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Diagnosing No-Starts on the 740 and 940 Volvo Models

2019-01-09 - ipd Staff

Disclaimer: Direct from ipd’s Tech Tip archive! This tech tip contains information from previous publications. Products mentioned may not be available or the information may not be accurate due to changes in supply, manufacturing, or part number association. Please contact ipd Customer Support if you have further questions.

© 2005 by Frederick Su

I've learned a lot working on Volvos over the years, especially after diagnosing and fixing our 1987 740 Turbo when it died last winter. I've compiled a succinct troubleshooting guide here to cover the most likely suspects for a no-start.

A dead engine has three possible culprits: mechanical, electrical, or fuel.

No or little noise on ignition "Start": Does the battery put out 12 volts? Also clean cable ends and contact surfaces. Other possibilities are the starter solenoid and ignition switch.

Engine cranks (but doesn't run) on ignition "Start": Be sure you have gas (and not water; I once had that problem!). Also check that fuse 1 (fuel injection) is intact.

Mechanical: Check timing belt. Lift oil filler cap to view camshaft while an assistant cranks engine. If camshaft rotates, the timing belt is fine.

Electrical #1: Pull the coil high tension wire from the distributor and hold its end about 1/8" from metal on the engine block. (Use a plastic grip or tongs, never "insulated" metal pliers or your bare hand! Do not stand in water!) With someone cranking the engine, check for good spark. Lack of spark means the ignition coil or the distributor's Hall sensor is faulty. A functioning Hall sensor detects camshaft rotation, sets the condition for delivering spark from the ignition coil, and closes EM2 on the fuel injection relay by completing the circuit through the ECU and the Charge Air Overpressure Switch connector (Figure 1). To isolate the fault, examine the fuel injection relay switch (details follow). If EM2 closes on ignition "Start," then the Hall sensor works and the ignition coil is bad. If EM2 is open on "Start," then the Hall sensor is bad. (Another option: bad wiring to components.)

Good spark implies a problem in the fuel injection system.

Fuel: To test fuel delivery, open the fuel rail and insert inlet into a jar or can. Remove fuse 11 for the in-tank pump and hit "Start." (The in-tank pump would still deliver fuel even if the main fuel pump were dead; engine will not run with a dead main fuel pump but can run with a dead in-tank pump. Don't overdo the cranking; a working main fuel pump was not designed to run solo.) If no fuel is delivered, then the culprit is the main fuel pump or a component in its circuit. If you get fuel, go to Electrical #2. Re-insert fuse 11.

Test the main fuel pump by setting the car up on jackstands, transmission in neutral, and wheels blocked. Underneath, first push on the pump feed wires to ensure proper seating. Cock an ear close to the pump while an assistant cranks the engine. A functioning main fuel pump emits a whiney, tinny whir. If there is no whir, check for 12 volts across the leads when engine is cranked. If you get 12 volts, then the main fuel pump is the culprit (which was my case).

Lack of 12 volts at the main fuel pump implies a faulty fuel injection relay. (It could conceivably be the Hall sensor, as well, but spark from the ignition coil in test Electrical #1 eliminates that possibility.) Lift and work the relay tray into the open after pulling out the ashtray and fuse cover plate and removing the cigarette lighter shelf (Phillips screw beneath plastic snap-off cover plate). Extract the fuel injection relay (leftmost, row 2, white cover) from its tray socket and gently pry the cover off to expose the two switches, EM1 and EM2 (coils of fine copper wire). Reinstall the relay sans cover into its tray socket. Turn ignition "On." EM1 should close and EM2 should remain open (Figure 1a). Turn ignition to "Start." EM2 should now also close. If it doesn't, hold EM2 closed with a nonconductive probe (plastic or wood), again hitting "Start." If the engine runs, the fuel injection relay is faulty.

Both switches should close on "Start," with EM1 closing even at ignition "On." If EM1 doesn't close at all, check for 12 V (should be always present) at junction 30 on the relay (Figure 1b). Then, with ignition "On," check for continuity between ground and 86/1 of the blue relay support socket. If the above two conditions are met, replace the relay. If not, trace backwards for wiring fault in these two feed circuits.

Electrical #2: If you get spark and fuel, then use the autoelectric wiring diagram to run voltage tests across each fuel injection component. Start by checking for 12 volts across the #1 fuel injector (green wire and ground) with ignition "On." If null result, work backwards using the diagram until you get a 12 volt reading. A 12-volt reading on the input end and none on the output end identifies the faulty component.

Fred is a physicist turned novelist who works on his Volvos when he has to. In one scene of his award-winning novel, An American Sin, the protagonist's 1970 145 Volvo breaks down outside Missoula, Montana, leading to a meeting with one of the pivotal heroes of the Vietnam War. For more information, visit www.bytewrite.com