Rover Efi Throttle Pot Setting

[honestjohntoo]

Owners of bog standard Rover Flapper Efi Systems know to adjust a throttle pot to 0.325 volts at idle position, but WHY? I've seen no written "reason" in any Rover document and of a few opinions heard, none ring true, until this! Full load enrichment ccts trigger when the throttle pot reaches 4.3 volts at 90 deg rotation, or full throttle, as described by Rover. Thus, I conclude, to guarantee an ECU sees a full 4.3 volts at 90 deg, a small offset at idle position is needed. Given normal 5% tolerances for analog cct components, if for some cct tolerance reason, full load enrichment ccts saw something lower than 4.3 volts, they may not trigger! 0.325 volts is exactly 7.5% of 4.3 volts so a 7.5% offset ensures the full load ccts always trigger at full throttle providing there is no overt mechanical reason preventing it. Without the benefit of the designers' knowledge or motives, this seems a logical explanation of why the setting is so specified. In the interests of research, can anyone confirm, or know different?

[TVRleigh_BBWR]

I would have thought it would have to be a low enough for the ECU to know there is no throttle.
Esp for the Air Con car, as the ECU would need to add extra fuel when it kicks in/out else the engine could stall.

[GreenV8S]

I guess that the idle throttle settings are used to enable the ECU idle speed / overrun control logic, but that's only a guess. People who know do take the trouble to set the throttle pot correctly at idle so I think it matters, regardless of whether you understand or like the reason.

[chedder]

If the TPS is set incorrectly either to high or to low, then the ecu doesn't like the voltage parameters on start up, leading to poor starting and rough idle.

[honestjohntoo]

TVRleigh_BBWR said:

I would have thought it would have to be a low enough for the ECU to know there is no throttle.

Esp for the Air Con car, as the ECU would need to add extra fuel when it kicks in/out else the engine could stall.

Efi A/C cars are fitted with a "fast idle solenoid" wired across the A/C clutch that bleeds a small qty of metered air into the plenum when the clutch is engaged to stabilise the idle speed during each cycle.

[adam quantrill]

The idle voltage is probably to get an input transistor into a linear region of its operation. If it were 0 it would be much harder to go from 0 to 0.5V and still have the input operating linearly.

[dickymint]

honestjohntoo said:

Owners of bog standard Rover Flapper Efi Systems know to adjust a throttle pot to 0.325 volts at idle position, but WHY? I've seen no written "reason" in any Rover document and of a few opinions heard, none ring true, until this! Full load enrichment ccts trigger when the throttle pot reaches 4.3 volts at 90 deg rotation, or full throttle, as described by Rover. Thus, I conclude, to guarantee an ECU sees a full 4.3 volts at 90 deg, a small offset at idle position is needed. Given normal 5% tolerances for analog cct components, if for some cct tolerance reason, full load enrichment ccts saw something lower than 4.3 volts, they may not trigger! 0.325 volts is exactly 7.5% of 4.3 volts so a 7.5% offset ensures the full load ccts always trigger at full throttle providing there is no overt mechanical reason preventing it. Without the benefit of the designers' knowledge or motives, this seems a logical explanation of why the setting is so specified. In the interests of research, can anyone confirm, or know different?

Back to basics for me - what does cct mean? (maybe then I can get my head around it).

[adam quantrill]

Circuit.

[Wedg1e]

Just been perusing the ECU circuit diagram and the throttle pot feeds two distinct parts of the proceedings, one being what looks like a comparator around an op-amp... so it may be that resistor values in that circuit demand a certain bias from the pot. Therefore blame the designer; maybe he was making it up as he went along

[honestjohntoo]

Wedg1e said:

Just been perusing the ECU circuit diagram and the throttle pot feeds two distinct parts of the proceedings, one being what looks like a comparator around an op-amp... so it may be that resistor values in that circuit demand a certain bias from the pot. Therefore blame the designer; maybe he was making it up as he went along

Also been perusing the same ECU ccts and the two distinct parts mentioned will both be for enrichment, Acceleration and Full Load.

One of them (Full Load??) has an op-amp reference input connected to a potential divider made up of a 15k and a 8.2k resistor on the 12 volt Vcc bus which results in 4.25 volts, so as a comparitor (which you suggest) it looks like its comparing the TP input voltage to 4.25 volts.

My electronic design career ended in 1968 so I'm very short on recall, but my perception regarding the "reason" for the TP offset (bias) might be correct. ie - to guarantee that full load enrichment is triggered at 4.3 volts just before the throttle reaches 90 degree rotation.