• The Forums are now open to new registrations, adverts are also being de-tuned.

Is using your mobile phone inside your car still a no no?

amwebby

MB Enthusiast
Joined
Aug 9, 2006
Messages
1,440
Location
Thorpe Bay
Car
CL500
My Manual states:

"Never operate radio transmitters equipped with a built-in or attached antenna (i.e. without being attached to an external antenna) from inside the vehicle while the engine is running. Doing so could lead to a malfunction of the vehicle's electronic systems, possibly resulting in an accident and personal injury,"

Now, using anything but a Bluetooth puck and phone with SAP would seem to invalidate this,i.e. you are using the mobile phone's aerial and not the car's aerial.

Yet Mercedes-Benz sell three Bluetooth pucks I am aware of, and only one supports SAP. Have they removed this warning? If I install a Bluetooth puck that only uses HFP am I invalidating my warranty?
 
Certainly would not invalidate warranty if a normal cell phone was used within the car through a BT HFP device or just as it is.

The user manual statement must be more of a liability statement and should be taken literally for other types of radio transmitters that transmit at significantly higher power than modern cell phones.

But the statement sounds contradictory to their product offerings, once the BT HF profile device came available (and the BT telephony support on the W204). I believe two of the European BT pucks are SAP and one is HF. In addition there are a few US versions, including a puck with a tail for pre-MHI cars. All the use BT devices use HF profile.
 
See the thread about airbags. I'd have thought there was reasonable chance of a mobile phone triggering an airbag detonator at very close range.
 
I believe this warning stems from a time when mobile phones did transmit at significantly higher rates than they do now, hence the exploding airbags. I'm sure their policy must have changed, hence MB suppling HFP Bluetooth devices, which, by their nature, use the mobile phone's aerial. I just wanted to make sure the MB wouldn't use it as a get out clause should something untoward occur. I just wondered if anyone had the official word from MB?
 
Hmm .. I think mobiles still radiate a fair bit of power - enough to affect nearby TV, computer speakers, radio etc when 'polling' (or whatever it's called).

I wouldn't personally put one anywhere near a live airbag, and current BMFA advice is still to keep them away from computerised r/c transmitters:

http://www.bmfa.org/news/bulletins/sb6_mobiles.html
 
Hmm .. I think mobiles still radiate a fair bit of power - enough to affect nearby TV, computer speakers, radio etc when 'polling' (or whatever it's called).

I wouldn't personally put one anywhere near a live airbag, and current BMFA advice is still to keep them away from computerised r/c transmitters:

http://www.bmfa.org/news/bulletins/sb6_mobiles.html

The link talks about hobby type transmitters that are not that well protected against external RF signals. Even professional quality transceivers are easily more sensitive to external RF signals than airbags that do not have to make use of those, they can simply deploy strong filters that easily attenuate radio transmissions. In any case, I am not promoting the use of any transmitters near airbags, just in case. There is no advantage from operating those extremely close to air bags, so why not leave some distance. Extending the distance from 1 cm to 10 cm makes a huge difference already.

Considering current cell phones power levels, a GSM 900 mobile would transmit at a maximum of 2 W peak power. On a speech call that would be with a 1:8 TDMA ratio. That counts to 0.25 W average power (approximately actually). The first car phones I have used were able to transmit at 20 W (that is compared to the 0.25W). In a standard installation they had a fixed antenna outside the car which makes the RF power low inside the cabin (lower than that from a GSM phone). But those devices could be used portable too. Then there are all sorts of amateur radio devices using relatively high RF power, I guess military can be left out of this discussion. In any case, the fact that (low quality design) audio devices get disturbed by cell phones is a completely different issue from an airbag circuitry.
 
Even professional quality transceivers are easily more sensitive to external RF signals than airbags that do not have to make use of those, they can simply deploy strong filters that easily attenuate radio transmissions.

The problem with r/c transmitters is nothing to do with RF interference - it's the on-board processors and/or memory being affected. Unless an airbag control module is shielded I'd have thought it quite possible for it to be affected in a similar way.
 
The problem with r/c transmitters is nothing to do with RF interference - it's the on-board processors and/or memory being affected. Unless an airbag control module is shielded I'd have thought it quite possible for it to be affected in a similar way.

But how do you think the on-board processors would get affected form a mobile phone if not from the RF signal from the phone? The RF signal from the phone to me is RF interference for the r/c.

Continuing on the allowed power levels, this is from my manual (that also has the same warning as in the first post.

Existing cell phones are well in the limits (operating mostly at 900/1800/2100 MHz).
 
Last edited:
This is a very specialized subject, and there are many ways that circuits can interact or be susceptible to outside sources from other signals. there is a capacitance effect, that is when cables are run close together as in a wiring loom. Microprocessors in the early days often had a tin shielding plate over the top, but these seem to be omitted these days. Any choke or coil of any form can take in any harmonic that is being radiated nearby, this be made worse or better when a ferrite core or ring is used to dampen or increase the efficiency of that part of the circuit. The CAN BUS is just a balanced low impedance pair of unscreened wires, supposed to be robust and suitable for car use. Alway the number one rule when adding anything to a car, never coil up surplus cables.

Like after eating a tin of baked beans that will make you break wind, the odd spike that did the damage is often impossible to find or know how it got in, just the same as you do not know which bean made you break wind
 
Modern mobiles transmission output is 02 - 0.8 W so they seem well within those limits.
 
