Engine tick over revs

[Nick Stubbs]

Haven’t noticed this before but with the current high ambient temps, using the air con I’ve noticed the engine revs are higher at tickover than usual, it isn’t doing a regeneration, is this usual ? Thanks. We were in France a few years ago and had outside temps of 45 deg but I don’t recall this happening.

 

[Alexlex85]

Regardless of the outside temperature, your engine should still be operating at roughly the same normal temperatures—around 90–100°C coolant and 110°C oil. The exception is if it’s overheating, which you’d normally see on the coolant temperature gauge.


If the idle speed is sitting at around 1,000 rpm, there’s a good chance it’s carrying out a DPF regeneration. If the DPF isn’t performing properly, it may be regenerating more frequently than it should. Over time, this can lead to diesel fuel diluting the engine oil, affecting its lubricating properties and potentially causing camshaft ticking noise.


If it were mine, I’d check the DPF performance using VCDS. A clogged or poorly functioning DPF can cause a range of issues and can become expensive if left unresolved.


Which engine is fitted to your car?

When was it last serviced?

 

[Nick Stubbs]

Thank you. It isn’t doing a regeneration it makes a different noise and carries on running when you stop, oil and coolant temps within range, it’s the 204 bi turbo, last serviced a couple of weeks ago by myself. It’s done just over 80 thou, the vag com isn’t giving any errors/faults, it did this during the last heat wave it was then normal it has done a recent regeneration. Noticed last night when doing a fuel run it had a raised tickover.

 

[WelshGas]

Thank you. It isn’t doing a regeneration it makes a different noise and carries on running when you stop, oil and coolant temps within range, it’s the 204 bi turbo, last serviced a couple of weeks ago by myself. It’s done just over 80 thou, the vag com isn’t giving any errors/faults, it did this during the last heat wave it was then normal it has done a recent regeneration. Noticed last night when doing a fuel run it had a raised tickover.

The AirCon uses power. The compressor is driven by the engine, mechanically, and the fans from the engine battery which will be recharged by the alternator. Both these systems put a load on the engine which will increase engine revs when idling.
By how much depends on the load. Is the aircon cooling the car down or is it at a steady state. Outside and inside temperatures all play a part. With the recent temperatures the aircon will be working hard. The load on the engine will be high.
When you see it next, switch off the Aircon. What happens?

 

[grumpyrs]

Mine has been doing this recently, never noticed it before. As above, if I turn off the air conditioning the revs drop back to normal. I did wonder if the air con pump was getting tight or maybe heading for failure, hopefully not and fingers crossed!

 

[Perfectos]

As WG: smart alternator will measure battery volts and usage of systems, engine revs are raised to compensate to ensure net effect voltage drain on starter battery is minimised whilst

 

[grumpyrs]

My Battery control module hasn’t worked for a couple of years, alternator is constantly at 14v. So the alternator appears to be dumb not smart, so I ruled this out as a reason for increased revs. Any thoughts welcome.

 

[Perfectos]

My Battery control module hasn’t worked for a couple of years, alternator is constantly at 14v. So the alternator appears to be dumb not smart, so I ruled this out as a reason for increased revs. Any thoughts welcome.

If the alternator is not adapting to voltage change , the Lin cable will have a break in it ?

Edit . Lin cable is the thin single cable from the negative terminal to the alternator

Edit edit : run a temp replacement to check (if you don't have means of checking in situ )

 

[WelshGas]

My Battery control module hasn’t worked for a couple of years, alternator is constantly at 14v. So the alternator appears to be dumb not smart, so I ruled this out as a reason for increased revs. Any thoughts welcome.

