Technician quick reference
EXPECTED VALUES.
General starting points for common automotive electrical measurements, organized with the conditions that make each number meaningful.
These ranges are not universal pass/fail limits. Confirm circuit design, operating state, temperature, battery chemistry, network topology and current OEM service information before condemning a component.
HOW TO USE THIS PAGE
Condition first. Number second.
A value without its test conditions can mislead. Load the circuit, identify the technology, record the operating state and compare with vehicle-specific information.
01 / REFERENCE GROUP
BATTERY & SYSTEM VOLTAGE
Use these only as first-look references after identifying battery chemistry, temperature and vehicle charging strategy.
| Test / condition | General reference | Use only when | Important caution |
|---|---|---|---|
| Rested 12 V lead-acid battery | About 12.6–12.8 V | Battery rested with surface charge removed at moderate temperature | A lower number can reflect discharge, temperature, battery condition or an active vehicle load. |
| Recharge-before-testing threshold | Below about 12.4 V | Open-circuit lead-acid battery check | Charge and stabilize the battery before judging capacity or cranking performance. |
| Cranking / load-test voltage | 9.6 V is a common 21 °C reference | Specified load and test duration | Temperature correction, battery type and OEM procedure control the actual limit. |
| Conventional alternator charging | Often 13.5–14.8 V | Engine running with a charged battery | Load, temperature, regulator set point and sense-wire drop change the result. |
| Smart charging system | Roughly 12.2–15.5 V may be commanded | Measured while watching generator command and battery current | A temporarily low charging voltage can be normal; use scan data and the OEM strategy. |
| READY-mode DC-to-DC output | Commonly in the 13–15 V range | EV or hybrid in READY with converter enabled | Exact target depends on 12 V battery chemistry, load and energy-management mode. |
02 / REFERENCE GROUP
LOADED VOLTAGE DROP
Measure with normal circuit current flowing. An open-circuit resistance check cannot reproduce a loaded connection.
| Test / condition | General reference | Use only when | Important caution |
|---|---|---|---|
| Single connection | Ideally 0.00 V | Circuit operating at normal current | Any measurable drop deserves comparison with current, connector design and OEM limits. |
| Wire or cable | 0.20 V maximum general fallback | End-to-end test while loaded | Long high-current cables may require a circuit-specific calculation or OEM limit. |
| Switch contacts | 0.30 V maximum general fallback | Switch closed and carrying normal load | Electronic drivers and solid-state devices are not ordinary mechanical switches. |
| Ground path | 0.10 V maximum general fallback | Loaded component ground to battery negative | Low-current module grounds may require substantially less drop. |
| Reference / signal ground | Near 0 V; compare under load | Sensor connected and operating | Millivolts can matter in precision sensor circuits; compare signal ground with battery ground. |
| Starter circuit total | Use OEM positive- and ground-side limits | Cranking with a capable battery | Split the test across battery posts, terminals, cables, solenoid and engine ground. |
03 / REFERENCE GROUP
CAN, CAN FD & LIN NETWORKS
First identify the exact network and topology. Resistance values do not apply universally to every vehicle bus.
| Test / condition | General reference | Use only when | Important caution |
|---|---|---|---|
| High-speed CAN termination | About 60 Ω | Network asleep and safely powered down; two 120 Ω terminations in parallel | Gateway isolation, switched termination and inaccessible branches can change a DLC reading. |
| One CAN termination visible | About 120 Ω | Same powered-down conditions | May indicate an open branch or one missing termination, but verify the topology first. |
| CAN recessive level | Both lines near 2.5 V | Typical high-speed CAN architecture | Bias varies by transceiver and fault-tolerant networks use different levels. |
| CAN dominant level | CAN-H about 3.5 V; CAN-L about 1.5 V | Typical high-speed CAN differential bit | Use two synchronized scope channels; a DMM only averages bus activity. |
| CAN differential amplitude | About 2 V dominant | CAN-H minus CAN-L | Edge quality, ringing and symmetry matter in addition to amplitude. |
| LIN bus | Recessive near supply; dominant near ground | Active 12 V/24 V LIN physical layer | Battery voltage, pull-up path and transceiver thresholds determine exact levels. |
| LIN data rate | 1–20 kbit/s | Standard LIN communication | Do not apply CAN resistance, bias or termination tests to LIN. |
04 / REFERENCE GROUP
COMMON SENSOR CIRCUITS
The circuit design matters more than the sensor name. Verify whether the signal is analog, frequency, PWM, SENT, LIN or a switch.
