Volvo 940 wont start.

[TheLeeds]

Originally Posted by Lolvo 940

Hi all worth a try with crank sensor then?
The car was apparently running when laid up. Only have a voltmeter.
So would a crank sensor mess with fuel and spark?
How prone are ignition amplifiers to faulting? And would that cause this issue?
Cheers for the help guys

I briefly had a Jaguar XJ40, 3.2s. In the paperwork was a garage invoice for a new crank sensor, a new ignition module and a new coil, dated about 4 months before I bought the car.

One day, whilst about 20 miles from home, it failed to start. For some reason there was an old bike wheel spoke in the boot, so I used it to shot an HT lead and check if there was a spark. There wasn't. I called the RAC, but after about 20 minutes of waiting, I tried again and the engine started, so I went home and cancelled the callout.

A few weeks later, I'd left it outside a mate's house while we'd gone somewhere in his car. When I got back to the jag in the evening, it wouldn't start, and again, there was no spark.
This time I had it recovered. I got a crank sensor off an X300 with the same engine, fitted it and it cured the problem. I would have bought a new one but I had no way of testing the original, and scrap yard bits are cheaper. Why it ate a crank sensor in a matter of months I still don't know.

I lent that car to my sister for a few months while her car was off the road, and when it came back, the MOT had run out and there was an ants nest in it. I remember driving it up the A259 to the MOT place and seeing ants crawling up the windscreen. I killed a load of them while it was on the MOT ramp as well. At least it only failed on anti roll bar bushes.

[Laird Scooby]

Somehow the relay is permanently fixed in my 760 or i'd have something similar by now, not that it's given me any trouble but i just hate the lack of initial priming when it can't be bothered.

Got a similar relay on my 827s which has some solder joints that fail - desolder, clean and resolder (with proper, non-EU solder) and refit, job done on those.

If i could get the Volvo relay out, i'd try the same and if it didn't play the game convert it using a 4 or 5 pin standard relay.
The 5-pin would be useful for another idea i had - fuel pump primes when car is unlocked through 87a and a timer relay feeding it - slightly unconventional as by rights 30 is the feed terminal but in this case, 87 would be the ignition fed terminal, 30 fuel pump feed and 87a fed from a timer relay set to output a feed for a few seconds when the doors are unlocked.

Might still do that and completely bypass the OE fuel pump relay!

[aardvarkash10]

Cool, you have a voltmeter.

Turn the ignition ON but don't start.

Check for 12v at the coil primary terminals (the spade terminals on the top of the coil). The red lead from the VM on the terminal, the black lead to a GOOD earth. It should read 12V or close to on BOTH primary terminals.

Check for 12V at the injectors. Disconnect ALL the injectors by removing the connector on them. On any one of the connectors touch (don't shove the probe into the terminal, you will damage it) the red VM lead to each terminal with the black lead on a good earth. You should get 12V or close on one, and nothing on the other.

Well done. You have just confirmed that you have ignition supply to the injectors and the coil. Any further tests need a bit of knowledge and some other gear.

At this point though, given its a Volvo with LH2.4 engine management, I'd go ahead and replace the crank sensor. I would personally recommend also replacing the ignition amplifier (the one on the inner guard that Dave described) as it WILL be the next failure if it hasn't already.

There are plenty of good threads on replacing both of these items. Look them up before you go ahead - the CPS especially can be a bit daunting, but it is actually straightforward as long as you are careful.

[Laird Scooby]

Originally Posted by aardvarkash10

Cool, you have a voltmeter.

Turn the ignition ON but don't start.

Check for 12v at the coil primary terminals (the spade terminals on the top of the coil). The red lead from the VM on the terminal, the black lead to a GOOD earth. It should read 12V or close to on BOTH primary terminals.

Check for 12V at the injectors. Disconnect ALL the injectors by removing the connector on them. On any one of the connectors touch (don't shove the probe into the terminal, you will damage it) the red VM lead to each terminal with the black lead on a good earth. You should get 12V or close on one, and nothing on the other.

Well done. You have just confirmed that you have ignition supply to the injectors and the coil. Any further tests need a bit of knowledge and some other gear.

At this point though, given its a Volvo with LH2.4 engine management, I'd go ahead and replace the crank sensor. I would personally recommend also replacing the ignition amplifier (the one on the inner guard that Dave described) as it WILL be the next failure if it hasn't already.

There are plenty of good threads on replacing both of these items. Look them up before you go ahead - the CPS especially can be a bit daunting, but it is actually straightforward as long as you are careful.

Thanks Ash - that was going to be the next lot after checking the fuses! Saved me a lot of typing!

[Lolvo 940]

Cheers guys will try later and come back I'm sure. Haha

[Lolvo 940]

Hey just swapped out crank sensor with a new one made no difference. I don't think I'm getting any power to coil.
Ive checked all my fuses and I appear to get power to the ignition amplifier.
Any ideas?

