How to increase V90 charging voltage?

[martin calva]

Question: How can I modify the voltage regulator of my V90's alternator to increase the battery charging voltage?

Background: My V90 when running, shows a battery voltage of around 13.7V. I want to increase it, so the battery is charged better (the car does not get run very often and usually only short, 30 minute max, journeys). I'd like the charging voltage to be around 4.5 volts.

I replaced the alternator regulator with a new one and the charging voltage increased, but only by a very small amount.

On my 940, I solved a similar problem by soldering a silicon diode into the regulator (so the regulator now thinks that the battery voltage is ~ 0.6V too low and increases the output of the alternator accordingly). Very quick easy and effective.

It's not obvious to me how to modify the V90's voltage regulator. It is different from the 940's regulator and I don't have its circuit diagram.

Also, whereas changing the 940's regulator is a 5-minute job, on the V90 it is getting on for a day's work so it's not easy to experiment with it. (Accessory belt, radiator hose, air-con pump and alternator all have to be removed and replaced).

I tried connecting a silicon diode in series with the D+ lead to the alternator but that made no difference to the charging voltage. Not surprising, as the current measured in that lead, with the engine running, was almost zero (perhaps a couple of mA).

Thanks for any advice.

[cheshired5]

Before doing anything, it's important to check the alternator output on the actual alternator.
You'll often find that the alternator is working absolutely fine and it's actually poor connections reducing the voltage that makes it into your electrical system. This video shows how to check voltage drop and improve system voltage.
https://m.youtube.com/watch?v=fd5LeTdxIzg

A failing diode can also reduce DC voltage output.
https://m.youtube.com/watch?v=dhf6IdBbE3Y

[JRL]

Excellent video.

[TonyS9]

Also 0.3V can make a fair bit of difference to the usual +2V charging. I would not recommend increasing the alternator voltage as not all the parts will be rated for 16.5V, or at least its an unknown.

The usual problem (940) is the battery crimps corrrode on the cable. If you are getting 13.7 normally, the lights might drop this to 13V which will give very poor charging (50%).

With the engine running turn the lights on high beam and measure the voltage at the battery and the alternator (rev at little if necessary), then trace the drop to each connector. The cables are very heavy duty and drop less than 0.01V from memory.

Th voltage at the alternator should be the total of all the drops and voltage at the battery. (eg 0.5V at each terminal +13V at battery = 14V at alternator)

The connectors can be improved by drilling a hole and filling with solder (or just replacing with standard aftermarket items.

[TonyS9]

Ah just watched that video that I have repeated somewhat, but I will say the guy doesn't actually understand how electricity works, and is talking some rubbish about whether the meter is showing + or - and how that indicates which side is faulty.. all it is is which way round the leads are and which way the current is flowing across the joint you are measuring.

He also should have measured alternator voltage directly across the alternator and not alt+ to chassis as bad contacts can happen in either side.

Overall he gets there in the end, but I am surprised it worked, normally a bolted connection like that (and the mating surfaces where shiny) would not be the problem.

In any case you can use the volt meter probes across smaller and smaller sectons to find where it is dropping, so stab the probe through the insultation and put the other one on the crimped connector, this is where I found my problems.

[cheshired5]

Originally Posted by TonyS9

Ah just watched that video that I have repeated somewhat, but I will say the guy doesn't actually understand how electricity works, and is talking some rubbish about whether the meter is showing + or - and how that indicates which side is faulty.. all it is is which way round the leads are and which way the current is flowing across the joint you are measuring.

He also should have measured alternator voltage directly across the alternator and not alt+ to chassis as bad contacts can happen in either side.

Overall he gets there in the end, but I am surprised it worked, normally a bolted connection like that (and the mating surfaces where shiny) would not be the problem.

In any case you can use the volt meter probes across smaller and smaller sectons to find where it is dropping, so stab the probe through the insultation and put the other one on the crimped connector, this is where I found my problems.

The point in the video about the display showing a + or - reading is to explain which of the two connections has the lower voltage.

A minus figure means the red lead is touching the connection with the lower voltage and a positive figure means the black lead is touching the lower voltage connection.
It's just a simple explanation of the reading for people who are new to testing.

Granted, the actual reason for any drop can be at any point in between but the video advises to take care of both ends.
Like you say, you can also test ever closer points if required but a simple cleaning of contacts was all that was needed in this case to bring the drop to an acceptable level.

The point is also made in the video at 9mins to test the ground side independently with a known good earth so testing the alternator b+ to chassis ground is absolutely fine if you've already established an acceptable voltage drop on the alternator ground side.

I don't think he's talking rubbish and I also think his knowledge of electricity is plenty good enough to diagnose an alternator fault.

[martin calva]

Many thanks for all the comments.

I'd already checked all the wiring. I can't remember exactly but it was a difference of about 200 mV between the alternator terminal and the battery + terminal, with headlamps, foglamps, heater blower, rear window heater, etc, all turned on.

The problem genuinely is that the alternator is not putting out a high enough voltage. I'd like 14.5 or thereabouts rather than the 13.7 I'm typically getting.

What I am after is either:
- detailed circuit diagram for the regulator, so I can figure out how to modify it. Or instructions on how to do the mod.
- source of a regulator giving perhaps half a volt more than the standard regulator.
- some other suggestion how to increase the alternator output voltage.

[cheshired5]

If the 13.7V is taken from the b+, your alternator should be putting out more voltage as standard.
It doesn't need modifying, it just needs diagnosing to work as designed.
What's the AC output?

[baggy798]

I've looked at a 940 and V90 wiring diagram and they are wired the same. What alternator do you have?

Dave Barton does adjustable regulators for external fan type alternators:
http://www.davebarton.com/AdjustableVoltage.html

Although I think yours will be the 100 amp internal fan type, which will have a regulator type similar to this:
http://articles.mbz.org/electric/regulator/

[martin calva]

Originally Posted by cheshired5

If the 13.7V is taken from the b+, your alternator should be putting out more voltage as standard.
It doesn't need modifying, it just needs diagnosing to work as designed.
What's the AC output?

Many thanks for the comment.


it just needs diagnosing to work as designed.
Sounds like good advice to me. Some months back, I swapped the voltage regulator and the output increased marginally. It is possible that the 3.7V was what it was with the old regulator and it is now at ~3.9V. (It was clear in my mind when I posted the query but now I'm not sure.)

What voltage should it be putting out? I have seen comments on the web that 3.7V is usual with a Bosch alternator and Bosch should have tuned its regulator to give more output volts. Up to now, I had assumed that the alternator was working as intended by Bosch.

What's the AC output? I don't know, but a very good question.

Please bear in mind that the rear of the alternator is essentially inaccessible on the V90. To get at it involves partly draining the rad, disconnecting the radiator hose, removing the auxiliary drive belt, removing the power steering pump and then removing the alternator (some of whose fixing bolts and nuts are hidden from view and have to be done by feel). To try the alternator then needs all the above to be refitted.

Ideally I'd like to find a solution that requires the minimum of the above to be done. Get hold of a S/H alternator and try a swap?