circuit that's 12/24 volts capable

So here is the problem, i install gps units for a company that sources their units from a japanese factory, anyway, the unit is 12/24 volts capable and have had few issues, one arose today however that i want to try figuring out, the    unit was installed in 1991 nissan b13, the customer got into a problem where the vehicle's engine did not wanna turn over, i interrupted the fuel line in the car, and after troubleshooting, found that the unit's 1 amp fuse had popped, this fuse is on the red constant 12 volt source. I also.found that the unit was allowing a ground signal to flow through the gwa wire, which caused the fuel cut relay to energise. My supervisor says, there was a voltage rise of 16.5 volts on the unit which caused the interior components to burn and thus allowing the ground to flow. My question is this, would a curcuit capable of operating at 24 volts burn if installed in 12 volt vehicle with a voltage spike that high. Bear in mind he did no physical checks to the unit, he claims to have checked it via a diagnosis software on his pc while in his office and i had the unit in question in the said vehicle.

Why are you not interrupting the starter wire? Interrupting Fuel or Ignition wires are very dangerous.

I agree with IAAI. Especially interrupting the fuel is the vehicle is in motion (lean detonation etc & engine or injector damage - as idiots with oil-pressure controlled fuel pumps sometimes have the pleasure of finding out LOL!). But a diode between the gwa and the relay solve the issue. Generally even automotive 12V devices will handle spikes of well over 16V (eg, 200V - 400V spikes), but that's for spikes, not a constant 16.5V, and it refers to input (power) voltages, not output circuits.

I know interrupting the fuel line is dangerous, but it's my job, thanks for the reminder anyway, appreciate it. However i'm simply looking for an answer to the question, if a unit operates 12/24 volts, would it be safe to say that any voltage under 24 volts would be under proper circumstances, not able to destroy any components, since they are rated to withstand voltage of 24 and under.

Stopping fuel for testing etc - and for stopping diesel - is fine. I and IAAI are talking about fuel cuts as a method of stopping engines - that can damage engines as well as injectors. (My point re (petrol) fuel pumps is that they should be controlled by spark, alternator charge lights, air flaps, ECUs etc but NEVER oil pressure. Oil pressure switches should control ignition kills, but cars usually employ the best and safest method, namely idiot lights or buzzers etc. I advocate the only improvement is a similar idiot light or buzzer that also triggers a (say) 3-second circuit that can be overriden else it kills the ignition. But I know monkeys that still persist with "traditional" oil-pressure fuel cut circuits!) Sorry, that's a bit of a hobby horse that has uncovered some truly moronic "industry experts". As to "12V rated for automotive applications" (ie, 12V battery charging systems), my general design rule it that 12V means 8V to 16V. I find that that is a common design rule or expectation. It doesn't necessarily mean that all things will operate in that range, but certainly they should not be damaged. Some devices might require above 9V or 11V etc to operate. With that 8-16V rule comes implicit typical 12V system transients and spikes, eg, up to 400V spikes. Domestic etc 12V equipment and devices are NOT the same as automotive 12V equipment. 24V systems merely double the voltage range, ie 16V - 32V, and maybe a proportional spike tolerance.    It is also expected that under-voltage and dips do not damage equipment, tho like AC brown-out situations, that can have complications and exceptions.     Hence IMO your 12/24V unit should tolerate and probably operate with an 8V to 32V "steady" supply plus typical automotive transients.   

However having something back-feed into the unit's output is a different issue. But that is part of device installation - ie, ensuring outputs are not overloaded by high-current loads, and that outputs are isolated from other power sources (hence the use of isolation diodes). Some outputs may require spike protection if relays are used (ie, a reverse-biased/connected diode across the relay coil else unit's output). Sorry for the long winded reply, but ensure your unit has not destructed because of back-feed or spikes into its output(s) etc.

I've never met a tracking unit that wasn't 9-28 volts! What caused the problem was probably no diode across the 85/86 terminals, the spike quencher also shuts down OS feed from the relay (86) which might have damaged the unit.

It's so nice having a succinct answer, and from such a knowledgeable person! Alas I got my diode solution from another (audio?!!) forum looking up what the heck GWA meant. But lacking specific knowledge, I emphasised the "12V design" power side of things. (Poor t&t tech. He'll be glad your back so I can refrain...)

16.5 vdc on a 24vdc is not too much voltage. They most likely are using a regulator circuit that can tolerate 30vdc or more. What is the point of this system? Is it for tracking and recovery (which I assume it is)? If so, then removing 12vdc from the unit and it rendering the car inoperable may actually be a fail safe. If you don't want it to operate this way you could always tap the 12vdc feed for the fuel cut relay in AFTER the fuse - if the fuse pops no voltage for the fuel cut.Kevin Pierson

Yes KP it's for track and recovery, thanks guys for responding, it's apreciated.