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2014 Sprinter Ambulance AM/FM Problem

This forum is dedicated to helping people with questions about installing radio equipment in vehicles. This can include antenna installs, electrical wiring questions/problems, and mounting systems. Pictures of installs are welcome.

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b18c_ej8
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2014 Sprinter Ambulance AM/FM Problem

Postby b18c_ej8 » Mon Dec 08, 2014 10:07 am

I installed a Motorola PM1500 into a customer's new Sprinter Ambulance a few months back. Due to garage height restrictions, putting the antenna on the roof was not an option (not even a low profile "transit" type), so I was forced to mount the 1/4wave VHF antenna on the drivers side fender using an "L" bracket. At time of install, the power out was right around 100 watts, with 2 or 3 reflected.

Antenna cable routes through the firewall, down driver sill plate to the chassis mounted behind the driver seat. Power cabling is routed rearwards to the main panel in the "patient area". Control cable runs behind seat and terminates to the control head mounted in the center console between driver & passenger seats.

The problem I'm having is that keying up the PM1500 to talk to county dispatch puts the aftermarket Kenwood AM/FM stereo into "Protect" mode. My previous experience with the car stereo world would tell me that the Protect message would indicate a short or significantly lower impedance in a/the speaker wire(s), and the internal amplifier has shut down.

Obviously the PM1500's RF is getting into the stereo causing the malfunction but I'm wondering what can be done to remedy this? If you unkey, and push the reset button on the stereo, it begins operating properly, but only until you key up the PM1500 again. Not a huge issue, but definitely a pain and inconvenience for the customer.

The vehicle's AM/FM antenna is mounted in the center on the cab portion (immediately in front of the light bar) and doesn't stick up past the rear roofline. I've never had this issue on any other vehicle so I assumed that separation between AM/FM and VHF antennas would be sufficient.

Any ideas on how to prevent this issue from happening? I'm wondering if a new ground from the chassis & cage of the Kenwood radio would help at all? I'm willing to bet that the harness ground is pretty tiny, and since the AM/FM antenna is mounted in the fiberglass tub of the roof, that's not exactly providing a ground either.

Thoughts/Advice would be very much appreciated!

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Wowbagger
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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby Wowbagger » Mon Dec 08, 2014 12:02 pm

You could try winding an RF choke on the cable, as close to the antenna as you can. It could be you are getting RF on the outside of the cable.
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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby b18c_ej8 » Mon Dec 08, 2014 12:28 pm

Sorry for the stupid question but should I try the choke on the AM/FM antenna? Thanks for the reply!

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Wowbagger
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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby Wowbagger » Mon Dec 08, 2014 4:42 pm

On the TX antenna. If you are getting an overload on the speaker wires, then it may be that RF is present on the shield of the TX coax and is getting into the AM radio's speaker leads.
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.

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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby Bill_G » Mon Dec 08, 2014 8:43 pm

Yep. Might be an easy fix. Might not. The fender mount antenna is prolly shining through the windshield and right into the AM/FM good time radio. Happy happy joy joy.

Well ... The first thing you want is for your shop manager and the customer to come to an agreement that it IS or IS NOT your problem to fix so that everyone is on the same page about who is going to pay for your time chasing this down.

Second - check your work. Make sure the coax connector is assembled correctly, the antenna is making good contact, and the L bracket is solidly attached. Verify your coax cable isn't running parallel to a vehicle harness that might have speaker leads in it. Cut your tie wraps and move your cable around to see if it changes the problem. If you bundled your coax someplace under the dash, spread that bundle out to see if it changes the problem. If you cut your coax to length, and the total length is under 11 feet, throw your wattmeter in line with a healthy length of test cable to see if it changes the problem.

Next, arm your self with some of those snap-on ferrite beads that you can clip around the speaker leads where they enter the stereo. And pray there's room to shove the head unit back into the dash when you're done. Buy a handful. Toss one on the end of the coax near the L bracket (like wowbagger said) to see if it changes the problem.

