I've got an unexpected and sudden no hot start. 90 740T.

Drove around all day, no issues. Haven't had any issues at all in the last several years.

Last night, I was working on getting some new battery cables ready to install. I noticed one of the larger (maybe 8awg) leads to the positive terminal had some heavy corrosion, and poked at it a little to check for voltage drop between the alternator terminal. I didn't make any changes other than to add a 3/4 of a quart of oil. The new cables are a long story, but they've been sitting around for ages, unneeded; I was just looking for something to fix.

Two things that were different today from other days:
1) It was warmer than it has been regularly and I drove about 30 miles. This happened after the car sat for about an hour and a half and the engine was still nearly warmed up.
2) I drove with the headlights *off* 90% of the time. I almost always drive with them on constantly. I did this because of a low-voltage under load issue I'm just beginning to look into.

I ran the car (LH2.4) through its self-test procedures modes 2 and 3. The crank sensor registered and the throttle switch did too. I can feel the fuel/hear the fuel pumps prime, and I can feel the RSR click on. In mode 2, it fires the fuel injectors without issue.
I backprobed a LED light into one of the injectors and found that it does NOT pulse when cranking. This seems rather odd. As for spark, the car fires and starts and runs for a few seconds on starting fluid.


My first suspect was the RSR, FI relay, and ignition amplifier. I have a spare ignition amplifier to try tomorrow and the RSR/FIR appear fine and to be working. Again, it seems very odd that the injectors weren't pulsing.

My real fear, if it's not the ignition amplifier, is that either leaking AC voltage from a failing alternator diode set or some other oddity damaged the FI computer. It's parked at a friend's house for the night.

Cheers!

PS I hope it's not this. I really don't want to re-open the ECU connectors: https://www.brickboard.com/RWD/index.htm?id=918069

How old is the fuel filter?

All right. It's starting again. I think it's a failing main fuel pump. Even with voltage, I had to whack it to get it to start running. I also redid the wiring pigtail (which I'd grabbed off a mid-80s model) because it was losing insulation.

In the process of fiddling with things, I pushed the right rear pin back out of the FI relay socket. That complicates things! So maybe the pin is more the cause than the pump itself. We shall see, but ~100-150K miles out of a pump isn't bad!

Cheers

Sounds like you're close to a fix. Couple of thoughts on your original post.

If you were getting brief firing with starter fluid, and good spark, the ignition amplifier is not likely to be the culprit. A failed ignition amp will cause a no spark condition.

Back probing an injector. If you hook up a test light to an injector in the usual fashion, e.g., one wire to ground and the other to probe, the results will be non-revealing for firing. Injectors fire on a constant 12V from the RSR, and a timed ground from the computer. Grounding the test light will produce a constant light when the injector 12v side is probed (which means the RSR is working), and no light when the injector ground side is probed.

Sounds like you have Bosch? One of the main causes of a accelerated failed or failing main pump is a failed or failing in-tank pump. The car will run (for a while) with a dead in-tank pump until the overworked main gives out.

Hall sensor.

Thanks! That's one thing I noticed - that the car fired on starting fluid that made me think it wasn't the ignition amplifier, but I couldn't find an exact cause.

I've used backprobing the injector before to monitor firing/non-firing. It works just like a NOID light. I can't remember if I had it bridging the injector or if I had it from injector + to ground. LEDs take much less power than typical test-lights, if that matters.
The light is off when bridging the connectors, then when the ECU grounds the other side, it blinks briefly. Using the self-test mode, it blinked when the injectors fired. I had a video of it on youtube at some point when I was trouble-shooting what turned out to be a shorting wire to the throttle switch (it shuts off injectors when closing the throttle at cruise!).

Cheers!

And yes, it's Bosch. The in-tank pump is running, but I will replace it when I get around to fixing the sending-unit.

Do not underestimate the socket. It put me on the path to working on Volvos.

13 years ago, I had an intermittent no start. Occasionally it would not start. Later, it would be start fine. Would never happen long enough to troubleshoot by a mechanic so it fell to me. Eventually it occurred long enough to troubleshoot and I found 10 ohms resistance at the fuel pump relay socket. I just had to tighten the spade in the socket a bit to make better contact and all was good after that.

I think pulling the relay tray out can stress those connections.

Voltage at the pump connector does not rule out the socket. The voltage could be dropping across a poor socket connection when you load it with the motor. Before current is flowing there is no voltage drop and the connector would show full voltage.

A whack to start the motor does seems to point to it though. I also have replaced a bad main pump. Before my main pump completely died, it gave out sporadically while driving, loosing power, before it finally gave up.

If you replace the pump, I would recommend getting a new cage and all the piece inside it as they will be in bad shape.

--
Paul NW Indiana '89 744 Turbo 180K/ '90 745 turbo 145K

Heh, funny you mention that.
I had previously removed the entire relay tray and fuse panel and cleaned/squished all the contacts. In the process, I bent the retainer pin a little for that particular female spade.

This is the 2nd pump I've had on the car. The other failed with a couple of no-starts. None have made the car die on me while driving other than the one and only junkyard pump I've ever used.

I replaced the bushings and everything, but I'll get a new filter too. The in-tank pump is about as old as the main and it's still running.

The main thing I want to do is to clean the underside of the car before I work on it. It's diiiirty and greasy.

Cheers!

Is the Tachometer bouncing when cranking? If not, it is the RPM sensor or possibly the power stage (ignition amp) which will not allow the fuel pump to turn on after priming.

If the power stage has not been replaced in recent memory, I would start with that. They are a wear item and it may be a good investment if it is not the culprit, yet.
--
Paul NW Indiana '89 744 Turbo 180K/ '90 745 turbo 145K

Yes, the tach is bouncing on cranking, and the CPS is barely 2 years old.

Power stage is my first suspect. Would that stop the injectors from pulsing?