940 Alternator Issues

[John Sweeney]

Afternoon all, I'd be grateful for any advice on a no-charge problem with the Denso 100A alternator in my 1992 940. I've had a look back through the forums and, having checked everything in the list below, I'm wondering if there is anything else I should consider before taking it to the mechanic.

The problem happened just as I returned from a 800 mile round trip two days before the lockdown. After all those miles through some pretty rough weather running as reliably as ever, at the end of my road the washer fluid level, bulb failure, battery, brake warning, handbrake and ABS lights came on whilst the engine was running and the alternator stopped charging.

I've looked into the following (I don't have much knowledge of auto-electrics so apologies for any confused terms or illogical thoughts):

- Check output voltage on alternator. Fault, 11.5v.

- Check voltage on exciter wire on key position II. Good, 11.5v.

- Check continuity from alternator ground terminal to lead ground point on engine block. Good, no resistance.

- Check continuity from lead ground point to battery negative terminal through all stages. Good, no resistance.

- Check continuity from alternator positive terminal to battery positive terminal. Good, no resistance.

- Check continuity from exciter terminal to alternator ground terminal. Good, no resistance.

- Check voltage drop between alternator positive terminal and battery positive terminal. Good, 0.1 volt.

- Check voltage drop between alternator body and battery negative terminal. Good, 0.2 volt.

- Check voltage drop between engine block and battery negative terminal. Good, 0.1 volt.

- Check continuity via brush terminals in situ through field wiring. Good, no resistance, brushes and field wiring OK.

- Check for magnetism on pulley bolt when on key position II. Good, field windings OK.

- Check bearings and belt. Good, no noise, no play in bearings, belt running as it should.

- Install new brushes and re-test. Still not charging.

- Install new voltage regulator. Now no warning lights on dash, re-test all connections. Still no warning lights so their route to ground is now faulty.
Remove new voltage regulator and test continuity against old one. New Voltage regulator faulty(!) - infinite resistance. Refitted old VR, warning lights now on again so their route to ground through alternator is now fine.

Still to check:

- Old VR might still be faulty - order 2nd new VR and install.

- Ignition switch could cause similar faults - fit new ignition switch and re-test.

Does anyone have any thoughts on what might be wrong? Have I made any bad assumptions? I'm nearly beaten and the mechanic beckons!

[Rversteeg]

You say "still no warning lights on dash" after you have replaved the voltage regulator. This is a bit confusing, there should not be any warning lights when the engine is running. Did you check the charging voltage of the new VR?

I once replaced the brushes. I am sure I did everything allright, but could not get the alternator to charge again. I took it to an old-fashioned alternator / start motor refurbishing workshop. They refitted the brushes, put it on their test bench and got it to work again. Maybe it is a good idea to have the alterantor tested before start suspecting other components.

Rob

[John Sweeney]

Thanks for the question Rob. Sorry, should have been clearer there - once the new regulator was installed, there were no warning lights on the dash at all, whether the engine was running or not. Grounding the exciter wire which runs from the warning lights to the alternator brought the warning lights back on so there was evidently a new ground fault after installing the new regulator. Refitting the old regulator solved that but brought me back to where I started - warning lights remaining on when the engine is running and no charge output from the alternator. Good thought about getting it professionally tested. It will probably come to that unless I can think of anything else.

[Laird Scooby]

Originally Posted by John Sweeney

Afternoon all, I'd be grateful for any advice on a no-charge problem with the Denso 100A alternator in my 1992 940. I've had a look back through the forums and, having checked everything in the list below, I'm wondering if there is anything else I should consider before taking it to the mechanic.

The problem happened just as I returned from a 800 mile round trip two days before the lockdown. After all those miles through some pretty rough weather running as reliably as ever, at the end of my road the washer fluid level, bulb failure, battery, brake warning, handbrake and ABS lights came on whilst the engine was running and the alternator stopped charging.

I've looked into the following (I don't have much knowledge of auto-electrics so apologies for any confused terms or illogical thoughts):

- Check output voltage on alternator. Fault, 11.5v.

- Check voltage on exciter wire on key position II. Good, 11.5v.

- Check continuity from alternator ground terminal to lead ground point on engine block. Good, no resistance.

- Check continuity from lead ground point to battery negative terminal through all stages. Good, no resistance.

- Check continuity from alternator positive terminal to battery positive terminal. Good, no resistance.

- Check continuity from exciter terminal to alternator ground terminal. Good, no resistance.

- Check voltage drop between alternator positive terminal and battery positive terminal. Good, 0.1 volt.

- Check voltage drop between alternator body and battery negative terminal. Good, 0.2 volt.

- Check voltage drop between engine block and battery negative terminal. Good, 0.1 volt.

- Check continuity via brush terminals in situ through field wiring. Good, no resistance, brushes and field wiring OK.

- Check for magnetism on pulley bolt when on key position II. Good, field windings OK.

- Check bearings and belt. Good, no noise, no play in bearings, belt running as it should.

- Install new brushes and re-test. Still not charging.

- Install new voltage regulator. Now no warning lights on dash, re-test all connections. Still no warning lights so their route to ground is now faulty.
Remove new voltage regulator and test continuity against old one. New Voltage regulator faulty(!) - infinite resistance. Refitted old VR, warning lights now on again so their route to ground through alternator is now fine.

Still to check:

- Old VR might still be faulty - order 2nd new VR and install.

