How to Test an Electric Range Surface Element and Infinite Switch
Why this matters
A surface element that will not heat, heats only on high, or heats in fits is one of three parts: the element, the terminal block (receptacle) it plugs into, or the infinite switch behind the knob. All three are inexpensive, which is why techs guess, and a guessed element plugged into a burned receptacle arcs and pits its new terminals within weeks. The switch is the one people skip because testing it means reading 240 V at the console. One gate settles it: does the switch put voltage on the element terminals, cycling on low and steady on high? Run that gate first and the remaining readings each have one job.
Power first, and the one live reading
Every resistance reading is power off. Unplug the range or turn its 240 V breaker off, and prove absence of voltage at the range terminal block with a tester proved on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition; check the current edition for its number) and 29 CFR 1910.333(b)(2). Remove the console back or the switch cover with power off. 'Appliance Electrical Safety Check Before Service' covers the receptacle and cord checks.
The voltage gate is live, at H1 and H2 only. With power off, clip insulated probe leads onto the switch's H1 and H2 terminals, route the leads out of the console so the meter sits on the counter, then restore power and turn the knob from out front. Your hands stay out of the console while it is energized: line terminals sit beside the H terminals on a crowded switch body, and a probe tip slipping between them shorts 240 V. Use a meter rated CAT III or higher. Power off again before unclipping.
Coils burn. A surface element stays hot enough to burn for several minutes after it is turned off. Let it go dark and cool before lifting it out of its receptacle.
How the switch drives the element
INFINITE SWITCH, TERMINAL MAP (one common layout)
L1 from range block L2 from range block
| |
| cycling contact, | line-break contact,
| bimetal opens and closes | closed whenever the
| on low, held shut on HI | knob is out of OFF
v v
H1 --[receptacle]--ELEMENT--[receptacle]-- H2
P pilot terminal, live when knob is out of OFF
|
pilot lamp -- other lead per the diagram
The infinite switch controls heat by time, not voltage. A small heater inside the switch warms a bimetal strip that opens and closes the contact feeding H1. At a low setting the cam lets the bimetal open the contact after a short on-time and keeps it open for longer; at higher settings the on-time grows; at HI the cam holds the contact closed. The second contact, L2 to H2, closes as soon as the knob leaves OFF, so the element is never left live on one leg with the knob off. The pilot terminal P lights the "element on" lamp; where its lamp's other lead lands varies by range, and the wiring diagram shows it.
So the reading across H1 and H2 at the switch is either full line voltage, about 240 V, or zero, and on a healthy switch at a low setting it alternates between the two. Some switches open both legs together; on those H1 to H2 still reads 240 V or zero, it just gets there differently.
The gate and the readings it routes to
Read H1 to H2 on a low setting, then on HI. Set the knob to 2 and watch for four minutes, then turn to HI. Pass: on 2, voltage appears for short periods and drops to zero for longer ones, repeating; on HI it holds steady. Fail one way: zero on every setting below HI and full voltage on HI, which is a switch whose cycling contact never closes. Fail the other way: steady voltage on every setting, which is a contact welded shut, and the element glows red on low.
Read the element against the wattage check. Element out, read across its two terminals. The rating is stamped on the element or in the tech sheet; R = V squared / W gives the sanity value. For an 8-inch coil rated 2,100 W at 240 V, that is 57,600 / 2,100 = 27.4 ohms. Within a few percent of the sheet value passes, OL is open. 'Estimating Heater Resistance From the Rating Plate Wattage' owns the method.
Each element terminal to its sheath. Highest resistance range. Both must read OL; any reading is insulation breakdown, and that element can energize a pan.
Inspect the element terminals and the receptacle. Look for blued or pitted element terminals, a cracked receptacle body, blackened contacts that have lost their spring, or melted lead insulation behind it. A burned receptacle is a high-resistance joint that heats itself and the element terminals together.
Swap a known-good element into the suspect position. Same diameter, from a position that heats normally. If the known-good element misbehaves in the suspect receptacle, the receptacle or its leads are the fault. If the suspect element misbehaves in a good position, the element is.
