940 B234F Hot start issue progressed to no start

We have a 91 940 B234F Bosch car that will no longer start. My Father's car, which he and the roadside Auto club did some initial troubleshooting on.

Started as a hot start issue, if driven and parked, then attempted to start again a short time later would turn over but not fire.

No spark at these times, with a replacement coil tried but no change. Initially after waiting for 10 or more minutes it would restart if tried again. This progressed to longer periods needed and eventually the car simply wouldn't start hot or cold.

There was some success with jumping the fuel pump via the fuse board at first. But then there was also some sort of "shorting" incident that took out central locking and that test (jumping) no longer works. This is second hand information, I wasn't present.

CPS has been tested for AC volts and resistance. Both present.

Power stage has been swapped out with a known working unit from my 740 B234F wagon, no change.

Coil has power - 13 volts dropping to 12 when cranking.

No sound from the fuel pump KP2 (run) or KP3 (start).

No power to the fuel pump wires when removed and KP2 and KP3 tried.

Two relays on the coolant expansion tank (Radio suppression and cooling fan) have been swapped with known good units from my running wagon. No change.

What I'm thinking of next is to confirm the relays behind the fuse panel - swapping in known good from my wagon.

I'm sure this is something small and fixable, but looking for advice from the knowledgeable ones in the TB community as to tests or steps to take next.

Hi,

My sig has a no start tbricks thread I wrote. With a DIY tool set I recommend seeing if it runs on ether.

Crank sensor could still be bad. Resistance and AC voltage tests are valid, but inconclusive. A scope would tell you for sure. $80 Hantek works OK.

No spark sometimes!? Or completely gone now!? Powerstage or crank sensor likely.

Fuel injector pulse? Noid light, test light, scope, screwdriver handle to ear and tip to injector, spray injectors with rail carefully pulled out and one injector into a cup, the rest unplugged, ground wires re grounded, and ignition disabled, etc are lots of home brew ways to check this.

When a crank sensor fails, the ignition and fuel timing can either be gone completely, or just at the wrong time. Timing light may show ignition timing way off. The cranking sound and backfires would also probably show this.

Report back and I will see what I can do.

-ASE Master Automotive Technician. Now also an Advanced Engine Peformance Specialist.

It's not usually that helpful, but checking for error codes can sometimes be useful. Do you have spark at all at the plugs? No spark means no fuel.

-Ryan

No spark should be easy to diagnose. I have figured out the circuit and how to test it with various tools over the years.

Here is my EZK thread with visuals:


http://www.forums.turbobricks.com/showthread.php?t=322395


Use a LED test light instead of a scope to check for 5v sqaura wave pulse from ezk box to the powerstage. It's up to you to determine which pin. I forgot, but post up diagrams if you can.

Thanks for the suggestions.

No spark at all. I have a new CPS on the shelf, which can be swapped in if fuel relay checks out. I've looked at the no start thread and there is more info in this thread already.

Error codes were checked, but I will redo them myself this weekend.

Ether won't be used - with no spark or fuel pump it won't give a result.

I'll concentrate on the Fuel relay first this weekend based on this:
https://www.volvoclub.org.uk/faq/HotStartProblemQuickReferenceGuide.html

Keep the suggestions coming, I'll keep the thread updated with progress and outcomes.

Do you have fuel injector pulse? If no, AND no spark, likely crank sensor.

No spark is gonna likely be crank sensor or power stage.

EFI/fuel pump relay won't be bad and cause a no spark IF you were able to pull codes. ; l

Hey guys...you missed a critical detail...no fuel pump prime or voltage. 2.4 should do a 2-3 second prime when you bump it to KP2 initially. If that's not happening, chasing down an ignition system failure is a moot point at this stage.

OP: You mentioned a short circuit on the central locking? Any details?

A couple things that come to mind right off, in lieu of additional info.
Ignition switches may be prone to failure. If you have a spare switch, that might be worth a shot, couple screws to pull the cluster loose, swap it out, see if it changes anything. Given the years and miles, they actually can loosen up so the bakelite comes loose from the potmetal casting and can cause intermittent random issues.
7/9 series have been known to melt the fuse block on the bottom side, without much reasoning. With minimal work you can get the fuse block up enough to peer underneath it, see if there's signs of melting.

Back to the 'shorting incident', given the central locking no longer works, sounds like there's a B+ supply issue somewhere. I haven't pulled the diagram up for it, not enough brainpower left right now to be of any use, but it might be worth checking for common circuits between the two, start your voltage testing there.