You are confusing the ' Smart ' function with the normal function of an alternator. The ' Smart ' function switches Off alternator charginging if not needed to reduce the engine load hence emissions.
In ' normal ' mode the
alternator voltage remains constant, the amperage is not constant. Amperage is determined by the electrical load (demand) connected to the system.
An alternator acts as a constant-voltage source that supplies a steady voltage (typically between 13.8V and 14.7V). The current (amps) that flows from the alternator depends entirely on what your devices or battery "pull".
Here is how the relationship works:

• No Load (0 Amps): If you have a fully charged battery and no accessories turned on, the alternator voltage remains constant, but it will output almost zero amps.
• Adding Load (Increased Amps): If you turn on your headlights, air conditioning, and seat heaters, the total electrical demand increases. The alternator will supply the same constant voltage but will now increase its current output (amps) to meet the new demand.
• Maximum Output Limit: Every alternator has a maximum amperage rating (e.g., 100A or 150A). If your electrical demand exceeds what the alternator can supply, the constant voltage will begin to drop, and your battery will be drained to make up the difference.

So the amps constitute the Load on the engine and so idling speed can increase to compensate.

 

[grumpyrs]

Thank you Perfectos and WG, I’ll try Lin temp wire, around the same time I connected a battery charger the wrong way round so thought I’d cooked the battery monitor.

WelshGas, that’s a great explanation. Much appreciated, I had indeed been confused by the ’smart’ function and normal function of the alternator. Thank you.

 

[WelshGas]

Thank you Perfectos and WG, I’ll try Lin temp wire, around the same time I connected a battery charger the wrong way round so thought I’d cooked the battery monitor.

WelshGas, that’s a great explanation. Much appreciated, I had indeed been confused by the ’smart’ function and normal function of the alternator. Thank you.

Just to finish off what a ‘Smart’ alternator actually does -

Unlike traditional alternators that output a constant 13.5V to 14.5V, smart alternators operate dynamically through the vehicle's Engine Control Unit (ECU).

Acceleration & Cruising: When you press the accelerator, the ECU drops the alternator's voltage (sometimes as low as 12.2V) to reduce the mechanical drag on the engine. The vehicle's electronics temporarily run off the main starter battery.

Deceleration (Regenerative Braking): When you release the throttle or brake, the alternator voltage spikes to high levels (sometimes up to 15V–17V) to harvest kinetic energy, rapidly charging the battery.

By reducing the engine load on acceleration less fuel is burnt and by increasing engine load on deceleration there is more engine braking, less mechanical braking and also some fuel saving.
Although these amounts of fuel saved are very small they add up over the life of a vehicle.

 

[Nick Stubbs]

The AirCon uses power. The compressor is driven by the engine, mechanically, and the fans from the engine battery which will be recharged by the alternator. Both these systems put a load on the engine which will increase engine revs when idling.
By how much depends on the load. Is the aircon cooling the car down or is it at a steady state. Outside and inside temperatures all play a part. With the recent temperatures the aircon will be working hard. The load on the engine will be high.
When you see it next, switch off the Aircon. What happens?

This is what I suspected, but having owned the van for 8 years I’ve never noticed before.

 

[Nick Stubbs]

Just to finish off what a ‘Smart’ alternator actually does -

Unlike traditional alternators that output a constant 13.5V to 14.5V, smart alternators operate dynamically through the vehicle's Engine Control Unit (ECU).

Acceleration & Cruising: When you press the accelerator, the ECU drops the alternator's voltage (sometimes as low as 12.2V) to reduce the mechanical drag on the engine. The vehicle's electronics temporarily run off the main starter battery.

Deceleration (Regenerative Braking): When you release the throttle or brake, the alternator voltage spikes to high levels (sometimes up to 15V–17V) to harvest kinetic energy, rapidly charging the battery.

By reducing the engine load on acceleration less fuel is burnt and by increasing engine load on deceleration there is more engine braking, less mechanical braking and also some fuel saving.
Although these amounts of fuel saved are very small they add up over the life of a vehicle.

And this is why your leisure batteries are sometimes not fully charged

 

[sidepod]

I’d love to know the exact amount of fuel this saves over a year?

 

[WelshGas]

I’d love to know the exact amount of fuel this saves over a year?

Make of it as you wish.

Using a smart alternator typically saves between 1% and 3% in overall fuel consumption, occasionally yielding up to a 5% improvement in specific stop-and-go driving conditions compared to a traditional alternator.

 

[Nick Stubbs]

Been out and about today, ambient temp less than 30 deg all normal