Problems with my digital dash from high energy interference

alloy

alloy

Dan, Retired old fart
H-M Supporter Gold Member
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Jul 5, 2014
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On my test drive a few days ago in the 55 my digital speedometer was reading wildly, up to 196 mph, and 38 mph in reverse. I contacted tech support and they said there is interference from some kind of high energy source. It could be the HEI igniton or I'm thinking possibly the plug wires. I've written back to ask him which one is the likely source of the interference.

Engine is a SBC with the distributor in the back and the oil pressure sending unit port right beside it, so that's where the sending unit is mounted. The plug wires run right by the oil pressure and tach wires. Oil pressure sending unit has 2 wires, one is a ground and that is run down to a bell housing bolt since it was the shortest point to a good ground and easy to access, second wire is to the dash. The tech suggested trying to shield the wires from the interference.

So he suggested shielding the wires, but no suggestion on how to do that. What ever I use will have to either slip over or wrap around the wires. I've really never ran across something like that. What types of shielding are available for this application?
 
MrWhoopee said:
Near as I can tell that's the only thing Ford got right. ;)
Click to expand...
I completely agree :grin:

Thank you for the link.

Do you wrap it like you do with electrical tape in a circle, or lay out out in a straight line and put the tape on the wire?
 
Google "shielding braid".
At one or both ends of the braid, solder a short wire to the braid and ground the wire.
Don't be surprised if shielding the OPS wires don't solve the problem. You may need more extensive shielding.
 
Are you using resistor plug wires? Most cars have those but for racing and customs sometimes non-resistor wire is used
Resistor wires radiate much less EMI noise.
Also the ignition wires should be separated from any sensor-type wires as far as possible
In severe cases a low pass filter is required in the power supply to the speedo or tach
Radio Shack used to sell one but you can make one yourself with a 10 mH inductor and a 500 uF cap:
12vLPF.jpeg
The inductor should be rated for the current you need- 1/2 amp or so would probably be fine
 
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I am doubtful that that type of shielding will work, first would be to change out the spark plug wires if they do not have a noise suppression core. The other thing may be to change the routing of the wire from the speed sensor.
 
If there’s just one component affected I’d shield that rather than trying to chase down every other thing that could be causing the problem.

Honestly, if the support I got from that manufacturer was “must be something else, you fix it” I’d be inclined to put it back into the box and send it back for a refund.

They should know it’ll be in a noisy electrical environment and should have engineered the required shielding into their product IMHO.

John
 
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