MACS Service Reports - 2015 - FEB2

between the relay and the clutch. And that really is
about as far as you can go with guessing.
But a lot of technicians will keep on guessing, jumping across a low-pressure and a high-pressure switch.
Maybe that will work occasionally, but it also doesn't
take long to at least look at the circuit, because we
see a lot of compressor clutch circuit failures that reflect some of the individuality in circuit design. And
it's not always on a new model. Pickup trucks stay in
use "forever," and here's one from a 1994 Dodge Ram
(it's "just Ram" now) heavy-duty (2500 series) pickup
(Figure 1). We chose the diagram from prodemand.
com because it managed to fit the A/C circuit into a
single diagram, whereas the OE is spread across two
diagrams. However, we're adding an OE illustration
as Figure 2, which shows the Power Distribution Center, which has the layout of the relays and fuses, and

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you can see that the Horn-A/C fuse is at the top right,
centered between three fuse holders.
In this case, reviewing the wiring diagram shows
that the wire to the compressor clutch is a tap off the
horn relay circuit, so if you believe in the logical approach to diagnostics, the obvious (and super-easy)
check is to just try to blow the horn. It doesn't sound,
so the next step is equally obvious: check the "B" fuse
for the horn, which is located in the same under-hood
Power Distribution Center with the relays for the
horn and compressor clutch. The fuse is blown, and
a new fuse restores the A/C clutch circuit. The blown
fuse could be just one of those things, but if blowing
the horn causes the fuse to blow again, there's apparently a current draw issue, and an addition to the job
ticket for more diagnosis. ■

Figure 2: The Ram Power Distribution Center layout: Fuse B is the one you're looking for and it's in the middle fuse holder at the upper right.

February 2015

2

MACS Service Reports



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Table of Contents for the Digital Edition of MACS Service Reports - 2015

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