Clamp the element current on HI. With power off, close the clamp around the H1 lead at the switch, leave the probe leads clipped, then restore power and turn the knob to HI from out front, hands out of the console as before. Expected current is W / V: 2,100 W / 240 V = 8.75 A, a little less once the coil is red, because its resistance rises as it heats. Voltage present with current near zero means an open you have not found yet, usually a lead burned off behind the receptacle. Current well above the calculation means a partly shorted coil. This reading also catches what the gate cannot: a switch whose contacts are burned enough to drop voltage under load reads 240 V with the element unplugged and noticeably less with it drawing current, so read H1 to H2 again while the clamp shows load.
Replace and repeat the gate. A switch gets its wires moved one at a time from old to new with the terminals matched by label, never by position on the body. A receptacle gets the kit with its own high-temperature leads and connectors; ordinary wire and wire nuts do not survive under a cooktop. A burned receptacle and the element whose terminals it pitted go together; either one alone will damage the other again.
Case one: the right front only works on high
The right front 8-inch coil heated fine on HI and not at all on any lower setting.
- Range unplugged, absence of voltage proved at the terminal block.
- Leads clipped to H1 and H2 on the right front switch, power restored, knob to 2: 0 V for the full four minutes, no cycling. Knob to HI: 241 V steady.
- That is the first fail pattern. The cam was forcing the contact closed on HI, and the bimetal never closed it on its own.
- Element anyway, with power off: 27.6 ohms against the calculated 27.4. Terminals OL to sheath. Terminals and receptacle bright and tight.
- New switch, wires transferred one at a time by terminal label.
Retest with the leads back on H1 and H2: on 2, voltage came on for about 10 seconds of every 40, repeating; on 5, roughly half on and half off; on HI, steady 240 V. A pan of water came to a boil on HI and held a simmer on 2. Ordering an element on the complaint would have been an element that tested good on the bench and a second trip for the switch.
Case two: the left rear heats in fits
The left rear 8-inch heated, then went dark, then came back, and the customer said it worked better when she pressed on the pan.
- H1 to H2 gate on the left rear switch: on 2 it cycled normally; on HI it held 240 V. The switch passed. The fault was downstream of it, and the gate had just sent the hunt to the element and receptacle.
- Element: 27.3 ohms, terminals OL to sheath. It passed on resistance, but both terminals were blued and one was pitted.
- Receptacle: one contact blackened and loose.
- Swap: the known-good right front element went into the left rear receptacle and heated intermittently there too. The receptacle was the fault.
The receptacle was replaced with the kit's leads, and the element with it, because pitted terminals in a fresh receptacle make a new high-resistance joint. Gate repeated: cycling on 2, steady on HI, and the coil held a steady glow on HI with no flicker when the pan was moved. Had the element alone been replaced, the new one would have arced in the old receptacle and pitted within weeks.
Where the answer changes
Glass-top radiant elements. You cannot swap one into another position, and many carry a limiter on the element that opens if the glass overheats. The gate still works at H1 and H2; the element and limiter are read power off from the tech sheet's diagram.
More than one element dead. Two or more elements out at once points away from switches and toward the supply: a lost leg at the breaker, the cord or the terminal block. Read L1 and L2 to neutral and L1 to L2 at the terminal block before anything at the console.
The element heats and the pilot lamp stays dark. The heat circuit is proven; the fault is the lamp or its lead from P. It still gets fixed, because the lamp is how the household knows a coil is live after it stops glowing. With power off, read the lamp for continuity per the diagram, and with the leads clipped and power restored, read P to the lamp's other lead with the knob on.
Electronic controls. Ranges with touch controls switch elements through relays on a board, not infinite switches; the tech sheet's relay output test replaces step 1.
How to verify you got this right
- On the repaired position, H1 to H2 cycles at a low setting and holds about 240 V on HI.
- Element within the sheet value, both terminals OL to sheath, and no pitting on terminals or receptacle contacts.
- Steady heat on each setting: a simmer that holds on low, a rolling boil on HI, no flicker when the pan moves.
- Readings and the sheet values on the ticket, console closed, power restored, pilot lamp lighting with the knob on.
References
- See related: Estimating Heater Resistance From the Rating Plate Wattage; Appliance Electrical Safety Check Before Service; How to Read a Wiring Diagram You Have Never Seen
- NFPA 70E, Standard for Electrical Safety in the Workplace, Article 120, current edition
- 29 CFR 1910.333, Selection and use of work practices (OSHA general industry)
- Manufacturer tech sheet for element ratings, switch terminal identification and the wiring diagram