Finally, try turning the power down. If you have to turn it way down (under 25 watts), the AM/FM is too sensitive and should be replaced with one a bit more immune. OR, you have to find a new location for the xmit antenna. If you only have to turn it down a little bit (like down to 75 watts), check your A+ and ground. You might not be getting as good of a connection as you think, and you're getting a DC return on the RF cable that is radiating into the stereo (like wowbagger speculated). If you have to turn it down to half (50 watts), and you have some extra coax you can pull into the engine compartment near the L bracket, trying carefully winding it three times in a 2 -3 inch coil as close to the L bracket as you can. Your match on the L bracket might look good on the wattmeter, but there's energy all over the coax getting into the stereo.

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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby canonninja » Mon Dec 08, 2014 10:58 pm

I cant say anything for certain about the Kenwood radio, but the ridiculous amount of hours I spend in Sprinters with OEM sound, I can say this.... in my truck at work, running a Kenwood TK-5820 with fender antenna and a 110w Motorola VHF XTL5000 on a roof whip antenna, we have zero issues with the OEM radio, no interference what so ever. The Sprinters all range between 09-15, and even the newer truck with lane guidance and blind spot monitoring and bluetooth OEM radio there are zero ill effects from the comms radios.

b18c_ej8
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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby b18c_ej8 » Tue Dec 09, 2014 6:49 am

Awesome info... THANK YOU everyone above for taking the time to reply. I will get back over there ASAP and see what I can come up with.

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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby KE7JFF » Thu Jul 07, 2016 5:04 am

I had sort of a similar issue with a Pioneer deck in one of my rigs once; the problem went away when I switched to a different stereo deck. I never got around to confirm what the exact issue was, but I have a hunch there was something weird like loose frame/board screws inside the deck itself.
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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby Bill_G » Thu Jul 07, 2016 5:30 am

RF does squirrelly stuff in a lot of vehicles. Some are seemingly immune, and some aren't. It's been a problem for a long time, and it takes patience and money to fix it. This is why we do our best to put the antenna top center of the (metal) roof whenever possible. These days the most common problem is the Trbo TDMA envelope is getting into consumer electronics causing a chuck-chuck-chuck sound in the speakers. Not bad if the volume level is low, but really irritating at full blast.

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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby KE7JFF » Thu Jul 07, 2016 6:42 am

Bill_G wrote:RF does squirrelly stuff in a lot of vehicles. Some are seemingly immune, and some aren't. It's been a problem for a long time, and it takes patience and money to fix it. This is why we do our best to put the antenna top center of the (metal) roof whenever possible. These days the most common problem is the Trbo TDMA envelope is getting into consumer electronics causing a chuck-chuck-chuck sound in the speakers. Not bad if the volume level is low, but really irritating at full blast.


I haven't run into that quite yet, but it wouldn't surprise me...I remember when "that buzzing" started to pop up when people started to use GSM phones around cheap speakers...

I do know that I talked with a tech at one of the car stereo shops that actually does good work in Portland and he tells me that from all the big manufacturers in this field, he runs into units right of the box that have quirks; so if he finds one that won't work with a customer's vehicle, he will put it a side and see if it works in another car; if it doesn't work in a second car, they send it back.
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Re: 2014 Sprinter Ambulance AM/FM Problem

Postby MTS2000des » Thu Jul 07, 2016 10:36 am

I have a Kenwood KDC-HD262 in my 2011 F-150. Uses the factory supplied wiring, speakers and whip antenna (one of the last years Ford uses a "for real" whip AM-FM antenna). I have two HP APX7500s, one V/7/8 and one U1/7/8. Keying both radios does nothing to the Kenwood (which I am surprised). Doesn't even both the FM reception when keying the VHF radio (ironically, the OEM Ford AM/FM basic radio went deaf when keying the VHF).
Antennas are both roof mount standard quarter waves for V/U and I have two 800 low profile antennas.

If your radio is going into protect, that is an amplifier shutdown- the ferrite beads might work to get RF off the speaker cable if that's where it's getting in from.
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