'96 940 LPT revs bog down and cut out under electrical load

[bmxJeff]

Originally Posted by Steve940estate

I've read about some cars having faulty ignition switches that can cause some strange faults, could be worth a look if nothing is wrong with the alternator.

Interesting, I'll bare that in mind although with the results I've got testing the system voltage I think the problem may lay elsewhere at the mo, cheers

[bmxJeff]

Originally Posted by xad001x0w

My car will certainly drop revs in idle when you draw a lot of current.

Example: at idle closing more than one electric window at once will drop the revs down by about 100rpm. It doesn't recover until I take my finger off the switch.

I've never noticed any other problems!

Thanks, it's nice to know that's normal. This is my first Volvo

Originally Posted by Rversteeg

The idling speed should be adjusted automatically by the CIS valve, located just below the inlet manifold. It is basically an elctrically operated air by-pass valve around the throttling plate. When closed, the throttle should just "leak" enough air for approx. 500 rpm idling, the additional air needed to make the engine tick over at 850-900 rpm should be going through the CIS valve.
If on idling the engine load increases (electrical components kicking in, A/C), the CIS valve is opened a bit further, bringing the idling speed back up again. A short drop in engine speed is normal, but it should recover itself within 1 second or so.
I would suggest to clean the throttle housing internally; to check the basic idling speed of 500 rpm (closing hose to CIS valve); to take out the CIS valve and thoroughly clean it; finally check the adjustment of the end switch on the throttle shaft.

However, a faulty idling control system does not explain the hesitations when the engine is at speed. It could be it's not the extra mechanical load on the engine, but an overall voltage level dropping to a too low value. Have you checked the earthing connection from the battery to the chassis? What's the battery condition? Maybe a voltage reading when driving (Voltmeter connected to eg. sigar lighter)?

Thanks for the suggestions. I blocked the pipe going into the CIS valve as you suggested as in pic below. Sealed intake and valve side by tightly taping plastic over pipe ends. I tried to start the car a few times normally and it was having none of it. Turned over and stopped dead the second I released the key. I forced it to start with a bit of throttle and gently lowered the revs but as soon as I fully lifted my foot, it died.



So this sounds to me like the CIS valve was working and probably working overtime to keep the car idling at 8-900rpm. Have I understood this correctly? Does this then mean my throttle body is not allowing enough air to pass at idle? I assume you mean the electronic throttle switch on the back of the throttle body and presumably I'd tape the CIS pipe back up and adjust it's position until I got the 500rpm I should have?

Sorry to bombard you with questions like an annoying child

Also I tested voltages in all conditions I could, please see below...

Originally Posted by Alasdair1

If the battery is good I wouldn't think an sporadic alternator fault would affect the car too much to begin with. My alternator belt snapped and I still managed to get home on the battery ok. It might be either a bad earth or broken wire? Give the battery terminals and earth straps a good clean and check for corrosion or damaged wires. Also check the alternator belt tension and condition.
Alasdair

This I am yet to do, but will. Thanks for the tips

Originally Posted by TonyS9

Please don't randomly replace big items, you need to diagnose the problem or take it to someone whoe can.

I'd suggest a voltmeter for a start, check the battery voltage with the engine running with minimum load and on high beam load. The voltage should not drop more than 0.3V across any cable at the battery (bad connections can occur in the +,- or both) between low and high load. Any more can be considered a fault, but to be honest these things gradually degrade and a good harness should not drop more than 0.1V. It should be close to 14V, 13V will cause charging problems. The crimp connections can suffer with high resistance as well as chaffing caused shorts. shorts are dangerous and can cause a fire, even with the engine off parked up.

The severity sounds like a short. You need to have it diagonsed before it starts a fire.

Woah! I just went back outside to test the voltage at the battery terminals and there was a bloody fire engine parked outside! Luckily no burning bricks! I understand what your saying, and I will make sure I'm ready to bail out at any moment! But seriously, thanks.

I definitely wont be replacing anything before proving as I mentioned above. Been there, got the T-shirt.


Ok, on to voltages as suggested by Rversteeg and TonyS9:

I rigged my multimeter up to the back of my cigarette lighter terminal (after trying to be lazy, poking the croc clips in the front and shorting the lighter blowing the fuse of course) and got this...

Ignition on (engine off) - 10.8v
Engine running - 13.8V

Then I went for a drive keeping an eye on the meter balanced in the dash. Voltage floated around 13.6V +/-0.2V for the first half of the journey. There was the usual 'power cut' (stereo reset/lights dimmed) and the voltage appeared to drop to around 9/10V before shooting straight back up. The problem is my meter only polls around once every 500ms so if the power dropped to 0V for a split second, It would be near on impossible to read although I now know there is a drop.

Now on the second half of the journey, voltage had dropped to around 12.5V and eventually creeped back up to 13.0V. This was under no additional load to the first half of the drive. Parked up I turned on full beam, stereo, fans, and played with the windows and there were again, drops to between 10-12V.

Ok, then I tested voltage directly on the battery terminals under these conditions:

Ignition off - 12.15V
Ignition on - 11.7V
Engine running - 13.3V
Engine running under load - The first time I tried this the voltage dropped to 12.8V then slowly dropped by .01V a second (ish) until it slowed right down to about 12.3V.
The second time it maintained a steady 13.15V


I'm no expert, but this doesn't seem healthy to me.

