CDM's and Ignition Sense

[SteveC0625]

I've got a client with three CDM1550's in their ambulance. All are set up for ignition sense and come on when the vehicle is started. Recently, two of them have stopped accepting ignition sense startup. I've verified that the ignition sense from the vehicle is correctly working by swapping in other CDM's. It's not the truck.

Here's the interesting part. The two failed radios both get their ignition sense voltage from the same circuit in the cab of the truck. The one that did not fail gets its voltage from a different circuit in the patient compartment. The truck recently underwent some major engine work. They didn't spot the ignition sense failure until sometime after the repairs were completed, but since the truck is shore lined when in quarters, it would have been easy to miss. My suspicion is that the ignition sense wiring got shorted out or grounded or some kind of surge, killing the ignition sense circuitry in these two units.

My question is: How much of a job to fix these radios are we looking at, and how costly? Does anyone know the Flat Rate Depot cost for these radios?

The truck is extensively wired with CDM remote cables so they'd like to stick with CDM's if at all possible. They also have invested in the CPS and OEM cables for programming so that's additional incentive to stick with the CDM's for a few more years. The county they are located in has a 100% inventory of Waris radios in the fire service which is even more reason to stay Waris for now.

So, repair or replace? Given the fact that CDM1250's are readily available for under $200, I'm pretty sure that replace is going to be a much less costly choice. The client never needed the CDM1500LS+ radios; CDM1250's are more than adequate for their needs.

[Bill_G]

Sounds like they need to do both. If their commitment to this model is that deep, then they don't really have a choice. They need to repair the ones they have to maintain their spares ratio, and they need to purchase as many good used radios as possible to increase it.

[ffexpCP]

Is there a tiny internal fuse on the ignition sense input? Maybe that fried.

I recall one on m1225. Can't remember if cdm has them too.

[jackhackett]

Is there a tiny internal fuse on the ignition sense input? Maybe that fried.

I recall one on m1225. Can't remember if cdm has them too.

Nope, no internal fuse for ignition sense. Fuses are usually used in models that don't have programmable ignition sense.

The first thing I would do is make sure they are programmed for it. If it took the users a while to notice it, maybe it never actually had it. In any case it's an easy thing to check before tearing into the radio or spending money on it.

If that's not the problem then the the radios probably took a surge or reverse polarity. There's a zener diode on the ign. sense line, if that shorted it would probably burn the circuit trace. Beyond that the exact circuitry depends on which board the radio has.

[SteveC0625]

Is there a tiny internal fuse on the ignition sense input? Maybe that fried. I recall one on m1225. Can't remember if cdm has them too.

Nope, no internal fuse for ignition sense. Fuses are usually used in models that don't have programmable ignition sense.
The first thing I would do is make sure they are programmed for it. If it took the users a while to notice it, maybe it never actually had it. In any case it's an easy thing to check before tearing into the radio or spending money on it.

Ignition sense worked just fine for several years on all three radios. But I did check programming first. All three use the same codeplug and were correctly programmed. The vehicle is shore lined and left with the windows up and doors closed. There's also a pa in the garage on the same radio channels so if you weren't standing right to the truck, you'd never hear the vehicle radios.

If that's not the problem then the the radios probably took a surge or reverse polarity. There's a zener diode on the ign. sense line, if that shorted it would probably burn the circuit trace. Beyond that the exact circuitry depends on which board the radio has.

That's my suspicion. When the engine work was done, they had to open up the cab side engine cover and remove some ambulance electrical components to complete the work. Since both radios get their ignition sense voltage from somewhere in cab, I figure they both took the same hit.

Thanks everyone for the info. It's off to the repair shop for a quote. If it's more than $150 -$200 per radio I think I'll have a chat with the Chief about simply replacing them.

[n1gtl]

$284 per radio would be what we get. This is a know issue with CDMs. If the voltage on the ignition pin spikes, it no longer works. If you are lucky enough to have the radio programmed for on/off and ignition, you can still power the radio up at least. For those who program "ignition only".... off to the depot.

You could always run the radios off a charge guard for about $90.

[Jim202]

$284 per radio would be what we get. This is a know issue with CDMs. If the voltage on the ignition pin spikes, it no longer works. If you are lucky enough to have the radio programmed for on/off and ignition, you can still power the radio up at least. For those who program "ignition only".... off to the depot.

You could always run the radios off a charge guard for about $90.

Voltage spikes are always a source of problems to the mobile radios installed in vehicles today. One of the questions I always ask is how old are the vehicle batteries. If the batteries are about 3 years or older, their internal resistance is getting on the low side. What this means is that starting the vehicle will have the engine turning over more slowly than with good new batteries. It also means the ability of the battery system to act as a good filter to noise and voltage spikes is now down in the pits. Another indication of old batteries is that they will cause the vehicle radio clock to start being reset to 12:00 as you try starting the vehicle.