Yeah I saw that about shorting, but don't know what that means and went over it and jumped to intermediate diag.

A test light and a DVOM would be where to start.

I could get the car to run without the fuse box by applying 12v to just a wire or two tactically, such as the fuel/EFI relay output wires.

Remember, only 4 wires to make it run!!! (My lh2.4 conversion thread)

First, one must understand that the EFI/Fuel pump relay is a dual relay. One side powers the fuel pump, the other the the EFI system. That's why I ask for a diagram to be posted so it can be reviewed by all.

If the EFI portion of the relay IS working, power would be found at places like the MAF, idle motor, injectors and whatever else is on that circuit. If codes are able to be pulled from both the EZK and the LH box that clearly means they are both powered up and it's not the relay.

Fuel pump prime, meh, maybe they popped a fuse during some diag. 5 minutes with a power supply like a jumper wire or a more professional tool like a power probe could easily diagnose that side of the circuit. Apply 12v at the fuse and listen for pump, check for 12v at fuel pump fuse, etc.

The no spark is the biggest red flag. I would first check for 12v at the coil, then a test light to coil negative for blinking during cranking, if no, I would then go further upstream to the powerstage. I coulld then use a LED test light or a scope to verify a steady 5v square wave signal from the EZK box. If THAT is missing, its either the EZK box (very unlikley, but possible) or the crank sensor. Crank sensor swap is parts darts. A scope could tell you 100% if it's working or not. If one had a good signal into the EZK box and NO signal out, it's the box.

Anyways, without the car it's tough for me to say what's happening, but I'm trying to provide the tools and methods to figure it out.

Good luck.

Ok, couple quick checks.
Given you have power at the coil, you should have power to the power stage, but it is worth a check, pin 4.

Check for power to fuse #1 in KP2. Should be 12v. This will prove out the 12v supply side to the ECU and ICU.
If you have 12v to fuse #1, proceed. If not, you have a 12v supply issue to the fuse block itself.

Check for power at fuse 11 when initially switching to KP2 (should show 12v for priming the pump for 2-3 seconds). Also check while cranking. This will prove out the ECU powers, grounds, and function, ignition switch powers, wiring to the relay, and the relay itself.
If good, check your wiring between the fuse block and the pump.
If not good, you have either an ECU fuel pump signal issue, a fuel pump relay issue, or a 12v signal to the ECU issue. If this is the case, check ECU pin 35 for 12v in KP2.

Report back on those, then we'll see where things are at.

gsellstr said:

Ok, couple quick checks.
Given you have power at the coil, you should have power to the power stage, but it is worth a check, pin 4.

Check for power to fuse #1 in KP2. Should be 12v. This will prove out the 12v supply side to the ECU and ICU.
If you have 12v to fuse #1, proceed. If not, you have a 12v supply issue to the fuse block itself.

Check for power at fuse 11 when initially switching to KP2 (should show 12v for priming the pump for 2-3 seconds). Also check while cranking. This will prove out the ECU powers, grounds, and function, ignition switch powers, wiring to the relay, and the relay itself.
If good, check your wiring between the fuse block and the pump.
If not good, you have either an ECU fuel pump signal issue, a fuel pump relay issue, or a 12v signal to the ECU issue. If this is the case, check ECU pin 35 for 12v in KP2.

Report back on those, then we'll see where things are at.

Click to expand...

Thanks, will do. Won't get to it until the weekend though. I will report back

Dont leave us hanging on the results OP. I put some effort into typing test procedures out. Please find a way to post a wiring diagram and we can go thru it. Posts that say "pin 1", "fuse 4" etc are confusing if there is no diagram.

It's really interesting to see different diag approaches. In this case just a test light would tell you a lot. I have a $20 LED test light and I have learned from other techs how much it can do.

Again, I maintain that if the computers are showing any codes you can skip looking for their power at their fuse. Front probing fuses is a way to damage the connector.

As a professional tech it's all about speedy diag. My approach, would be to maybe rack the car, pull the fuel pump wire and have a buddy crank and see if there is voltage to the pump. Ok, check, fuel pump relay works. No? Incorrect rpm signal or lack of one to tell the computer to power the fuel pump is plausible.

No sprak!? Power to coil, ok. Coil negative switching on a test light? No? Go to powerstage. 5v square wave there? Power to it also, as sellster mentioned? Power and no square wave or blinking on LED test light. I would then suspect crank sensor.

Ok back to work. Let us know.

