• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

740 '89 744t No start, injectors keep ticking with key out of ignition

daniels740

Member
Joined
May 8, 2019
Location
South Florida
On Friday morning last week, my '89 744T refused to start. It cranked as normal, but wouldn't start. It had spark but I was pretty sure the fuel pump wasn't running.

So, I swapped the fuel pump relay. It wasn't immediately after, but after a little more fiddling and pushing in the new relay that the engine started. After that, it ran fine. I drove a couple miles, parked, then started back up just fine and made my way home.

The next day, Saturday, it also started and drove fine. Sunday, I just left it parked and didn't touch it.

Now, Monday morning, the battery was randomly drained. Power locks still worked but it didn't have enough juice to crank. I tried jumping but couldn't get a good enough connection with the jumper cables for it to crank over fast enough.

Very strangely, after I stopped cranking with the cables connected, I heard a click-click-click-clicking noise coming from around the intake. Removing the injector ballasts, the clicking stopped, which likely means it was the injectors firing, as if the engine was running; except that the key was completely out of the ignition. The fuel pump wasn't also running.

What could this be? ECU gone bad?

I'll try a different battery tomorrow and see if it'll start when it can crank properly.
 
I started to ask about the cold weather but . . . .

Another guess is the ignition switch. I have had one fail internally

The ECU will have had power to continue to operate. The power being on could have weakened the battery.
 
I will say the onset of any issues was right on a colder morning but that's 60-70?F. Coldest it's been this winter so far was 52? this morning. Might that be harmful to old electronics?

Also, I replaced my ignition switch with an OE Volvo one just under two years ago. Nothing else has an indication of getting power when it's not supposed to: No blower motor, radio, power windows, lights, etc unless the key is in the correct position. Is the engine control unit supposed to get power at all times?

I'll swap the battery in a couple hours and see where that takes us.
 
First off, the ECU on my 1987 745 T did not have any on board diagnostics so it did not have a keep alive circuit for storing error codes. I don't know when Volvo started implementing ECU diagnostics; but, if there are no diagnostics on your 1989 then it is unlikely that it has keep alive power.

The operation of the injectors will depend on how the firmware in the ECU is coded. It would be more common practise that the ECU does not start firing the injectors during cranking until it picks up an engine RPM signal indicating that the engine is turning. The ECU uses the same RPM signal to start up the fuel pump. With this set up, if the engine isn't turning the injectors should not be firing. However, if the ECU uses an explicit start signal from the ignition switch to go into cranking mode then if it is getting an errant start signal of some kind it may be firing the injectors; however, if that is the case then I would also expect the pump to be running.

My 1987 745 T and its service manual are long gone so I can't check the wiring diagram. With your new battery, if the clicking problem persists with no key in the ignition, as a first easy step I would just unplug the ignition switch. If the clicking stops then you have a likely culprit. I really doubt that the ECU uses an explicit start signal and that the switch is the cause of this problem; but, it is easy to check and rule out as a cause.
 
Last edited:
Charged battery in - it fired for a moment then died immediately, as neither of the fuel pumps were running. After that, injectors kept pulsing. I pulled the wires from the ignition switch, and they continued to pulse.

Sounds like the ECU has gone bad to me. Any other possibilities?
 
Last edited:
Charged battery in - it fired for a moment then died immediately, as neither of the fuel pumps were running. After that, injectors kept pulsing. I pulled the wires from the ignition switch, and they continued to pulse.

Sounds like the ECU has gone bad to me. Any other possibilities?

Is it a lh2.2 or 2 4 car? If it's lh2.2, like previously stated it should not fire the injectors or run the fuel pump without an rpm signal from the pickup in the distributor. I would say bad ecu if that's the case.
 
Lh2.2, sorry. Last year as it was extended to '89 for the Turbo cars. I put a wanted ad for a new ECU on here. If nobody has a spare, I'll just grab one off of eBay. Thanks, guys.
 
When it's clicking with the key out, try tapping on the top of the RSR Radio interference Suppression Relay, and on the fuel pump relay, with the handle of a screwdriver to see if it stops clicking. With the key off, the LH2.2 ECU should not be getting any power.
 
Bench tested fuel pump relay and it's working fine. The Radio Interference relay was also clicking properly. I just can't imagine it being something other than the ECU gone bad right now.
 
I pulled the ECM last night and it did stop the injectors. Visual inspection revealed no corroded contacts nor any melted or heat-fatigued areas on the circuit board.

