Electronic Siren Troubleshooting Tips
Electronic sirens use high power-transistorized amplifiers that do not forgive mistakes. Therefore, you must be careful when installing a new siren, and must quickly repair any wiring or other problems that develop in an old installation. Often, by the time you are aware of a problem, electronic damage has already resulted.
I hope the following tips will help owners of antique and classic emergency vehicles who may not be familiar with electronic sirens.
GENERAL TIPS:
Before installing a siren, it is important to know whether the vehicle is positive or negative ground. Some older vehicles, and some large trucks are positive ground. All electronic sirens I've seen are designed for negative ground installation, and installing them on a positive ground vehicle will destroy them instantly.
When installing a siren, the speaker wattage must at least equal or exceed the siren amplifier's output power (rated in watts). You can use a 100 watt speaker on a 58 watt siren, but not a single 58 watt speaker on a 100 watt siren.
Speaker impedance (rated in ohms) must be correct! A brief impedance mismatch, short or ground in the speaker wiring will destroy the siren amplifier almost instantly
High-power transistorized amplifiers require speakers to be connected to them to provide a load to dissipate the output transistor's heat. NEVER turn on the siren without a proper speaker connected, or you may damage the siren.
INSTALLATION TIPS:
We will assume that both your siren and speaker are correctly matched to work together and that both are in good working order.
1. Mount the siren and speaker, but do not connect the siren to power. Run your speaker wiring first, but do not connect either end.
2. Always run new wire. Heavy-gauge lamp cord (sometimes called "zip cord") with stranded copper conductors works well. Make sure it has thick insulation, because vehicle wiring is subject to more wear than home wiring. It's cheap to buy (much cheaper than a new siren), and can save having to replace your siren. I prefer not to splice wiring unless absolutely necessary. The more splices you have, the greater the chance for future problems. If you must splice the wire, it should be firmly twisted together and soldered. After soldering and trimming the joint to remove stray strands, wrap the splice with a several layers of good quality plastic electrical tape to prevent future shorts or grounds.
3. Once the speaker wire has been run between the siren and the speaker location, it is best to check the wire before connecting it. Please see "Check the Speaker Wiring" below.
4. After the speakers are installed and connected, connect the positive 12 volt wire from the siren to a suitable power source.
TROUBLESHOOTING TIPS:
Volume is low, siren may sound "garbled" like it is under water.
This usually indicates blow power output transistors. It is also possible that the speaker coil is defective (or both). To troubleshoot, do the following:
1. If possible, connect the siren to a speaker that you know is good. If it works OK, the problem is the speaker coil. You can replace the coils on Federal and some other speakers, but the cost used to be pretty close to a new speaker. Be sure to use the same power and impedance coil if you replace it. The coil unscrews from the back of the speaker cone.
2. If the siren volume is still low when connected to a speaker you know is good, the power output transistors have probably blown. Unless you have the repair parts and an electronic background to know how to properly replace them, you are better off to take the siren to a repair shop. Frankly, even though we repaired lots of sirens and other amplifiers with no later problems, it is not uncommon to have a siren go out again if the output stage blows. If it were mine, and if I could find a cheap used siren to replace it with, I would just swap sirens.
3. Before reinstalling the siren, check the speaker wiring as shown below.
Siren fuse blows:
While fuses occasionally blow for "no reason," more likely than not a blown fuse indicates another problem. If you replace the fuse and the replacement fuse blows, whether immediately or some time later, identify and repair the cause of the problem before inserting a second replacement fuse or risk causing further damage. Fuses generally blow because of blown siren output transistors, short/open in wiring, or defective speaker coil.
1. The factory installed a certain type and size of fuse for a reason - to protect the siren's electronic circuits. Look for a sticker on the siren or fuse holder that specifies the correct fuse size and type, or check the owner's manual if you have one. Don't simply replace it with the same size that was in it (someone else could have installed the wrong fuse), and NEVER replace the fuse with a larger one (the old penny-in-the-fuse box scenario). Driving a fire truck with smoke coming out of the dash is only funny to the folks who are watching.
Most of the fuses are fast-blow. Don't replace a fast-blow fuse with a slow-blow, and don't replace a slow-blow fuse with a larger fast-blow fuse.
2. Check the siren's power wire to be sure that the insulation hasn't been pinched or rubbed, causing a short to ground. If you find one, wrap the worn insulation with several wraps of electrical tape and try to re-route the wire so it doesn't get pinched or rub in the future.
3. Check the siren's speaker wires as shown below.
4. If the fuse blows again, after checking to be sure that the speaker wires are not shorted or open, there is an internal problem with the siren electronics. Unless you can do electronic repair, take the siren to a qualified repair shop for service.
CHECK THE SPEAKER WIRING:
IMPORTANT: The wiring must be disconnected from both the siren and speaker before doing this test. The same test is used for new installations and existing ones.
You need a volt-ohm-meter and a jumper clip to check the speaker wire. Any cheap volt-ohm-meter (VOM) will work, as we will not be doing any precise measurements. The jumper clip is a short insulated wire with alligator clips on each end.
Strip between ¼" and ½" of insulation from both sides of each end of the speaker wire.
Set the VOM to the Rx1 scale. When you touch the leads together, the meter should give a reading. You don't care what reading, just so the meter deflects when you touch the leads together. If you don't get a reading, check the meter's internal 1.5 volt battery. It is required for this test. They usually use a AA alkaline battery - replace if necessary.
Connect one end of the jumper clip to a bolt or other unpainted part of the vehicle's body, then connect the other end of the jumper clip to one of the meter's leads (it doesn't matter which meter lead). Touch the other meter lead to an unpainted part of the vehicle's body. You should get a reading (again, we don't care what). If you don't get a reading, check the connection of your jumper clip. Move the jumper clip or your free meter probe to another part of the body if necessary until you get some kind of a meter reading. When you get a meter reading, move to the next step.
Touch the free meter lead to one side of the speaker wire. You should NOT get a reading. Do the same for the other side of the speaker wire. Again, you should NOT get a reading. If you get a reading from either side of the speaker wire, the wire's insulation has been nicked and is grounding to the vehicle chassis. This will destroy your siren. Find and repair the nick in the insulation until you do NOT get a reading from either side of the speaker wire. Try to re-route the wire so it doesn't get pinched or rubbed in the future.
Now, go to the opposite end of the speaker wire and twist both stripped leads together. Go back to the end where your meter is connected and remove the jumper clip from the vehicle's body. Clip it to one side of the speaker lead. Touch the free meter lead to the other side of the speaker wire. With the speaker wire twisted together on the opposite end, you SHOULD get a reading. This indicates that the wire is not broken inside the insulation and had not been cut in two anywhere.
It is now safe to connect the speaker wire to the speaker(s) and siren unit. At the siren, be VERY careful that no stray strands of wire spread between the speaker terminals, causing a short. I prefer to twist the ends of the speaker wire tightly and coat them with solder or to crimp insulated "Y" lugs on them before connecting them to the siren to prevent problems.
It is important to connect the speaker(s) correctly. I prefer to solder the wire joints and wrap them with several wraps of quality plastic electrical tape. I use Scotch 3-M. Don't cut corners. Cheap tapes like RadioShack will come loose, either letting moisture in the joint or allowing them to short against the body. If you use crimp connectors instead of soldering, they should be jelly filled or securely wrapped with electrical tape to keep out moisture.
