Battery/Alt Issues needing diag assistance

Good Morning All!

Recently my 1991 Volvo 740 Non Turbo Automatic has been giving me some dead battery in the morning or after sitting a while issues.

Here's whats going on:

• I have a sub woofer connected up properly to come on only with key on power

• I have done the "big 3" power and ground cable upgrade (man those stock ones are tiny)

• I have converted from the clutch fan to a 2 speed 940 electric fan. Instead of having it go off of the 3 wire bmw coolant sensor (which I have and can do) I have the low speed come on with key on ignition and high speed come on when I turn the AC on. (how that works is when the AC pressure sensor says pressure is good and sends the 12V to the clutch, I have tapped into that and send it to a relay as the signal for power to be sent to the OEM volvo electric fan relay (safety redundancy) that is fused.)

• Checking voltage (I have an HKS turbo timer (in prep for +T) installed that has a voltmeter) some days it will be at 13.8 at idle with not accessories on, and around 13.2-3 with ac on full blast.

Other days it will be much lower (13.0 no accessories and 12.2-3 with ac on full blast)

• My alternator field is working (magnetized when on) but it is very hot (too hot to touch bare) and I removed it and examined the voltage regulator and brushes. The top brush was worn down quite low so I got some new ones and replaced. The bearing in the alternator spins nicely

Still getting same voltage. I have since pulled the alternator and will get it tested today

• I had the battery pulled and checked and it tested out good.

• On my dashboard my battery light does not come on when the key is turned forward, and it does not come on when I ground the igniter (D+) wire from the alternator. I have the open housing later design alternator.

Connected to the dash, I have always had a funky speedometer that did not read the correct speed.



That is as much that I can think of right now. I want to give you guys as much information as possible so we can eliminate the unknown

Thanks for your help! I can add pictures of anything needed

The alt warning light not working is a pertinent piece of information here.

On Volvo's, the warning bulb circuit is used as the exciter circuit for the alternator. It provides a trickle of current (as the bulb provides resistance) to jump start the alternator.

Without external excitement, the alternator *often* will self-excite - as residual magnetism in the steel parts of the alternator make enough current to get it going, even without field currents electro-magnetizing them. But sometimes it won't, sometimes when the alternator is hot, or if you didn't rev it high enough, or who knows why. Sounds like in those cases you're just running off the battery.

I'd start by pulling that warning lamp wire off the alt and see if it has 12V+ with the key on. Then probably go to the cluster and check for a burnt out warning bulb.

Does it make sense that a burnt out warning bulb could itself cause problems with the system it's supposed to warn you about? Not particularly, but it is what it is.

JohnMc said:

The alt warning light not working is a pertinent piece of information here.

On Volvo's, the warning bulb circuit is used as the exciter circuit for the alternator. It provides a trickle of current (as the bulb provides resistance) to jump start the alternator.

Without external excitement, the alternator *often* will self-excite - as residual magnetism in the steel parts of the alternator make enough current to get it going, even without field currents electro-magnetizing them. But sometimes it won't, sometimes when the alternator is hot, or if you didn't rev it high enough, or who knows why. Sounds like in those cases you're just running off the battery.

I'd start by pulling that warning lamp wire off the alt and see if it has 12V+ with the key on. Then probably go to the cluster and check for a burnt out warning bulb.

Does it make sense that a burnt out warning bulb could itself cause problems with the system it's supposed to warn you about? Not particularly, but it is what it is.

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OK thanks for this. I thought I may have an issue with the bulb so I pulled it out and from visual the filament looks ok. I will check resistance first and then 12V to make sure it illuminates.

Also in my diagnosis I checked the voltage of the exciter wire and when plugged into the alternator it consistently made .5V less than the main power wire on the alternator (ie if alternator + was 13.8, the exciter was 13.3 and so on)

When unplugged voltage on the wire was 0 (this makes sense)

I will test with the cluster plugged in but pulled out that 12V is being sent to the terminals for the battery bulb.

Follow JohnMc's direction. The trickle of current through the charge indicator light is critical to reliable boot strapping of the alternator field winding on start-up.

Per a recent similar thread (but on a 240) - you might be better off checking that exciter wire with a bulb than with a multimeter. Multimeters can be misleading - showing you N volts, but with the faintest whisper of amperage, not enough to really do anything. But if it can light a test bulb, it's enough current.