This is contrary to what a bad relay usually causes, right?

What I know from experience is that many circuit boards fail without any visual aspect. Just last year I helped a buddy diagnose a bad ECM on his Astro Conversion Van. It's quite annoying because the circuit board looked like new.

I'll see what I can do to measure parasitic drain. It's going through the #1 fuse so it shouldn't be all that difficult to measure.
 
As I recall, the LH 2.2 injectors are batch fired and ground switched by the ECU. I think it is a single ground switch (transistor) for all 4 in the ECU ????

In order for the injectors to be operating, two things need to happen. The ECU has to be firing the output transistor and the radio suppression relay has to be energized. The radio suppression relay only gets powered up if the main fuel injection relay is powered up. Normally the main relay gets powered up by a ground signal from the ECU after the ECU has received a power on signal from the ignition switch. If the radio suppression relay is powered up then the ECU is also likely powered up even though the ignition is off.
Since things still click with the ignition switch disconnected, I suspect that you may be getting some kind of false ground which is powering up the main relay or the relay has seized in the on position. It is also possible that the radio suppression relay is stuck on; but, with no power to the ECU it is not obvious why the ECU would be firing the injectors.

When the fuel injectors are clicking with the ignition off, start at the radio suppression relay to check for +12v there (should normally be 0 volts with ignition off). If you have +12v at the radio suppression relay, check the coil terminal on the relay to confirm that it is powered up and if it is, go to the main relay and check the 87 terminal. If you have 12 v at the 87 terminal, the relay is being powered up through a false ground on terminal 86/1 or the internal contact is stuck. You will have to check to determine the specific problem.

Here are wiring diagrams. Scroll down to find LH 2.2 or 2.4.

http://www.autoelectric.ru/auto/volvo/740/1989/740-89.htm
 
Turns out the ECU is getting power at pin 21. Not good at looking at schematics, but it appears power is going to pin 30 of the fuel relay, through the electromagnet to 86/1 and then to pin 21.

Should power be making its way to pin 21 like this normally?

jaeuuS5h.gif
 
Turns out the ECU is getting power at pin 21. Not good at looking at schematics, but it appears power is going to pin 30 of the fuel relay, through the electromagnet to 86/1 and then to pin 21.

Should power be making its way to pin 21 like this normally?

jaeuuS5h.gif

Look at the diagram - where does pin 21 on the ECU connect to? That will tell you whether it's continuous, switched or ground (thru another device/relay, etc...

That will help guide you. I don't have an LH2.2 diagram handy to evaluate.
 
From what I can see, Pin 21 straight comes from the number 1 fuse, into the fuel pressure relay, through an electromagnet, then into the computer. I think the computer should be grounding this pin when the fuel pumps should be running.

Instead, I think the power on that pin is back-feeding (bad diode?) and causing the injectors to pulse and other nonsense. Also explains why the pumps don't run when cranking, though the relay is good; the 21 pin isn't getting grounded.
 
They used the wrong symbol for pin 21 in that diagram. It should have been the arrow with the ground inside it. This symbol means that the ECU can ground the pin to energize external parts. For pin 21, the ECU grounds it to turn on the main/fuel relay.

Pins 5,11,25 are the real ground connections. Speaking of which, it would be good to check that the ground wires are good going to the intake manifold. I'm not sure what LH2.2 uses, but LH2.4 has 2 pairs of black wires going to ring terminals on the front&back of the intake manifold.
 
Turns out the ECU is getting power at pin 21. Not good at looking at schematics, but it appears power is going to pin 30 of the fuel relay, through the electromagnet to 86/1 and then to pin 21.

Should power be making its way to pin 21 like this normally?

When everything is switched off, it would be normal to have +12v on pin ECU 21. From the diagram, it appears that terminal 30 on the main relay has unswitched 12v, so power at terminal 30 would be normal. Terminal 86/1 on the main relay (which is connected to ECU pin 21) would float at 12 volts until such time as it is pulled to ground by ECU pin 21. As such, measuring 12v at ECU pin 21 when everything is turned off would be normal. When ECU pin 21 is pulled to ground that causes the main relay to energize which powers up everything else. The fact that you measure 12v at ECU pin 21 means that you do not have a false ground. Check for voltage at terminal 87/1 on the main relay (or terminal on the ECU). If you have 12 volts there, that likely means the main relay has stuck contacts or an internal short.
 
Back
Top