The warnings are there for the same reason as sell-by dates on food: to protect the vendors should anything go wrong. If there really was a problem with phones in cars we would have heard about it by now.
 
Modern mobiles transmission output is 02 - 0.8 W so they seem well within those limits.

Doesn't make a difference here really but the output power range for GSM 900 is (for power class 4, which is in practise the only one on the market) 33 dBm "peak", that is 2 W. Average power would be 0.250 W when transmitting at peak 2W because the phone only transmits in a 1:8 duty cycle. The peak is one that may be relevant here, another point is the on-off pulsing which creates the 217 Hz audio buzz on poor radios etc. because of the pulsing RF envelope getting demodulated by non-linearities in the affected circuitry.

The minimum power on a GSM 900 phone would be +5 dBm (about 3 mW). On 1800 MHz the peak power is 1 W, the minimum power is 0 dBm (1 mW or 0.001 W). GSM power class 5 would transmit at 0.8 W peak, I guess you were referring to this figure.

UMTS phones would transmit at +21 dBm or +24 dBm peak, but they transmit continuously. The average form a +33 dBm GSM 900 phone would be exactly the same +24 dBm as the average of a UMTS phone (when both transmit at max power).

Again, just to put more figures, would not make a difference on the topic and the conclusions.
 
I just checked my manual for the page shown above. Although it looks different from that it does state the same figures and warning. There is a preamble however, which states:

"The installation of telephones and two-way radios with the maximum transmission output listed below is approved by Mercedes-Benz, provided that the installation is completed in a professional manner and a reflection-free, tuned external aerial is used."
 
But how do you think the on-board processors would get affected form a mobile phone if not from the RF signal from the phone? The RF signal from the phone to me is RF interference for the r/c.

What I meant was that the problem wasn't direct RF interference e.g. mobile phone transmission interfering with output signal of the r/c transmitter. The issue was corruption of data stored within the tx, leading to internal logic errors etc.

I still don't see why it would be out of the question for the elctronics in an airbag control module to be similarly affected. The fact that it doesn't transmit anything is neither here nor there.
 
What I meant was that the problem wasn't direct RF interference e.g. mobile phone transmission interfering with output signal of the r/c transmitter. The issue was corruption of data stored within the tx, leading to internal logic errors etc.

I still don't see why it would be out of the question for the elctronics in an airbag control module to be similarly affected. The fact that it doesn't transmit anything is neither here nor there.

I agree on the above, just that the R/C device processor board is poorly engineered against RF signals while I'm pretty sure the airbag circuitry is professionally designed against RF signals, and it is relatively easy to do that both for the airbag circuitry as well as the R/C device.

Think about a cell phone itself for an example, who would sit closer to the radio transmissions and the cell phone processor would not get upset from its own radio.

Like you clarified, it would be different if the interfering device would emit at the RF operating range of the victim. Honestly, cell phones have wide band transmitter noise that would block the useful received signal of a victim system, depending on its operating frequency (I did not check the case for these R/C devices), but only if operated (on a call) extremely close to the victim device (one metre distance makes a world of difference).

Still, I would avoid using the phone extremely close to any airbag electronics, or any other essential electronics, when no need at all to do that. Who knows about an odd combination of events and why try one's luck on that.
 
Given that Mercedes-Benz now supply at least two Bluetooth devices that mean you will be using a mobile phone (and accompanying aerial) inside the car, what warnings or restrictions do they place on that?
 
This is very unlikely, given that it's designed to be an inch or two away from the transmitter aerial!

Come on, I guess we agreed that it gets interfered from the cell phone RF signal, not from the smell of of it or anything else. Now if it gets interfered from this RF transmission, I would say it is "poorly designed" compared to devices that do not get interfered.

OK, I admit that this is not the primary design feature of an R/C controller and I'm not claiming that the device in question couldn't be the best on its class etc. "Poorly" here is a very relative term.

Considering the transmission from the controller, that power is still lower than the cell phone transmit power, it must be a device operating on unregulated frequencies (citizens band or similar) and would not be allowed to use more than perhaps +17 dBm or so, could be +13 dBm.

I understand that on the R/C system there is no RF receiver in the remote control device but only on the controlled device? In this case the cell phone transmitter wide band noise would be critical if the phone was used close to the controlled device. If it did interfere the controller, then it would be affecting the processor or base band circuitry, like you said.

Sorry that I was not clear, probably still not very clear but I'm afraid it is more of fixed position on the issue, better stop arguing, I believe others got the points they need to make their own conclusions.
 
OK my final post on it, as I agree we're not really getting anywhere.

The processor in an r/c transmitter is definitely designed to be resistant to RF, on account of the fact that it's sitting inside an RF transmitter ... right next to the aerial. How resistant? Well, obviously the answer is 'well enough for the expected operating conditions' - just as the processor in a mobile phone is.

So, we know for sure that a device which is specifically designed to be RF resistant (to some extent) CAN be adversely affected by a mobile phone at close range.

On that basis it's not totally impossible for something like an airbag controller (which we're only guessing/assuming has some degreee of RF shielding) to be similarly affected. Which is perhaps why the manufacturer specifically says not to use transmitters within the vehicle unless connected to an external antenna.

Whether the risk is high or low (and I would think fairly low), it's still there IMO.
 

Users who are viewing this thread

Back
Top Bottom