Ballast resistor questions almost always come down to three numbers: how many ohms your coil reads, how many ohms sit in front of it, and what your ignition wants. This page puts the factory numbers for GM, Ford and Mopar next to PerTronix, MSD and Accel instructions, then lets you run the math for your own setup. If you already suspect a bad resistor, start with my step-by-step test guide: How to Test a Ballast Resistor on a Classic Muscle Car With a Multimeter.
Quick answers
- GM points cars use a resistance wire of 1.8 ohms (service manual figure) feeding a Delco 1115202 coil of about 1.24-1.46 ohms. [1][16]
- Ford points cars use a 1.30-1.40 ohm resistor wire with a 1.40-1.54 ohm coil. [4]
- Mopar electronic ignition (1972-79) uses a dual ballast: 0.5-0.6 ohm coil side and 4.75-5.75 ohm ECU side. 1980-82 uses a single 1.25 ohm resistor. [9]
- PerTronix Ignitor needs at least 1.5 ohms total primary on V8s and 3.0 ohms on 4 and 6 cylinders; Ignitor II 0.45 ohm; Ignitor III 0.32 ohm. [21][22][23]
- An MSD Blaster 2 (0.7 ohm) on points needs a 0.8 ohm ballast; with an MSD 6 or 7 box it does not. [24][14]
- HEI needs full battery voltage; bypass the resistor wire. [29]
Ballast resistor and coil calculator
Pick your ignition type and coil, enter what sits in front of the coil, and the calculator gives total resistance, coil current, voltage at the coil, and heat in the resistor, then checks it against the manufacturer rules in the tables below.
Ballast Resistor and Coil Calculator
Ohm’s law estimate only (I = V/R, P = I²R). Real resistors and coils change resistance with temperature. Specs and sources are in the tables on this page.
What does a ballast resistor do?
A ballast resistor (or resistor wire) limits current through a points-type coil and the points during normal running, and is bypassed while cranking so the coil gets full battery voltage for starting. [25][11] Without it, a ballast-type coil and points run at much higher current and wear quickly. [25][31]
How many ohms is a factory ballast resistor or resistor wire?
GM: 1.8 ohms. Ford: 1.30-1.40 ohms. Mopar points: Standard RU-11 reads 0.5-0.6 ohm cold and is rated 1.75 ohms operating. Mopar electronic: 0.5-0.6 ohm thermal or 1.25 ohm non-thermal coil side. [1][4][7][9]
| Make / system | Years | Type | Resistance (ohms) | Part numbers | Crank bypass | Notes | Source |
|---|---|---|---|---|---|---|---|
| GM (Chevrolet, Corvette, Corvair and other GM points cars) | Most 1957-1974 (PerTronix); 1965 Corvette and Corvair manuals cited | Resistance wire in harness (glass/cloth braided on Corvair) | 1.8 ohms (service manual figure) | Used with Delco 1115202 coil | Starter solenoid R terminal feeds coil + during crank | Year range differs by source: PerTronix says most GM 1957-1974; Hemmings says a resistance unit was required on GM cars newer than 1963; Corvair resistance wire from about 1960. One Corvair owner measured originals and a new replacement at 2.2 ohms, so real wires can read above 1.8. | [1][2][3] |
| Ford (points ignition) | 1957 shop manual spec; most 1957-1974 (PerTronix) | Resistor wire (pink) in harness | 1.30-1.40 ohms | C0LF-12250-A (pink, 61.49 in, #20 gauge) | I terminal on the 4-post starter relay feeds coil + during crank | Ford points spec quoted: amp draw 4.5 A engine stopped, 2.5 A idling. | [4][5][6][3] |
| Ford DuraSpark II | 1977-1991 FoMoCo | Resistor wire (red with green stripe) | 1.30-1.40 ohms | D7AZ-12250-A (Motorcraft DY-213), 49 in, #16 gauge | I terminal on starter relay | Part data quoted from Ford parts catalog by a forum member. | [6] |