[Rversteeg]

For cleaning and adjustment of the throttle you can also have a look at:
http://www.swedishbricks.net/700900F...h%20Adjustment
As the engine stalls with a disconnected CIS valve (or pinched off hose), it seems the basic throttle valve setting of 500 rpm is not correct. I'd first clean the throttle housing as it usually does not run out of its correct setting by itself. Take off the air intake hose, open the throttle valve by rotating the shaft and clean the housing with a degreaser.

The intermittent voltage drop while driving does not seem OK either. Battery voltages with alternator charging also seem on the low side (usually 13 to over 14 V). I suggest you check all earthing connections, especially the one between battery/chassis and the others carrying the full current of all electric consumers. Also earthing braided straps between engine / chassis (rear of rocker cover).
A faulty connection should show up on your voltmeter as a voltage difference when measured across (one clamp on battery earth / one clamp before earthing connection). You can also compare the voltage reading with both clamps directly on the alternator against the reading on the battery, to check the alternator wiring to the battery.

[James_EG_940]

I'll second Rversteeg's post.

For the voltage to drop like that, suddenly and intermittently, it just screams bad connection. Something, somewhere is either inhibiting or stopping completely the flow of current. Rust/corrosion, loose screw, cracked cable, dirt, oil, air!

[bmxJeff]

Originally Posted by Rversteeg

For cleaning and adjustment of the throttle you can also have a look at:
http://www.swedishbricks.net/700900F...h%20Adjustment
As the engine stalls with a disconnected CIS valve (or pinched off hose), it seems the basic throttle valve setting of 500 rpm is not correct. I'd first clean the throttle housing as it usually does not run out of its correct setting by itself. Take off the air intake hose, open the throttle valve by rotating the shaft and clean the housing with a degreaser.

The intermittent voltage drop while driving does not seem OK either. Battery voltages with alternator charging also seem on the low side (usually 13 to over 14 V). I suggest you check all earthing connections, especially the one between battery/chassis and the others carrying the full current of all electric consumers. Also earthing braided straps between engine / chassis (rear of rocker cover).
A faulty connection should show up on your voltmeter as a voltage difference when measured across (one clamp on battery earth / one clamp before earthing connection). You can also compare the voltage reading with both clamps directly on the alternator against the reading on the battery, to check the alternator wiring to the battery.

Thanks for the pointers and the useful info. Really helpful. I've got a busy work schedule right through till next week now but I'm gonna try and squeeze this in over the next few days. I'll post my findings when I'm done. Cheers

[bmxJeff]

Ok, so I'm getting a steady 14.1V at the alternator with the engine running and only 13.4V at the battery terminals. I read that I should have no more than a 0.2V drop from the alternator to the battery.


I have gone round and cleaned up all the earths I can find. These include:

• Battery earth lead to chassis rail
• Battery earth lead to block
• 2 x earth straps from firewall to cam cover
• 3 x spaded terminals on the inner left wing coming from light loom
• blue lead from back of alternator

Are there any more you know of?


I have tested voltage at the terminating points of all these earths and I read a steady 12.41V everywhere (engine off). Upon writing this, I have realized I may be being an idiot. Should I have had the earths disconnected at the terminating point to get a true test of the lead so that I'm not just testing an electrical 'bypass' through the chassis etc? Please confirm!


The only voltage drop I can recreate 9/10 times is the major 3-4V drop when I flash my full beams so I figure I'll narrow my search to that fault and start there.

[wrinkles]

I had the xmas tree lights on the dash If the altenator stops charging for e second or so it upsets the ecu and it can throw up all sorts of faults then the charge comes back and it all goes back to normal It is probably the regulator pack on the altenator nick one from the scrap yard off an 850 like I did. or get one off Flebay.

[bmxJeff]

Originally Posted by wrinkles

I had the xmas tree lights on the dash If the altenator stops charging for e second or so it upsets the ecu and it can throw up all sorts of faults then the charge comes back and it all goes back to normal It is probably the regulator pack on the altenator nick one from the scrap yard off an 850 like I did. or get one off Flebay.

I did wonder about the voltage regulator. I'll add that to my list of bits to scavenge from Silverlake, cheers

[TonyS9]

If the voltage is good at the alternator your regulator is fine, it is a voltage regulator.

You are getting too much drop in the alternator-starter-battery-chassis harness, however I would not rule out a faulty battery or intermittent short. My fault was in the cable crimp on the big + terminal at the battery. I drilled a hole in it to the copper and soldered with a blow torch. Solved the drop and slow charging problems. It was very consistent though.

The lighter socket goes through alot of harness so it doesn't give super accurate readings for simple volt drop issue. Working in the engin bay with tickover and high beam should be enough to identify high resistance joints.

You may well have a high restance joint(s) and they will be common on cars this age, but I still think you also have a short problem. I would also try and look at the alt voltage when driving, you need a bit of extra cabling into the cabin for the meter.

The alternator can provide about 100 amps, and the normal low load is about 25amps, mainly from the fuel pump and injectors, with maybe 5-10 A battery charging. If the alternator is dropping you have a short type of fault.

[rogerb]

Check between the lead part of the positive battery post and the starter stud.
I have had several cases of high resistance in the positive cable , usually where it is crimped at the battery end.
Also there was a service note about the positive battery cable rubbing through and shorting out , if yours is only a bit worn it may only short out for a second or so as the engine rattles about .
There should be very little volt drop across the lead , even with lots switched on . Check for volt drop across alternator positive lead and negative.
Sometimes the lead will get hot at the ends if they are poor.
You have way too much volt drop before the battery.
Only once every thing is ok at the battery ,should you start looking other things such as the key switch ,relays or fuse box ect.
Roger