While we discussed a no-crank here, we may encounter a car that cranks fine, but does not start. We now have a crank no-start!

The car may crank nicely, turning the engine at the rate we expect, or it may crank, but slowly. If it cranks slow, that needs to be fixed first.

Slow crank:

This can be caused by a few things:

• Low battery voltage
• Bad battery capacity
• Bad connections in the battery plus or the ground
• Bad starter motor

Normal crank, no start:

This can be caused by:

• Mechanical issue
• No spark
• No fuel
• No air

Air:

Any restrictions in airflow can be investigated quickly. Is the air filter clogged? Is something stuck in the intake tubes? Are the intake tubes damaged?

Fuel:

Next is to check for no fuel. Will the engine start if fuel is provided? To test this:

• Undo the intake air tube
• Spray in a flammable fluid, such a brake cleaner
• Put the tube back
• Start the engine

If the engine now runs: the engine doesn’t get fuel. Next, check fuel delivery! Time to dust off your test light and the scan tool.

If it still does not run: probably not a fuel problem. Next, check for a spark!

Spark:

To check for spark, remove an HT lead or, indeed, an ignition module. Attach it to a spark plug, then crank the engine and check for a spark. Got a spark? The engine should run…or is it mechanical?

Or got no spark? Time to dust off your test light and the scan tool.

Mechanical issue:

While the car may have many electrical and electronic modules and sensors that all can go bad and break the car, mechanical issues do still occur. A camshaft timing problem, lack of compression on one or more cylinders, burned valves, bad piston rings, all of it still exists and will find its way to your workshop now and then. Be prepared to not only blame sensors, wiring, and electronics, but keep an eye open for the mechanical part of a car.

When cranking the engine while not letting it start, you may get an impression of the state of the engine.

• Does it crank evenly?
• Does it crank irregularly?
• Does it seem to go faster or slower per revolution or per two revolutions?
• Does it attempt to start, or is it not trying at all?

The result of listening to the engine crankling may give you a lead if there’s a compression problem, or an ignition or camshaft timing problem. As a mechanic, you know how to handle these.

Diagnose the issue:

For a no-spark or an engine that is fuel-deprived, the scan tool will very likely give you more clues. Do you get codes pointing to fuel-related matters or ignition-related matters?

Usually, it’s the CSPS (camshaft position sensor) that provides the signal for both timing ignition and fuel injection to the ECM. But in many cases, the CPS (crank position sensor) can take over this function if the CSPS fails. In such cases, the engine light will be on, but the engine runs seemingly normally. Obviously, with both sensors out of service, the engine has no way of knowing in what cycle it is, and the engine will fail to run. Similarly, other sensors failing can be replaced with a predetermined fixed value in the ECM. This can be the case for the mass air flow sensor, but also for the sensors for air temperature and pressure – lighting the engine light in all these cases.

Ignition:

The ECM signals each ignition module to fire at the correct moment. In order to find out where the crankshaft and camshaft are, sensors provide the ECM with that information via the CSPS and CPS.

Check a module:

The most basic test for each module found in any car electric system is to check for battery plus and grounds. For this, nothing more advanced than a test light and a voltmeter is required – and obviously, a wiring diagram!

Some brands provide a power distribution diagram. This will provide you with an overview of all pluses and grounds of all modules in the system. If you have this, then use it, it’s great! If not, no worries, we’ll go about the old-fashioned way instead – which always works, by the way.

Another way to simplify the search is if there’s a layout of the pin connectors for each module. In our case, for the ECM. This will show you the numbered pins in the connectors to the module, and a list of what each number connects to.

Now check all pluses with your test light with the ground lead connected to a solid ground – preferably the battery. As a rule, no high currents are handled by the ECM. Instead, a relay is used that is activated by the ECM. So if your test light lights fine on all the pins expected to have a battery plus, then that’s ok.

For grounds, switch the test light to battery plus and check the pins expected to be grounds.

The most usual is to have a missing or bad plus or a missing or bad ground. In such cases, trace back where the lead comes from to solve the problem.

But, if all pluses and grounds are ok,

 

 

Paul
Sweden
2025-09

This page is part of a series. You can find the main page for the series here.