| Chrysler / Mopar points (2-terminal ceramic ballast) | Fits 1953-1973 Chrysler applications per SMP listing | Ceramic ballast resistor, 2 terminals | Cold 0.5-0.6 ohms; operating 1.75 ohms (SMP spec) | Standard RU-11; Echlin ICR12; Chrysler 1689336, 1842522, 2095501 and others | Ignition switch START circuit feeds coil + side of resistor | Thermal (iron-wire) resistor: resistance rises as it heats. Calculator uses the stated value you choose. | [7][8] |
| Chrysler electronic ignition, dual ballast: compensating side | 1972-1979 (5-pin ECU); first on late-1971 340 manual-trans per TSB D71-8-9 | Dual ceramic ballast, 4 terminals (coil side) | 0.5-0.6 ohm thermal; 1.25 ohm non-thermal replacement | Echlin ICR24; Chrysler 3656199 (per forum citing TSB) | I2 lead (START) feeds coil side, bypassing the 0.5 ohm section | Chrysler service reference says new non-thermal 1.25 ohm resistors can substitute for thermal ones. Dave's Place lists 0.50-0.60 ohm at 70-80 F. | [9][10][11][12] |
| Chrysler electronic ignition, dual ballast: auxiliary side | 1972-1979 | Dual ceramic ballast (ECU auxiliary feed, not in coil circuit) | 4.75-5.75 ohms | Echlin ICR24 | n/a | Feeds ECU pin 3 only. Not part of coil primary resistance. | [9][10][11] |
| Chrysler electronic ignition, single ballast | 1980-1982 (4-pin ECU) | Ceramic ballast, 2 terminals | 1.25 ohms | Echlin ICR23 | START circuit | [9] | |
| MSD 8214 (aftermarket) | Universal | Ballast resistor for MSD Blaster coils on points | 0.8 ohm | MSD 8214 | Not stated by MSD | MSD: not necessary when an MSD 6 or 7-series ignition is used. | [13][14] |
| Accel ballast supplied with 140001 SuperCoil | Universal | Ballast resistor | 0.85 ohm | Supplied with Accel 140001 | n/a | Same retailer listing also says 'Includes Ballast Resistor: No'. Conflict in listing; confirm contents with Accel/Holley. | [15] |
AMC, Jeep and International: Hemmings reports AMC used the GM coil with an external resistor through 1974, and the 1975-77 AMC, Jeep and International Prestolite electronic system still used a Delco-style externally resisted coil. We did not find a sourced ohm value for these, so none is listed. [18] Imports (VW, Datsun, British cars) are not in the table because we could not verify factory figures from primary sources.
What is the primary resistance of my ignition coil?
Factory points coils read about 1.24-2.05 ohms depending on make and source, and need external resistance; PerTronix Flame-Thrower canister coils at 1.5 and 3.0 ohms are designed to run with the ballast removed; low-resistance coils (0.32-0.7 ohm) are for electronic or CD ignition, or need a ballast on points. [16][4][20][14]
| Coil | Used with | Primary (ohms) | Ballast needed? | Notes | Source |
|---|---|---|---|---|---|
| Delco-Remy 1115202 (GM points coil, external resistor) | GM 12-volt points, 1955-1974 era | 1.24-1.46 ohms (alt. 1969 CSM figure 1.77-2.05) | Yes: 1.8 ohm resistance wire | Conflict: NCRS forum post gives 1.24-1.46; a CorvetteForum post quotes the 1969 Chassis Service Manual at 1.77-2.05 ohms. Both shown. | [16][17][1][18] |
| Delco-Remy 1115207 (GM transistor-ignition coil) | GM breakerless/T.I. (1960s) | 0.35-0.55 ohms (1966/1969 CSM); 0.38-0.51 (other post) | Use with factory T.I. system only | Not a points coil. | [16] |
| Ford conventional points coil | Ford points ignition (1957 manual spec) | 1.40-1.54 ohms at 75 F | Yes: 1.30-1.40 ohm resistor wire | [4][5] | |
| Ford DuraSpark II coil | Ford DuraSpark II | 0.8-1.6 ohms | Yes: 1.30-1.40 ohm resistor wire (D7AZ-12250-A) | [19][6] | |
| Chrysler electronic ignition coil (1972-82) | Chrysler EI with dual/single ballast | 1.34-1.79 ohms (alt. 1.41-1.79) | Yes: dual ballast 0.5-0.6 ohm section (or 1.25 non-thermal) | Conflict: Chrysler 300 Club reference 1.34-1.79; Dave's Place 1.41-1.79 at 70-80 F. | [9][10] |
