Rooftop Unit With an Electrical Fault and No Cooling: Diagnosing Contactors, Caps, and Controls
A packaged rooftop unit calling for cool but blowing nothing but air is almost always an electrical story — a contactor that won't pull in, a run cap that's lost its microfarads, or a control circuit that opened somewhere between the thermostat and the contactor coil. R-Pro's field app walks you from symptom to cause to repair steps right on the roof with no signal, while the office ERP turns that same job into a quote, an invoice, and a posted accounting entry without you typing anything twice.
On the roof: isolating contactor, cap, or control
- Confirm the call first: 24V at the contactor coil (R to Y/C) means the control side is good and the problem is downstream; no 24V sends you back to the thermostat, transformer, low-pressure/float switch, or a tripped safety in the control string.
- Contactor not pulling in with 24V present = bad coil or stuck/welded contacts; pulling in but compressor and condenser fan dead = pitted, burned contacts dropping voltage across the points — check line voltage in vs. load voltage out.
- Compressor hums and trips on overload while the fan runs (or vice versa) points straight at a weak or open run/dual-run capacitor — verify the microfarad rating against the nameplate, not just "it looks fine."
- Don't miss the simple killers: blown 3- or 5-amp control fuse, tripped condenser high-pressure switch from a dirty coil, a chewed low-voltage wire, or a transformer that reads 0V on the secondary.
- R-Pro's AI diagnosis is built on 600+ real HVAC and refrigeration cases, so "RTU, calls for cool, contactor won't engage" returns the likely causes and the order to check them — and it all works fully offline when the roof has no signal.
Capture the job once, on site
- Scan the RTU nameplate with your phone and R-Pro auto-fills the make, model, serial, voltage, and compressor data into that customer's record — no squinting at a faded plate and retyping later.
- Log the actual finding (e.g., "40/5 MFD dual cap reading 31/4, contactor points pitted") as a quick voice memo instead of thumb-typing on the roof in the wind.
- Everything attaches to that site's per-customer service history, so the next tech — or you in six months — sees exactly what failed and what was swapped.
- Pull the part off your truck and scan the supplier receipt; R-Pro reads it and drops the cost straight into the job, ready to flow to the books.
- Issue a service receipt to the customer on site the moment the unit is cooling again, before you've even climbed down.
In the office: quote, invoice, and book it
- Turn the field findings into a clean quote — itemized for the dual-run capacitor, the contactor, and labor — and send it for approval before you order parts on a bigger repair.
- Raise a proper invoice or tax invoice using YOUR country's tax setup (VAT, GST, or sales tax — your own name and rate), not a generic template that doesn't match your filing.
- The capacitor and contactor you pulled off the truck come out of inventory, and the supplier receipt you scanned posts to purchases — your parts cost and margin on this RTU are accurate, not a guess.
- Accounting updates automatically: this job's sale, the part purchase, and any expenses roll into sales, purchases, and net profit, and onto the customer ledger.
- Because the field app and the ERP are one system, the roof job and the office books are the same job — no double entry, no end-of-month reconstruction from crumpled receipts.
Two tools, one RTU call, zero double entry
R-Pro pairs a field app and an office ERP as two equal tools that work together. On the roof: AI diagnosis from 600+ real cases, nameplate and receipt scanning, voice memos, per-customer history, and on-site receipts — fully offline. In the office: quotes and itemized estimates, invoices and tax invoices with your own country's tax rate, accounting, inventory, suppliers, and customer ledgers. One subscription, 10 languages, and every rooftop job flows straight into your books.
Get R-Pro →FAQ
There's 24V at the contactor coil but it won't pull in — is the contactor bad?
Almost certainly. With confirmed 24V across the coil terminals and the contactor still not engaging, the coil is open or the armature is stuck. Swap the contactor (match the coil voltage, pole count, and FLA rating to the nameplate). If it pulls in but the compressor still won't run, check for voltage drop across pitted load contacts — burned points can hold the contactor closed yet starve the compressor.
How do I know if the run capacitor is the cause and not the compressor itself?
A weak or open run cap usually shows as the compressor humming and tripping on its internal overload, or the condenser fan straining or not starting, while everything else has power. Read the cap's actual microfarads with a meter and compare to the nameplate rating — a 45/5 dual cap reading 30/3 is failed even if it looks fine. A genuinely failed compressor typically shows a grounded or open winding or a locked rotor that a new cap won't fix.
The control circuit has no 24V at all — where do I start?
Work the control string backward: check the low-voltage transformer secondary (should read ~24V), then any inline 3–5A control fuse, then every safety in series — low-pressure switch, high-pressure switch, float switch, and the thermostat itself. One open safety or a single chewed low-voltage wire drops the whole circuit, and the compressor never gets the command to start.
Can I do all the diagnosis and paperwork without signal on the roof?
Yes. R-Pro's field app — AI diagnosis, nameplate and receipt scanning, voice memos, service history, and on-site receipts — runs fully offline. When your phone reconnects, the job syncs and flows into the office ERP, so the quote, invoice, parts, and accounting are ready without re-entering anything.