Thread: Bus is overheating.

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Originally Posted by johnboy

I've got a problem with the bus overheating around town, long steep inclines, or in heavy traffic.

No disrespect meant and you probably have already checked, but I gotta ask.
Idle, low speed and light load over heating can have several different cause, one, several or all can contribute.
* Low initial timing (sometimes moving vacuum advance from port to manifold fixes)
* Too lean fuel mixture at idle and transition circuits (richening the idle circuit sometimes helps)
* Insufficient low speed air flow, this one gets tricky because there are several causes of this,
A). Low voltage/amperage output at low speeds
B). Undersized wiring
C). Air flow restrictions (small grill opening, A/C condenser)
D). Undersized electric fan motor (there’s more to this)

JMHO;I would check and adjust initial timing to 12° -15° (this will probably require adjusting the distributor’s mechanical curve). Then follow that with adjusting the vacuum advance to add 10° at idle and cruise. I would then check/adjust the AFR at idle, transition and cruise circuits. Next I’d check the front of the radiator for air flow restrictions, often it is the A/C condenser mounted too close to radiator blocking air flow (1” space between rad and cond with sealed edges preferred).

After checking/adjusting the above, if the heating issue is still exists, I would check the wiring from the battery to the alternator, and the supply wiring to the fans to be sure they are sized large enough for the current demands.

Next I would check the alternator’s output to see if it is providing the current needed at low speeds.

Lastly (and this could be argued it should be checked first), I would check the CFM output of the fan and compare it to the current demand of the motor (I’ll explain this better below).

Many electric fan manufactures use a misleading rating for their fans “CFM" at no load. To me this is an OXYMORON, as there can not be a CFM rating worth its salt at NO LOAD. The load would be the power required to turn the fan blades at speeds sufficient enough to output the rated CFM (they cheat by using formulas on paper comparing the motor’s RPM at no load and the coefficient of the blades pitch and draw at that RPM), however as the load is applied the motor speed drags down from insufficient horse power (amps).

AND, many (too many actually) fan companies do this NO LOAD rating, resulting in unsatisfactory performance and consumers left scratching their heads trying to figure out why it’s not cooling. 2000CFM from a 5 amp motor AIN’T gonna happen.

If your fan is rated at xxxxCFM, but the motor’s current demand is too low (current demand is what the motor uses to operate and another way of gauging horsepower), it just can not perform adequately. Your fan’s electric motor’s demand should be at least 15 amps for a 16” fan (25 amps on a high output fan), and this leads into the next part of the equation (supply wiring and current available).

With higher current demand, higher outputs are required, the alternator and/or wiring may be undersized, and this leads into the old saying “there are no FREE lunches”. If the current demand is not available, the voltage drops, which in turn increases current draw and the cycle spirals downward.

Everyone buys into the ELECTRIC FANS gain horse power, while it's true when the electric fan is OFF, it's not true when operating. When all things are equal horse power is horse power wether mechanical or electrical.

Sorry for the book……….