| PerTronix Flame-Thrower 40011 / 40001 / 40111 (1.5 ohm) | Ignitor, 12 V, 8 cyl (also 6 V 4/6 cyl) | 1.5 ohms | No: remove or bypass ballast/resistance wire | [20][21] | |
| PerTronix Flame-Thrower 40511 / 40501 / 40611 (3.0 ohm) | Ignitor, 12 V, 4 and 6 cyl | 3.0 ohms | No: remove or bypass | Chart also lists 28010 at 2.8 ohms for 1-6 cyl in one row. | [20] |
| PerTronix Flame-Thrower II 45011 / 45001 / 45111 (0.6 ohm) | Ignitor II (also Ignitor 6 V 8 cyl per chart) | 0.6 ohms | No: remove or bypass | [20][22] | |
| PerTronix Flame-Thrower III 44011 / 44001 (0.32 ohm) | Ignitor III | 0.32 ohms | No: PerTronix says the ballast needs to be removed when this coil is used with the systems it lists (Ignitor III) | [23] | |
| MSD Blaster 2 8202 | MSD 6/7, electronic, or points (with ballast) | 0.7 ohm | With points: 0.8 ohm ballast MUST be used (MSD 8214). With MSD 6/7: no | [24][14][13] | |
| Accel SuperCoil 140001 | Points (with ballast) or breakerless | 0.7 ohm | With points: supplied 0.85 ohm ballast plus original ballast if equipped | Retailer listing of Accel specs; ballast-included field conflicts. | [15] |
Do I need a ballast resistor with electronic ignition?
It depends on the unit. PerTronix Ignitor (I) sets a minimum total primary resistance; Ignitor II and III want the ballast removed and set coil minimums; MSD says a ballast is not necessary; HEI must not have one. [21][22][23][27][29]
| Ignition | Rule | Minimum primary (ohms) | Source |
|---|---|---|---|
| Breaker points (stock) | Ballast or resistance wire required with a ballast-type coil. Points life guidance: keep primary current around 3.5-4 A. | not specified | [25][26][5][14] |
| PerTronix Ignitor (I), 8 cyl | Minimum 1.5 ohm total primary resistance. If the coil alone has 1.5 ohm or more, remove/bypass external resistors; if lower, keep them. Red wire to ignition-switch side of the resistor when a resistor is kept. | 1.5 | [21][3] |
| PerTronix Ignitor (I), 4 or 6 cyl | Minimum 3.0 ohm total primary resistance. | 3 | [21][20] |
| PerTronix Ignitor II | Coils rated 0.45 ohm or greater (installation text); FAQ in the same sheet says 0.6 ohm or greater. Remove ballast for optimum performance. | 0.45 (0.6 in FAQ) | [22] |
| PerTronix Ignitor III | Coils 0.32 ohm or greater. Ballast removed with Flame-Thrower III coil. | 0.32 | [23] |
| MSD 6 / 7 series (CD) | Ballast not necessary. MSD coil chart: can be left in-line if originally equipped. MSD 6AL instructions: recommended to bypass. | not specified | [27][28][13] |
| GM-style HEI distributor | Feed wire must not be a resistor wire or contain a ballast; full battery voltage required. Test: running voltage at the coil feed within 1.5 V of battery means not a resistor wire. | not specified | [29][30] |
Where the manufacturer is inconsistent, both numbers are shown: the PerTronix Ignitor II sheet says 0.45 ohm in its installation notes and 0.6 ohm in its FAQ, and MSD’s coil chart (leave the ballast if original) differs from the 6AL sheet (recommended to bypass). [22][27][28]
How do you calculate coil current and ballast wattage?
Total primary resistance is the coil plus everything in series: R = Rcoil + Rballast. Static current is I = V / R. Voltage at the coil + terminal is V × Rcoil / R. Heat in the resistor is P = I² × Rballast. When the resistor is bypassed while cranking, cranking current is Vcrank / Rcoil.
Worked example 1: GM points (1.35 ohm coil + 1.8 ohm wire, 13.8 V)
R = 3.15 ohm. I = 13.8 / 3.15 = 4.38 A static. Coil + = 13.8 × 1.35 / 3.15 = 5.91 V. Heat in the wire = 4.38² × 1.8 = 34.5 W static. Cranking at 10 V with bypass: 10 / 1.35 = 7.41 A.
