EC240B 81066 Intermittent Fail to Crank

I have a problem with my Volvo EC240B 81066

It has a intermittent fail to crank fault. Turn the ignition switch, and it bumps the starter, but only for a moment. Holding the key to the crank position makes it hit the starter momentarily, on off on off on off. Likely doesn't get a revolution out of the engine before dropping out, then waits for a second or three, and hits it again momentarily.

I had a technician replace the small relay on the starter. The starter has the large heavy power relay, but also has a smaller power relay to energize the heavy relay. Two power relays in series sort of, Anyway, the lighter of the two seemed weak so he replaced it, and it didn't' change anything.

The only way to get it to crank is to jumper one of the relays using a length of copper wire, in the same way you would do it in the old days.

The ignition lead that should supply power to the start relay on turning the key has solid 24V when you turn the key. But when you actually attach it to the relay terminal, the 24 volts becomes intermittent. and the starter motor starts bumping on off on off like I said.

You get a steady fast crank with the jumper wire by hand.

Coupled with this problem is a fail to start. We have cranked the living daylights out of this engine and it won't fire either. Smoke from the exhaust and some slight coughing, but no fire.

Many weeks ago we fired it up, warmed it up for an hour, then drove it to the fuel tanks to fuel. It got there and died on the spot. Hasn't run since.

We have verified it has fuel pressure.

Dealer techs are weeks away......HELP!

Check wiring to alternator. Pay particular attention to tiny wires. D+ or W if broken or loose will tell the V-ECU that the engine is running and disallow starting. You also have a crank relay above the batteries on a ledge behind a tin guard. Diode protected. There are 2 diodes flanking that relay. Should be a blue black 2 wire connector that just terminates. Check the diodes. One failed will cause fail to start.
Do you have any error codes?
Good luck

I should have included, there are no error codes

Then I would proceed as advised
BTW, if you put the key in the ignition switch and go straight from off to crank in one motion, will it crank? This is important.

I have mailed but maybe some others may like to have the response.
Kind regards
Uffex

In reply to the illustration provided by Uffex, I would add that the pic in bottom right corner looks more like terminating resistor behind E series I-ECU. Especially with that part number on it. It should be a small oblong bulbous thing about the size of your fingertip. One on each side of the safety relay. The one that looks like a saucer. Good luck.

Very interesting fault you have there.
The alternator is out of the picture as it would not have the rpm and time for the field to saturate and produce voltage at D+.
The problem could be a faulty starter safety relay which is the one in yellow on the diagram but i suspect the problem is over on the VECU side.
As it and the alternator has the ability to energize that safety relay (which stops the cranking BTW) and it also controls the fuel shut off solenoid which sounds like that is not been energized properly ether.
You can try a couple of things to eliminate the safety relay at least.
If you short out the two wires in the start circuit on that safety relay and it cranks ok it proves it is that relay or some thing powering it causing the cranking issue.
If that works re-connector the wires and remove remove the D+ wire from the alt and the wire in the diagram marked control lock switch from the safety relay.
if the issue remains it is the safety relay faulty.
If the problem is gone it is over on the VECU side.

I would caution against "shorting " anything on a Volvo. The diodes really don't appreciate it.

If you switch to manual mode it will take the VECU out of the frame.
KR
Uffex

Switching the computer to manual doesn't change any of the symptoms

Thanks for your help, this is helping a lot.

I'm gonna ask again. If you roll the key from dead off straight to crank with no hesitation, does it crank, THIS IS IMPORTANT?

Yes it cranks momentarily.
Unlike a pure mechanical moter, it does take a second for the computer to boot, but before thats done, the starter engages, just for a second.

Its almost as if something sees revs and quits cranking

Have you checked D+ at the alternator?
Disconnect it and ground the wire. See if it cranks

Alternator has large pos and neg leads, plus a 4 pin connector. I unplugged connector. Symptoms unchanged

That is the updated alternator. Does it appear that the harness is spliced in?

No. Ive owned this since near new. Its all factory

Ok, it's a later version than, would be more fair to say. Is your fuse box behind the cab like on a C model?

Yes

Son of a gun. I'll look at diagrams this evening. That's an animal of a different color

Well I looked and I do not have any diagrams or schematics on that particular model that was a late version just before they made the full transition to a C model. The earlier diagram posted up by Uffex Is 4 in early model B series up to the middle generation the later versions which you have are the closest to AC without having fully integrated servo sensors. We'll have to wait until someone who has prosis can provide a diagram or the correct schematic with your serial break