$80 Hantek labscope is fun to use and actually works. I would rather spend $80 on a tool than, for example $80 on a crank sensor that doesn't fix the problem.

OK, had a chance to get back to this.

• No power to fuse 1 in KP2 or KP3.
• Jumping power to fuse 1 starts the fuel pump priming.
• Re-checked RSR relay, CPS and power stage connectors. Found one wire failing in the RSR relay plug, pushed that on properly to get a connection. Used RP7 on all the connections checked.
• Then noticed that the fuel pump would prime if fuse one had pressure applied - while rechecking for 12 volts. Fuse socket is a little dodgy, we modified the fuse a little to ensure fuel pump primes every time. This will be revisited after retesting with a charged battery.
• With the fuel pump jumped we get an initial spark from the coil on cranking, only the one spark though that we could see. Haven't retried with the modified fuse one yet - flat battery.
• Checked the back of the fuse block, all wires and connectors look like new. There is one wire from fuse one to the fuel injection relay that has had an extra length of wire inserted with three crimp connections (two bits of same gauge wire). This looks odd but does look sound, when we have a charged battery this will be kept in mind.

Rain and flat battery stopped us for the day.

And checked for codes - no ignition or fuel system error codes stored.

Again, time for a wiring diagram. I dont even know what fuse 1 does; a wiring diagram would tell us. I dont see how EZK and LH would blink a 1 1 1 at you without power and ground.

Ok so you saw A spark. That happens some times. That tells me your EZK likely IS powered up.

Next diag step is....IdK....do you have fuel injector pulse..?


I have learned which 4 or 5 wires would be needed to make lh2.4 happen, but I don't currently remember so that's where the wiring diagram comes in.

45 seconds of proof:

https://youtu.be/vzQxpwOHb0c

...

Come on Zach, you and I know not everyone is good at reading a diagram if provided and being able to translate that into a diagnostic path, you have access to them as well I'd like to assume, cut the crap dude. I won't get into a pissing match with you, we've BTDT. You also know I can't link to the diagrams I have access to, company policy that I will NOT break as I like my job.

So, OP found a fuse issue causing the pump prime/run failure (fuse 1 is fused power from the pump relay). Given it will prime once you resolve the fuse block issue including whatever the stuff spliced in is (may be related, may not, worth making sure it's resolved first), that tells me the power supply to the ECU is fine as it is powering up, doing what it should initially. Now time to figure out the spark issue. Typical causes are usually power stage or crank sensor. Did you get a chance to check for power supply to the power stage (pin 4) and ground at the power stage (pin 2)? Next will be checking for a signal to the power stage from the EZK, as that will validate the crank sensor and EZK operation. Power stage pin 1 is the output to the coil, pin 5 is the signal from the EZK. A test light on pin 5 will tell if you're getting a signal to the coil (coil is ground triggered, FYI). I'd need to do some digging, see what the input side signal is but Zach may already have that info at his disposal, been too long since I've had to check the input.

One of the impacts of troubleshooting is disturbing things that may have not been part of the problem initially. But if disturbed and not handled carefully then cause the issue to morph. I'm happy today that we are back to where the issue started - my Dad had noticed when the car was going through the unreliable stage that if he didn't hear the fuel pump at KP2 it wasn't going to start. But I don't know if the fuses had already been looked at before this was noticed.

Yep, we have power to pin 4 of the power stage.

Will check for ground pin 2, a signal at pin 5.

Will try a restart probably tonight before any more troubleshooting next weekend, as the battery should be fully charged again. Trying that because of the RSR faulty plug finding.

Failing that, more troubleshooting on the weekend.

Again, signal from EZK to powerstage is a 5v square wave. What would this show up as a reading on a DVOM?

ZVOLV said:

Again, signal from EZK to powerstage is a 5v square wave. What would this show up as a reading on a DVOM?

Click to expand...

Test light will confirm presence or absence of a signal. (Thanks gsellstr).

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Might be a moot point - Dad came around today while I was at work and tried starting the car, it started and ran briefly. Not sure how long that was.

Having found the wire pushed out of the RSR relay, and having fixed the fuel pump prime issue I had thought this might be the case. Flat battery recharge was what I was waiting on.

• Question - I wasn't sure of which position the fuel pump wires came from - could they be reversed and still run the pump but not in the correct direction? I knew it was a 50/50 thing when I replugged the wires after testing voltage earlier, and it was something I was going to reverse if it did attempt to fire or run for a short time only.