Worked example 2: Ford points (1.47 ohm coil + 1.35 ohm wire, 14.0 V)
R = 2.82 ohm. I = 14.0 / 2.82 = 4.96 A static. Heat in the wire = 33.3 W static. Coil + = 7.30 V.
Worked example 3: MSD Blaster 2 on points (0.7 + 0.8 ohm, 13.8 V)
R = 1.5 ohm. I = 9.2 A static. Heat in the ballast = 67.7 W static. MSD specifies this combination, but the current is well above the 3.5-4 A range cited for normal point life, so the calculator flags it. [14][25]
These are static numbers with the points closed and the engine stopped, the worst case. With the engine running, average current is lower because the points are open part of the time and the coil’s inductance slows current build-up; a quoted Ford points spec lists 4.5 A engine stopped and 2.5 A at idle. [5] Thermal (iron-wire) ballasts also rise in resistance as they heat, which is why an RU-11 reads 0.5-0.6 ohm cold but is rated 1.75 ohms operating. [7][26]
How does the crank bypass work?
Ford uses the I terminal on the 4-post starter relay; GM uses the R terminal on the starter solenoid; Chrysler electronic ignition feeds the coil side of the ballast from the I2 lead in START. In each case the resistor is bypassed only while cranking. [4][2][30][11]
How do I test a ballast resistor and coil?
- Coil primary: key off, all wires off the coil, ohmmeter across + and -. Compare with Table 2. [21]
- Resistor: key off, meter across the resistor terminals (Mopar dual: test each section). Compare with Table 1. [9]
- Voltage, key on, points closed: meter on coil + to ground. A ballasted system reads well below battery voltage (Corvair owners report about 6-7 V). With points open it reads battery voltage. [32]
- Cranking: coil + should rise to cranking battery voltage when the bypass works.
- HEI or full-voltage feed check: engine running, feed within 1.5 V of battery voltage means it is not a resistor wire. [29]
Full procedure with photos: How to Test a Ballast Resistor on a Classic Muscle Car With a Multimeter.
Frequently asked questions
Do I need a ballast resistor with a PerTronix Ignitor?
Only if your coil reads below the PerTronix minimum: 1.5 ohms on 8-cylinder and 3.0 ohms on 4- and 6-cylinder engines. If the coil meets the minimum, PerTronix says to remove or bypass the ballast or resistance wire; if it is lower, keep it. [21][3]
What ohm ballast resistor do I need for points?
Match the factory total. GM paired a 1.8 ohm resistance wire with a roughly 1.24-1.46 ohm Delco coil; Ford paired a 1.30-1.40 ohm resistor wire with a 1.40-1.54 ohm coil. MSD specifies a 0.8 ohm ballast for its 0.7 ohm Blaster 2 on points. [1][16][4][14]
How many ohms is a GM resistance wire?
GM service manuals list the points-era resistance wire at 1.8 ohms. Measured originals can read higher; one Corvair owner measured 2.2 ohms. [1][2]
How many ohms is the Ford pink resistor wire?
Ford specifies 1.30-1.40 ohms. The original pink wire C0LF-12250-A and the later DuraSpark II wire D7AZ-12250-A have the same resistance. [4][6]
What are the two sides of a Mopar dual ballast resistor?
The compensating side (coil feed) is 0.5-0.6 ohm thermal or 1.25 ohm non-thermal; the auxiliary side (ECU feed) is 4.75-5.75 ohms. It was used with the 5-pin ECU from 1972 to 1979. [9][10]
Does an MSD 6AL need a ballast resistor?
No. MSD says a ballast is not necessary with any MSD ignition; its coil chart says it can be left in-line if original, while the 6AL instructions recommend bypassing it. [27][28]
Can I run an HEI distributor on the resistor wire?
No. Mallory says the feed must not be a resistor wire or contain a ballast; run a full-voltage feed. With the engine running, the feed should be within 1.5 volts of battery voltage. [29][30]
Why does my engine start but die when I let go of the key?
On a ballasted system the resistor or resistor wire is bypassed while cranking. If the resistor or wire is open, the coil only gets power in START, so the engine dies in RUN. [11][2][31]
What voltage should I see at the coil positive with the key on?
