when they should of course be off. The injector drivers alternate. Pin 23 on U102 was showing Vcc - Pin 24 0V. But, of course, when attempting to look into it further, that fault went away.

According to the 4CU PDF upthread, U101 (the LFC1041AE) has various digital functions:

RPM to digital converter Digital Memory (fuel map) Number-to-time counter Voltage Controlled Oscillator (fuel trim)

and the input to the fuel map memory is 7 bits coming from an ADC in U102 (LFC1071AA). Apart from the ADC, U102 does look mostly analogue.

So it's digital, but it's not programmable. I assume the fuel map is hardcoded in a ROM (of that vintage, to be programmable it would need a quartz window for a UV-erasable EPROM) or maybe a PROM (writable one time only).

Meanwhile the 14CU has a microprocessor in it, so it is programmable via software in the modern sense.

Theo

I've been in contact with the owner and he regards it as scrap. He's currently using a Land Rover version from an engine with the same CR. So I've changed U101 for one from a scrap Vitesse unit. No charge if he tries it and not OK. But now have 5v Vcc.

That's interesting. So it suggests U101 pin 11 was inhibiting the 5V rail.

Theo