[Laird Scooby]

Originally Posted by Lolvo 940

Hey just swapped out crank sensor with a new one made no difference. I don't think I'm getting any power to coil.
Ive checked all my fuses and I appear to get power to the ignition amplifier.
Any ideas?

Originally Posted by aardvarkash10

Cool, you have a voltmeter.

Turn the ignition ON but don't start.

Check for 12v at the coil primary terminals (the spade terminals on the top of the coil). The red lead from the VM on the terminal, the black lead to a GOOD earth. It should read 12V or close to on BOTH primary terminals.

Check for 12V at the injectors. Disconnect ALL the injectors by removing the connector on them. On any one of the connectors touch (don't shove the probe into the terminal, you will damage it) the red VM lead to each terminal with the black lead on a good earth. You should get 12V or close on one, and nothing on the other.

Well done. You have just confirmed that you have ignition supply to the injectors and the coil. Any further tests need a bit of knowledge and some other gear.

At this point though, given its a Volvo with LH2.4 engine management, I'd go ahead and replace the crank sensor. I would personally recommend also replacing the ignition amplifier (the one on the inner guard that Dave described) as it WILL be the next failure if it hasn't already.

There are plenty of good threads on replacing both of these items. Look them up before you go ahead - the CPS especially can be a bit daunting, but it is actually straightforward as long as you are careful.

Work through Ask's method above, i'm guessing you had tried putting the voltmeter across the coil LT (low tension) terminals and got no voltage and hence your comment?

The way the circuitry is designed, the coil is only powered up when it needs to be, in other words with the engine cranking and/or running.

[Dirty Rooster]

Originally Posted by Lolvo 940

Hey just swapped out crank sensor with a new one made no difference.

Does the rev counter activate during cranking?

[aardvarkash10]

Originally Posted by Lolvo 940

I don't think I'm getting any power to coil.

Why do you think this?

Originally Posted by Lolvo 940

Ive checked all my fuses and I appear to get power to the ignition amplifier.

How did you identify this?

Some background information and a qualified suggestion.

The coil is supplied with a 12V live feed any time the ignition switch is in the IGN position. This supply is NOT fused. If you do not have 12v here the most likely fault is the ignition switch.

The negative side of the coil is the "switched" side. The "switch" is the ignition amplifier. The ignition amplifier turns on and off (closing and opening a circuit to earth) in response to a signal from the EZ-K ignition computer. This in turn switches in response to a signal from the crank position sensor.

There are only two components that are vulnerable in this circuit - the CPS and the amp. The cps is vulnerable by location - behind the cylinder head it goes through a heating/cooling cycle that eventually breaks down the materials in it allowing moisture in that ultimately degrades the fine wire winding that is its heart.

The ignition amp is vulnerable to heat because its an electronic device that generates heat in its normal operation. This heat has to be bled off somehow, and so it sits on an aluminium heatsink with a heat transfer cream in between the two. This cream dries, hardens and stops doing its job allowing the amp to get too hot. Eventually its internal components fail.

The ignition computer is operating at very low currents and so tends not to get hot by design. Its also protected inside the passenger compartment (unless you have a really bad water leak). Without a catastrophe, its pretty robust and unlikely to fault.

Upshot - without a dual trace oscilloscope and the skill to use it, and assuming you do have a confirmed 12v supply to the coil and the ignition amp, your next step is to replace the ignition amp.

I do stress that this is a best guess. I don't like guessing as Dave will tell you. Your replies to my suggested tests don't make sense at the moment - one or the other is very likely wrong. Go and test again.

[Laird Scooby]

Originally Posted by aardvarkash10

The ignition amp is vulnerable to heat because its an electronic device that generates heat in its normal operation. This heat has to be bled off somehow, and so it sits on an aluminium heatsink with a heat transfer cream in between the two. This cream dries, hardens and stops doing its job allowing the amp to get too hot. Eventually its internal components fail.

I don't like guessing as Dave will tell you.

I've only ever seen a spot of heatsink compound at the most behind an original ignition amp Ash - on any car!

More often than not, there has been none at all, relying simply on the physical contact. Normally one of my first preventive maintenance tasks on any car is to remove the ignition amp and clean up the mating surfaces then apply a bit of heat transfer paste/heatsink compound/cream.

In some cases this has even cured/revived failing ignition amps when i've started getting an odd misfire soon after taking ownership and a long run.

Either way, the old compound needs cleaning off with something non-abrasive, degreasing and drying and new stuff applied.

Sometimes on remote diagnosis such as we perform on this (and other forums) a "best guess" is the only way, usually because the person who has the problem has no real prior knowledge of the intricacies of fault finding. Very often that person has little or no technical knowledge either so much of it has to be descriptive (eg the little black box on the inner wing behind the headlamp etc) which obviously doesn't make life easy.

Guessing tends to work out expensive at times though and as we both know, very often wrong.

However, given the other evidence so far, it is pointing at the ignition amp as the best guess. I'd still like to see all the earths removed, cleaned, checked and refitted, just to eliminate them as well.