With points closed (or coil negative grounded) a stock ballasted system reads well below battery voltage; Corvair owners report about 6-7 V with the 1.8 ohm wire. With points open you read battery voltage. [32][33]
How much current can breaker points handle?
References cite about 3.5-4 A for reasonable point life; above that, point wear speeds up. A quoted Ford points spec lists 4.5 A engine stopped and 2.5 A at idle. [25][26][5]
Related guides: setting ignition timing with a timing light; testing the alternator and grounds with a multimeter. More in the Car Maintenance Hub.
Safety notes
- A ballast resistor carries tens of watts of heat (see the worked examples), so let it cool before touching it.
- Do not leave the key on with the engine stopped for long: PerTronix warns this can damage the Ignitor, and a ballast-type coil with no resistor heats quickly. [3][31]
- Disconnect the battery negative before rewiring. [14]
- Use high-temperature terminals on ceramic ballast resistors. [5]
Sources
Every value above comes from one of these. Forum posts are used only where they quote a factory manual or parts catalog, and are labeled. Where sources disagree, both figures are shown.
- CorvetteForum: C2 ignition coil application (service manual 1.8 ohm resistance wire) (forum quoting service manual)
- Performance Corvair: Let's talk about Corvair ignition systems (PDF) (enthusiast reference quoting shop manual)
- PerTronix 1181LS 12-volt negative ground instructions (PDF copy) (manufacturer)
- Y-Blocks Forever forum: 1957 Ford shop manual coil and resistor specs quoted (forum quoting shop manual)
- Ford Truck Enthusiasts: Ford conventional point-type ignition specs quoted (forum quoting shop manual)
- Ford Truck Enthusiasts: Duraspark II wiring (Ford resistor wire part numbers) (forum quoting parts catalog)
- Standard Ignition RU-11 specifications and cross reference (FinditParts listing of SMP data) (aftermarket spec)
- Standard Ignition RU-11 listing (PartCatalog), fitment years (aftermarket spec)
- Chrysler Corp. Electronic Ignition System 1972-82 service reference (PDF, Chrysler 300 Club) (service reference)
- Dave's Place: Chrysler Electronic Ignition fault table (reproduces factory specs) (service reference (secondary))
- Chrysler Electronic Ignition System overview (PDF, Ramcharger Central) (service reference (secondary))
- For C Bodies Only forum: ballast resistor do you need it (cites TSB D71-8-9) (forum)
- MSD 8214 0.8 ohm ballast resistor product page (manufacturer)
- MSD Blaster Coil PN 8202 installation instructions (PDF) (manufacturer)
- SFX Performance listing for Accel 140001 SuperCoil (reproduces Accel specs) (retailer listing of manufacturer spec)
- NCRS forum: 202 ignition coil resistance specifications (forum quoting service manual)
- CorvetteForum: ignition coil specifications (1969 Chassis Service Manual quoted) (forum quoting service manual)
- Hemmings: Ignition coils explained (reputable reference)
- Ford service manual via Operation CHARM: DS II Test 12, coil primary resistance (1987 F-250 5.8L) (factory service manual)
- PerTronix Flame-Thrower coil instructions and coil chart (PDF) (manufacturer)
- PerTronix Ignitor 1181LS installation instructions (PDF) (manufacturer)
- PerTronix Ignitor II installation instructions (PDF) (manufacturer)
- PerTronix Flame-Thrower III coil instructions (PDF) (manufacturer)
- MSD 8202 Blaster 2 product page (manufacturer)
- Curbside Classic: History of electronic ignition, losing the points, part 1 (reputable reference)
- Triumph Herald Tips and Tricks: basic points-type inductive ignition (enthusiast reference)
- MSD coil compatibility chart (PDF) (manufacturer)
- MSD 6AL PN 6420 installation instructions (PDF) (manufacturer)
- Mallory 75-series billet HEI installation instructions (PDF) (manufacturer)
- Team Chevelle: points to HEI conversion (forum)
- MG Experience: why are coils 3 ohms? (forum)
- Corvair Forum: coil input voltage (forum)
- Corvair Forum: 4 volts at coil (forum)
Photo: ignition coil by Nick Ares, CC BY-SA 2.0, via Wikimedia Commons (background extended).