| Test / condition | General reference | Use only when | Important caution |
|---|---|---|---|
| 5 V reference | Commonly 4.8–5.2 V | Sensor connected, reference measured to sensor ground | Some systems use 3.3 V or another regulated value; one shorted sensor can pull down a shared reference. |
| 3.3 V reference | Commonly near 3.3 V | Applicable controller and sensor design | Do not condemn it for failing a 5 V expectation. |
| Three-wire analog signal | Often about 0.5–4.5 V through range | Known mechanical input swept smoothly | Endpoints, direction and transfer curve vary; look for dropout and compare redundant channels. |
| Hall digital low | Often below 0.5 V | Correct pull-up, ground and air gap | Some outputs do not pull fully to ground and some are current-modulated. |
| Hall digital high | Near pull-up voltage | Correct supply and controller pull-up | The pull-up may be 5 V, battery voltage or another value. |
| NTC temperature sensor | Resistance falls as temperature rises | Sensor disconnected or voltage interpreted through known pull-up | Use the OEM temperature/resistance table; circuit voltage is not temperature by itself. |
| Inductive speed sensor | AC amplitude rises with speed | Correct air gap and reluctor condition | No single voltage applies across cranking, idle and road speed. |
05 / REFERENCE GROUP
STARTING, CHARGING & RIPPLE
Compare repeated events and synchronized channels. Meter readings can hide short transient failures.
| Test / condition | General reference | Use only when | Important caution |
|---|---|---|---|
| Alternator AC ripple screening | Often below about 0.5 V AC | Battery healthy, engine running and electrical load applied | Meter bandwidth and AC coupling vary; a scope pattern is more useful than one number. |
| Healthy rectifier pattern | Evenly spaced repeating peaks | Scope AC-coupled at an approved low-voltage point | Missing or unusually tall groups can suggest diode or stator faults; compare frequency with alternator speed. |
| Relative compression current | Even repeating peaks | Stable cranking speed, fuel/ignition disabled as required | This is a cylinder comparison—not an absolute compression-pressure specification. |
| Cranking battery trend | Initial dip followed by stable repeating load | Known-good battery and consistent cranking | Slow speed, temperature, oil viscosity and starter condition affect amplitude. |
| Starter current | No universal amperage | Correct engine, temperature and battery condition | Use OEM current specifications and compare current with cranking speed and voltage drop. |
06 / REFERENCE GROUP
PARASITIC DRAW & SLEEP
Confirm the vehicle is actually asleep and preserve module wake conditions during the test.
| Test / condition | General reference | Use only when | Important caution |
|---|---|---|---|
| Older/simple vehicle after sleep | Often about 20–50 mA | Doors latched, key removed, timers expired | Alarm, radio memory and accessories affect the result. |
| Modern connected vehicle after sleep | Often about 30–80 mA | All networks asleep and remote services inactive | Some vehicles legitimately exceed this; OEM sleep-current specifications take priority. |
| Sleep transition | Temporary higher current is normal | Observed from shutdown through complete sleep | Opening a door, polling with a scan tool or moving a key can restart the timer. |
| Sleep time | Commonly 20–60 minutes; sometimes longer | Vehicle secured using a non-intrusive measurement method | Telematics, battery conditioning and delayed accessories can wake periodically. |
| Battery-life estimate | Capacity ÷ average current gives ideal hours | Current and capacity expressed in matching units | Real usable time is shorter because temperature, aging and starting reserve matter. |
07 / REFERENCE GROUP
INJECTORS, COILS & SOLENOIDS
Resistance is only a preliminary check. Current shape, mechanical response and driver strategy provide stronger evidence.
| Test / condition | General reference | Use only when | Important caution |
|---|---|---|---|
| High-impedance port injector | Often roughly 12–16 Ω | Injector disconnected and temperature considered | Applications vary; compare identical injectors and use OEM data. |
| Low-impedance / peak-and-hold injector | Often roughly 2–5 Ω | Correct injector type identified | The driver limits current electronically; never substitute a saturated-driver expectation. |
| Direct injector | No universal resistance/current | Correct gasoline or diesel system and approved equipment | High-voltage drivers and boost circuits require system-specific testing. |
| Port-injector current ramp | Smooth rise to driver limit | Current clamp with synchronized voltage command | Slope reflects inductance and supply; pintle movement may appear as a subtle change, not on every design. |
| Ignition-coil primary resistance | Often too low to be decisive | Meter-lead resistance removed | Current ramp, dwell, supply and secondary evidence are more useful than resistance alone. |
| PWM solenoid | Current follows duty and inductance | Known command, supply and mechanical load | A DMM average cannot show saturation, shorted turns, sticking or driver current limiting. |