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I own a 1992 Bluebird bus which has a Hurri-Hot heater that recently stopped working and I'm running out of ideas why. When I turn on either switch to activate either of the two fans, a circuit breaker on the panel trips immediately. I can hear it click and I can also see that the voltage cuts off. After roughly 10 seconds the breaker tries to reset and instantly trips. I can hear the periodic click if I leave either of the switches on.

To clarify, there are two fan motors on the heater with two separate switches that are both protected by a single 20amp breaker. Turning on either fan on any setting trips the breaker. Also, I haven't talked about the heat source because it's basically just a radiator (warmed off the engine fluid) that the fans blow air across. They warm up just fine. So I think we can disregard the plumbing.

Here is an overall diagram of the wiring. enter image description here

The battery power passes through this breaker. enter image description here

These two switches energize wires 151&152 and 154&155 which lead to the heater fans. enter image description here

Finally, here are the two fans on the heater that aren't working. I've removed the cover to give us a better look. enter image description here

Things I have tried:

  • I did the standard short circuit test by splicing my 12v tester between the positive battery terminal and the hot wire... no evidence of a short. I tested by turning on the headlights etc and the light goes on.
  • I replaced the circuit breaker with a brand new one. No change at all.
  • I "hot wired" one of the fans by running a wire straight from the positive battery terminal to wire #151 (spliced right by the fan) and it worked fine. So I know the fan motors work.
  • I bypassed wire #151 from the switch to the fan. With this I can run that fan on HI but it still trips the breaker if I turn it on low.
  • I bypassed wire #152 from the switch to the fan. With this nothing changes... both settings on that fan trip the breaker immediately. Not sure what this indicates.
  • I have closely inspected the wires all the way from the heaters back to the switches into the panel where the breaker is. There is no damage to the housing or anything resembling a short. There is a 3 foot span of wire I can't inspect but it's very well protected under factory-installed piece of trim that has never been removed... so I'm pretty confident there's no short under there. (This fan worked recently and nothing has been disturbed near that 3 foot span since then.)

So I'm stumped! My mechanic mentioned there might be a short IN the fan motor, but I don't think that's likely since there's TWO motors and the odds of BOTH developing internal shorts at the same time seems unlikely. Also remember, they worked recently and they haven't been messed with since.

What am I missing?? Thanks in advance.

  • Have you a current clamp? – Martin Jan 3 '18 at 8:09
  • You have not said anything about the heat itself. How is heat generated? Is it by electric element or heat from the engine coolant? If it is electric, that is a major factor. – CharlieRB Jan 3 '18 at 13:58
  • @CharlieRB Thanks. The heat is from the engine coolant pumped through the system from the engine through the heat exchangers in the unit. The fans just blow air across that to distribute the heat. The heat part is working fine, as the hoses and the heat exchangers get nice and warm to the touch. I'll update the question. – emersonthis Jan 3 '18 at 18:12
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When driving an inductive load like a motor, if the voltage goes down, the current goes up. I suspect you aren't giving the motors the full voltage. Since both fans seem to suffer the same problem, I would start checking for high resistance/voltage drops in the shared wiring (from the circuit breaker back to the battery, and the grounds).

Also, just to verify, those aren't 24 volt fans are they?

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Well this is one of those head-smackers...

Long story short: there WAS a short in wires 151/152.

Most likely they heated up and melted together but didn't show any visible damage from the outside. This was confirmed with lots of testing, but basically my initial experiment of bypassing the wire from the switch was correct. In the end, the real mystery was that I was looking for exposed wire touching metal or something visible. It didn't occur to me that the paired wires were shorting against themselves from inside the housing. Live and learn.

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