- Ignition switch could cause similar faults - fit new ignition switch and re-test.

Does anyone have any thoughts on what might be wrong? Have I made any bad assumptions? I'm nearly beaten and the mechanic beckons!

It's the rectifier, almost certainly. https://www.ebay.co.uk/itm/NEW-ALTER...L/392733899488

If you have a local autoelectricain that can order from Wood Auto, the part number is RTF49899, it's a screwdriver job to change and make sure you clean the slip-ring end bracket (aka casing) where the voltage reg attaches. Also check your slip rings for wear.

Very often if the rectifier has fried itself, the new voltage reg appears not to work so it's worth changing the rectifier first and then retesting with the new reg as well.

[John Sweeney]

Dave, many thanks for your advice. I've spent three days looking at the inside of the back of the alternator and I didn't even notice there was a separate rectifier! From your eBay link I recognise it now and I'll investigate tomorrow.

[Laird Scooby]

Originally Posted by John Sweeney

Dave, many thanks for your advice. I've spent three days looking at the inside of the back of the alternator and I didn't even notice there was a separate rectifier! From your eBay link I recognise it now and I'll investigate tomorrow.

You're welcome John, i've recently rebuilt one with the exact same symptoms (but not a Volvo one, still Denso though) and the problem was definitely the rectifier. Using a new voltage reg also gave the same results as you got with a new one on the dodgy rectifier, fitted a new rectifier and all worked as it should.

You should be able to test the rectifier with your multimeter on the diode setting, if you get any short circuits (you almost certainly will) then the rectifier is fried. Note you'll need to undo the screws linking it to the stator windings so they can be removed as to not cause false readings.

[TonyS9]

Could well be the rectifier but just to check on the testing..

When you checked the voltage drops was this with a large load, or using the meter continuity?

The best sensitivity is to put the high beam headlights on with the engine running and measure the voltage across the cable.

[NI_Volvo_Nut]

The single spade wire on the rear of the alternator controls the dash lights, have you check this is connected and the connection is good?

[Laird Scooby]

Originally Posted by TonyS9

Could well be the rectifier but just to check on the testing..

When you checked the voltage drops was this with a large load, or using the meter continuity?

The best sensitivity is to put the high beam headlights on with the engine running and measure the voltage across the cable.

Originally Posted by NI_Volvo_Nut

The single spade wire on the rear of the alternator controls the dash lights, have you check this is connected and the connection is good?

All the clues needed to work out it's almost certainly the rectifier (subject to confirming by testing out of circuit) are highlighted in a quote of the OPs OP below :

Originally Posted by John Sweeney

Afternoon all, I'd be grateful for any advice on a no-charge problem with the Denso 100A alternator in my 1992 940. I've had a look back through the forums and, having checked everything in the list below, I'm wondering if there is anything else I should consider before taking it to the mechanic.

The problem happened just as I returned from a 800 mile round trip two days before the lockdown. After all those miles through some pretty rough weather running as reliably as ever, at the end of my road the washer fluid level, bulb failure, battery, brake warning, handbrake and ABS lights came on whilst the engine was running and the alternator stopped charging.

I've looked into the following (I don't have much knowledge of auto-electrics so apologies for any confused terms or illogical thoughts):

- Check output voltage on alternator. Fault, 11.5v.

- Check voltage on exciter wire on key position II. Good, 11.5v.

- Check continuity from alternator ground terminal to lead ground point on engine block. Good, no resistance.

- Check continuity from lead ground point to battery negative terminal through all stages. Good, no resistance.

- Check continuity from alternator positive terminal to battery positive terminal. Good, no resistance.

- Check continuity from exciter terminal to alternator ground terminal. Good, no resistance.

- Check voltage drop between alternator positive terminal and battery positive terminal. Good, 0.1 volt.

- Check voltage drop between alternator body and battery negative terminal. Good, 0.2 volt.

- Check voltage drop between engine block and battery negative terminal. Good, 0.1 volt.

- Check continuity via brush terminals in situ through field wiring. Good, no resistance, brushes and field wiring OK.

- Check for magnetism on pulley bolt when on key position II. Good, field windings OK.

- Check bearings and belt. Good, no noise, no play in bearings, belt running as it should.

- Install new brushes and re-test. Still not charging.

- Install new voltage regulator. Now no warning lights on dash, re-test all connections. Still no warning lights so their route to ground is now faulty.
Remove new voltage regulator and test continuity against old one. New Voltage regulator faulty(!) - infinite resistance. Refitted old VR, warning lights now on again so their route to ground through alternator is now fine.

Still to check:

- Old VR might still be faulty - order 2nd new VR and install.

- Ignition switch could cause similar faults - fit new ignition switch and re-test.

Does anyone have any thoughts on what might be wrong? Have I made any bad assumptions? I'm nearly beaten and the mechanic beckons!

John (the OP) has already done all the testing to confirm it's the rectifier but good practice is always to confirm before replacement, just in case. However being sat at a computer miles away i don't have that luxury.

[John Sweeney]

Originally Posted by TonyS9

Could well be the rectifier but just to check on the testing..

When you checked the voltage drops was this with a large load, or using the meter continuity?

The best sensitivity is to put the high beam headlights on with the engine running and measure the voltage across the cable.

Thanks all. Tony, I checked using the meter with a moderate load (I think it was heater fan on full and heated seats).

I'm planning to test the rectifier tomorrow and at least then I'll know what the problem is or be able to admit defeat!