Worst ign noise I've ever seen; 1997 Chev Silverado 350 V8

[ozindfw]

I'm trying to help a friend solve an ignition noise problem. His truck is so bad that it tears up anything on UHF that's not putting more than 20 uV into the RX. It's just as bad on a talkie 5 feet away.

I've run some informal measurements with a field strength meter and a small beam at about 10 feet on 220 Mhz. It's 30 db worse than my worst car.

I'm looking for suggestions about what to look for. I've never seen one this bad. Anatomically unlikely recommendations ignored

The vehicle just had new plug cables (supressor type) and resistor plugs installed.

[Pj]

What make/model radio?
Where is it getting A+ from and where is it grounded?
Where and how is the antenna mounted?
Where is the radio mounted?
If the radio is turned on via the ignition, where is the sense power coming from?

I own a 1997 Suburban with the 350 (basically the same truck...just with a top ) and I have 4 antenna's NMO on the roof, LB on a lip mount on the hood, and had a glass mount and had no problems with truck noise. My power is coming off the AUX A and B off of the fuse block and grounded back to the sheetmetal nearby the fuseblock. Ignintion sense is coming off the car radio via a fuse tap.

[apco25]

Back when I had my 95/96 Yukon (it was a hybrid year) it was very quiet RFI wise except I would occationally get a low level carrier on one specific VHF freq that was carrier squelch.

I replaced a ground strap that was missing and it went away. never had another bit of noise from the truck again.

[ozindfw]

> What make/model radio?

It's an ICON 2720, but my GP2000 and any other talkie is torn up at 10 feet.

> Where is it getting A+ from and where is it grounded?

Direct to the battery. Problem remains if power and antenna are disconnected.

> Where and how is the antenna mounted?

Glass mount Larsen on rear window behind driver (yecch)

> Where is the radio mounted?

Under the second seat behind the driver.

> If the radio is turned on via the ignition,
> where is the sense power coming from?

Accessory fuse block Aux A

But this thing is putting out enough noise that it tears my talkie up on a channel that is putting 10 uV in to the front end.

[ozindfw]

>Back when I had my 95/96 Yukon (it was a hybrid year) it was very
>quiet RFI wise except I would occationally get a low level carrier on
>one specific VHF freq that was carrier squelch.

>I replaced a ground strap that was missing and it went away. never
>had another bit of noise from the truck again.

And I think this will be key, we need to find a similar truck and look for what's different. .

[wx4cbh]

Check to see that there's a substantial ground wire (preferably decent sized braid) from the battery negative post to the fender, the frame, battery to engine block, engine block to the fire wall, and body to the frame rail that the battery ground is tied to, all with star washers and lock nuts, and use high quality fasteners. If not, put 'em on there and see if that doesn't take care of the noise. Recently found a fire department command Tahoe that had none of these, had gawdawful ignition noise, and they had tried numerous "solutions" to rid the noise vermin by using caps and coils suppressors everywhere and replacing the ignition wires and plugs. Radios were installed by the competitors and it was first class all the way, but they hadn't checked the factory radio noise supression grounds, not even to see if those were there - they weren't. One clue was the noise in the AM/FM on the AM band as well as the FM on weaker stations and the static that occured on the AM radio when I simply opened and closed the doors, or operated any of the electrical accessories, including the dome light.

We did the ground thing and it all went away, even without the suppressors. It would appear that nothing had a common ground.

Oh, and solder the connectors on the braid, don't just crimp 'em.

[KitN1MCC]

maybe the fuel pump as well

[ozindfw]

On the advice of a friend, I pulled the distributor cap and found the "Button" missing. New cap and rotor made 30 to 40 db improvement. The radios are now useable. There is still detectable ignition noise on high band on signals at threshold, but even then the radio is usable.

The freind gave me a few tips, all of which were useful.

1) listen to the rate of the noise pulses. For an 8 cylinder vehicle at idle (750 to 1000 RPM) the pulse rate will be 50 to 60 hertz if all the plugs are involved, so the *rate* should sound similar to power line hum if all plugs are involved. If only one is involved it will be around 10 hz. The difference is easy to hear now that he mentinoed it. I can tell the difference listening tho other cars at stop lights. Remember that on a four cycle car engine the plugs only fire every other time, so there is a factor of two in the rate calculation.

2) If all the plugs are involved the problem is most likely in the circuit before the distributor, even the low voltage side.

3) ground straps are rarely an issue at VHF and UHF, though the complete absence can be a problem.

In this case the distributor button (the little carbon button that contacts the center of the rotor) was completely gone and spark had to jump an additional 1/8" gap. The electronic ignition happily compensated and there was no noticeable degradation in vehicle performance beyond the "enhanced" ignition noise.

As a side note, the AM broadcast radio was uneffected. It worked fine even in the high noise case. It must have a really good noise blanker.