Motorola mobile power problems

Discussion in 'General Technical Questions and Answers' started by KH7RD, Sep 10, 2019.

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  1. KH7RD

    KH7RD Ham Member QRZ Page

    I am a school bus driver, trying to help my employer with motorola business radios in our school buses. It seems that all of a sudden we are killing radios. Contractor is slow to respond, so I am looking for a solution. It appears we are blowing internal fuse on circuit boards. Buses are 12 volt, and occasionally they have dead batteries. It seems when we jump start, and or charge battery, radios die. Power to unit is fused per installation instructions/and installer design...25 amp blade in line.

    I am looking for an option...voltage surge suppressor, or a voltage regulator or conditioner that we can insert between source voltage and radios. Waiting for service supplier is frustrating.

    Radio is XPR 2500 a little over three years old, out of warranty. Now down 3 units...20 more in use!

    Brookings OR
  2. KU4X

    KU4X Premium Subscriber QRZ Page

    Stop-gap measure: Turn the radio OFF prior to starting the vehicle and jumping/charging the battery.

    K4AGO and WA8FOZ like this.
  3. KA2IRQ

    KA2IRQ Ham Member QRZ Page

    Hmmmm... very curious indeed. What is the state of the radio when this jump starting or charging is going on? Do the drivers leave them on? Do they go on automatically when the key is inserted? It would seem to me if they were powered off, they should be fine.

    I get the impression from your post that this is a new thing... but I'm sure bus batteries die all the time and have been jumped and charged over the years. So what is different about now? New model of bus? New model of battery charger?

    Pet peeve of mine is when people leave any electronics or vehicle systems in the "on" state when they turn off the ignition. Anything from radios to air condition, etc. It's just laziness to not power things off... but that's just me.

    My girlfriend is a school bus driver and she tells me some of the maintenance headaches the mechanics have with these things. From commercial use to kids spilling soda on the dashboard on their way out of the bus, they can be a real challenge. Some buses get used every day, some sit for months at a time. It's a tough environment.
    KU4X likes this.
  4. K7JEM

    K7JEM Ham Member QRZ Page

    Power to these radios is usually routed directly to the battery, and there are circuits inside the radio that have DC voltage applied even when the power is turned off. I do not have a schematic of this radio, so I don't know what internal fuse may be affected. One thing that seems out of line is the 25 amp external fuse, that seems very high for a 45 watt maximum radio. Should be more like a 15A fuse, with typical current draw around 10 amps at full output power.

    There is no easy "fool proof" method to fix this problem, short of installing a relay in series with the power lead that is controlled off when the hood or access door to the battery is open. This also sets up an additional failure point for the DC power, but it would be economical to implement.

    KB0MNM and W9GB like this.
  5. W9KEY

    W9KEY XML Subscriber QRZ Page

    In the "old" days, I believe tow trucks used a starting system that could put out way over 12 volts - maybe 18 or even 24 to give a "big" jump for starting cold vehicles in the winter. You might want to actually measure the voltage used during the jump starting process. Of course, it would be prudent to either turn off or completely disconnect the radio and any other user installed electronics before doing such a test. Normal automotive electronics should be able to tolerate 16 Vdc, as specified by the the "load dump" spec (when battery accidentally disconnects from an alternator running at full speed, causing a voltage overshoot).
  6. KB0MNM

    KB0MNM Ham Member QRZ Page

    w9KEY probably said it best, yet all of the other ops were correct. There are a couple of other things that you should be aware of: 1. Those older busses that originally did not have air-conditioning, yet have been retrofit- frequently become 'radio-killers' because the systems were not fully designed for the additional loading. This explains unexpected cut-outs as the radio can be 'voltage starved' by wiring that is really insufficient. If you add a very-small guage ( say 28 awg ) pair of wires directly to the feed for the radio ( usually a big solenoid to the left as seen by the operator ), you can monitor the voltage with a multimeter as the bus makes a few stops. Guess what? The opening of the door also creates a big electrical disturbance in some busses. No need to have the radio on for this test, unless you are ready to sacrifice one in front of other witnesses on the bus. Let them see the voltage during a jump start or a run, and then look at the specs. 2. B.T.W. double-check that your installers picked good grounding. I worked with one nit-wit who wanted to expedite things by grabbing a more convenient ground connection on a circuit breaker. It turns out that since he was not allowed to power up the bus, the breaker 'ground' was really the ignition circuit. That will fry an antenna coax shield, when the radio 'ground' is really hot. Getting back to the issue-Many modern 'jump-start boxes' actually put out about 17 VDC to 20 VDC. Anything over 14 VDC is certainly 'out-of-spec'.
  7. KB0MNM

    KB0MNM Ham Member QRZ Page

    BTW- That relay schematic from k7JEM is maybe the best thing you can do if the mechanics can not be taught. Be aware that most of the better 30A and 40A automotive relays also have a diode inside. You might be suprised at the color-coding ( white and black ) for the coil, yet probably counter-intuitive if you are thinking red and black for positive and negative. What? A white ground? ( yes, in some cases ). Best to fuse it rather than have the diode short the circuit down.
  8. KH7RD

    KH7RD Ham Member QRZ Page

    Thanks for input.....Radio;s are wired hot all the time, and using ignition sense trigger to turn on/off with ignition key. As noted, would be great to put a cutout switch in circuit, but the human factor would forget to turn radio on or off. I agree that the 25 amp fuse seems excessive. When we finally got a hold of radio tech, he was dumb..founded. This has happened on one older (10years) and two newer model buses. When I added ignition sense wiring, I did double check installers work, and even rewired a few shoddy installs....BTW school buses use white wire as positive!!!

    Welcome additional comments!! Thank you!
  9. WA8FOZ

    WA8FOZ Premium Subscriber QRZ Page

    Whenever I have to jump start a car, I disconnect all accessories. I learned to do this a long time ago after frying some gear. Any accessory that I install can be easily unplugged - PowerPoles, y’ know. So simple to do.
    G4COE and KU4X like this.

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