With the larger alternators on emergency vehicles, you can see poor voltage regulation with load changes. What I am talking about is when you turn on or off the emergency lighting systems. With the use of the old incandescent lights, the higher the current switching caused by this lighting load. This high current draw will cause voltage peaks and dips in the vehicle battery voltage busses. If your using the new strobe packages, then your current draw in so much less and doesn't stress the voltage regulation any where as much as with the incandescent lights.

Vehicle grounding can be a source of problems. I always use a digital volt meter and do some checks when someone reports radio problems. I start by measuring the voltage between the negative terminal of the battery and the engine. Set your meter on a low scale if you have that option. You should not see anything except when trying to start the vehicle. Then check between the negative battery post and the frame of the vehicle. You need to do this with the entire emergency lighting load turned on.

Lastly, I do a battery voltage check. Normally I try to do this before the vehicle is started for the first time in the morning after sitting all night and not used. Your looking for the battery idle voltage. Put your meter between the negative and positive battery posts. You might also try from one battery post and the wire connector on the other battery post. Poor connection between the battery post and the connector can cause problems also. Then start the vehicle with no lights turned on and record the battery voltage. Now turn on the headlights on low beam. Record the voltage. Now turn on the high beam headlights and record the voltage. Now turn on the air conditioning on high and record the voltage. Now that you have gone through these steps, turn on the emergency lights, with the loads you have already measured turned on and record the battery voltage.

What your looking for is the knee of the current output of the vehicle alternator. If the idle is set according to the vehicle manufacturer, normally around 650 RPM, you should see the battery voltage stay fairly much the same. If it starts to drop with the load, you may have a problem with the alternator output capacity.

One other test I do with the heavy load on is to put your digital meter on the AC selection. Measure on the battery posts and see just how much AC voltage you may have. This is an indication that shows if you may have a bad diode in the alternator. You should not see more than around 100 to 150 millivolts of AC voltage. Much more than that indicates you may have a failed diode in the alternator.

On another point, most of these vehicles normally have 2 batteries with a selector switch to choose the #1 or #2 battery. It is not a good idea to run the vehicle in the both position. This becomes a problem with training as many dealers tell you to run this switch in the both position. This goes against why there are 2 batteries. The intent is to have a spare battery to start the vehicle, if the selected battery is dead or lacks enough power to be able to start the vehicle. The guide lines are to provide a backup battery in an emergency situation.

The second problem with running the vehicle battery selection switch in the both position is that if you have new batteries in one side and old ones in the other side selection, you can cause a problem when you select the both position. What can happen is is the 2 battery banks are of different ages, you could cause such an inrush current that you could cause an explosion of one of the older batteries. The inrush current is from the higher voltage batteries trying to equalize the lower voltage (old) batteries.

What most people don't know is that on the emergency vehicles, there should be a diode isolation block assembly that allows charging both batteries when the engine is running, regardless of which battery is selected, not both. This is the reason that the diode isolation unit is installed. It keeps the different battery units isolated from each other, but allows them to be charged when the engine is running.

While I am on the subject of dual battery banks and starting a vehicle, I have seen bad habits used by fire fighters while starting the fire trucks. On this type of vehicle, you generally will find 2 separate starter motors. On the dash will be 2 start buttons right next to each other. Once a bad habit is started, it is hard to break. What they were shown by the dealer rep was to press both starter buttons at the same time. This is about the worst you can do to the electrical system. First, if there is a jam of some sort on one starter motor, you now cause major damage to the other starter motor and maybe even damage the flywheel ring gear. Then there is the huge current draw that the electrical system was not designed for. Remember earlier I told you that the dealer rep told everyone to put the battery selector switch in the both position. Now you have compounded the possible damage to the vehicle.

Enough on bringing some of the electrical issues to the surface. Please come back to the group here and let them know what you find with your radio problems.

One last comment is to have a sign in the vehicle that can be flipped from #1 to #2 to indicate which battery bank to be used next. This way the next person using the vehicle will know which battery bank to select. Hang it over the starter button or some other place that it is in the way of starting the vehicle so you can't miss it. When I say flip the sign, you put the number 1 on one side and the number 2 on the other side. Even a 5th grader can understand that.

Jim

[ve3nsv]

Are you using emerg? If not you could jumper pin 7 & 9 together and just switch the main line to switched ignition power. This will turn the radio on whenever power is applied to the radio power connector. I know this solution isn't ideal but will keep this radio in service until they decided to switch to something a little newer.

[Bill_G]

I've had after market rooftop air conditioner condenser motors toast ignition sense circuits when they switch off. Chocolate stains and moon craters in the open collector devices. 100PIV diodes inline with the sense lead stopped that nonsense.