Bob Krantz

I had a funny episode yesterday that left me wondering what happened, and if others have had similar experiences.

In Houston, we have toll roads and EZ Pass, a RFID device for electronic payment at toll points. These things work by picking up the radio signal at the toll booth, which induces a magnetic field strong enough to create a small burst of current, which in turn makes the tag transmit its ID back to the booth.

I recently replaced the windshield, and came out of the office yesterday to find the tag had fallen off its spot by the rear view mirror (apparently the shop had not replaced the double side tape, but hoped the few sticky bits left would be sufficient). To remind myself to hold it up at the toll booth, I placed it on the seat between my legs. Of course, on a LHD car, this is inches from the DME.

No problems at all until the tool booth. I held the tag up ond then dropped it back between my legs. Just after the booth, the car lost power for a moment, then back on, and repeated this for about 3 more minutes. It felt like a soft ignition loss, that would respond to clutch in and hitting the accelerator.

Anyone else ever try this? Any double-E types out there who can tell us what signal strength comes out of these RFID tags, and if the DME might be succeptible?
Bob K
1989 C4

Adrian

Dear Bob,
Ah the wonders of modern technology. A mobile phone and your computer will act in a similar way.
Model year 1989 and 1990 964s were not fitted with DME electrical suppression apart from shielded cables. In model year 1991 Porsche introduced an addtional suppressor installed between the two plugs in the engine bay electrical panel. These two plugs carry the engine sensor data forward to the DME. This is explained in my book. I would advise two things.
1/. Fit this suppressor and
2/. Check the condition of the DME harness under the left seat and in the engine bay.
Ciao,
Adrian
964C4

DaveK

Useful post. Your description of the power loss is similar to a problem I've had intermittently for the last year or so. It didn't do it before that a9nd I'm RHD so my mobile is a fair way away from the DME), so my problem is probably something else - but, I do have a 1990 car so I won't have the suppressor that Adrian mentioned.

Car is in for a service next week - will make them fit the suppressor. Can't do any harm.

Adrian

Dear Dave,
I was not actually talking about a mobile in the car. However now you bring it up distance from the phyiscal location of the DME is irrelevant. A mobile phone creates quite an EMF as does the handsfree unit it should be installed in. If you have damaged shielding or bad grounds this can still create problems. It is less of a problem in cars but we had this problem in aircraft where mobile phones left turned on in checked baggage used to set off the fire warning systems and pilots using mobile phones whilst in flight did horrible things to the flight guidance systems.
This also applies to installing aftermarket Navigation systems with GPS as well.
Ciao,
Adrian
964C4

garrett376

Let me advise a third thing - with that much EMF going on, I'd keep it away from your crotch area when going through the toll booth!!! Who knows what else it's doing to YOU!

DaveK

Ah - well I have GPS as well. Not for navigation, but for speed cameras. I'm surprised my car runs at all!

Will definitely get a supressor fitted - will look up details in the book tonight so I know what to tell my mechanics.