On the 240, if the bulb is burnt out, or there's an issue on the cluster printed circuit, you can get other bulbs that light up, and you can get 12V backflowing through other circuits on the cluster, but not really enough to reliably kick-start the alternator.

Maybe swap the bulb for another 'known good' bulb in the cluster? If it still doesn't light up when you ground that wire by the alternator, start looking at the printed circuit on the cluster, and look at a wiring diagram.

JohnMc said:

Per a recent similar thread (but on a 240) - you might be better off checking that exciter wire with a bulb than with a multimeter. Multimeters can be misleading - showing you N volts, but with the faintest whisper of amperage, not enough to really do anything. But if it can light a test bulb, it's enough current.

On the 240, if the bulb is burnt out, or there's an issue on the cluster printed circuit, you can get other bulbs that light up, and you can get 12V backflowing through other circuits on the cluster, but not really enough to reliably kick-start the alternator.

Maybe swap the bulb for another 'known good' bulb in the cluster? If it still doesn't light up when you ground that wire by the alternator, start looking at the printed circuit on the cluster, and look at a wiring diagram.

Click to expand...

Do you have or know the location of the gauge cluster wiring diagram and charging circuit? If I can bypass the cluster completely and have a bulb isolated I can check to ensure it lights (or read inline amp draw on that circuit)

UPDATE:

Got my alternator tested at autozone and the machine said it failed completely. I asked for a printout with the specs but the guy operating it said that wasnt possible...

How could it fail completely when I had it on the car and the car will run? I just want a readout showing the load and its output :/

I am going to nashville this weekend to pick up a new alternator, one from a junkyard, and multiple voltage regulators so I can have some backups. I have also heard of people installing ford, vw, and GM units? I would love to have a 120a VW bosch alt on there

I've had these alternators fail internally with problems in that exciter circuit so they either are intermittent or not charge at all. Eventually becoming completely non functional. Your car will run fine and start fine on the battery. It just won't continue to be that way as the voltage on the battery decays. You can usually get about 20 minutes to an hour of driving when you are only using battery power. Depending on accessories.

I would buy new voltage regulators. Like on ebay. You can get voltage regulators with a slightly higher voltage set point than the original ones. This is helpful because modern batteries like a bit more charging voltage. Get something like a 14.4v or even a 14.6v set point.

Once you have a known good battery perform a parasitic drain test with a good meter. I wouldn't wanna see more than 30 milliamps drain. If you are up at 100-300 milliamps your batt can easily die overnight.

FYI - most cars, or at least anything after the 1960s, use the alternator exciter wire to turn on multiple different console check lights before the engine is started. If only one bulb is burnt out, the alternator field still gets charged through the other bulbs and the alternator will work fine.

If you look at the 240 Greenbooks, they'll show the diodes in the console circuits that allow the alternator exciter wire to turn on multiple lights -- the diodes isolate the exciter from the normal check-light sensor. As time has gone by, even the detailed wiring diagrams have stopped showing the diodes and circuit board details. I don't know if the 740 Greenbooks still show the diodes in the console PCB.

TLDR - at key on engine off, if just your alternator light is off then the lamp is burnt out. If multiple lights are off, then the alternator exciter wire to the console is most likely broken or disconnected.

Slap 12v to exciter terminal with a jumper once per drive and it will charge.

UPDATE:

Found 2 bricks in the junkyard in nashville (1990 REX wagon & 94 940 turbo)

I pulled alternators from both, gauge cluster from the 90, and a bunch of small parts I needed for my =T ill be doing in July.

Installed the gauge cluster from the 90 and the 100a Bosch I got from the 940 turbo and we are sitting at 14.4 V idle no accessories and 13.8 fan and AC and lights maxed out. Looking good to go guys

Recently my 1991 Volvo 740 Non Turbo Automatic
Found 2 bricks in the junkyard in nashville (1990 REX wagon
Installed the gauge cluster from the 90

Click to expand...

How did you manage that? The 740 clusters are completely different between 90 and 91.

MasterBlaster said:

How did you manage that? The 740 clusters are completely different between 90 and 91.

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I know my car is a 91, the one at the junkyard had 1990 written on the side. My 91 has the older style gauge cluster plugs.

As to how i did it:

1) Remove gauge cluster from dash
2) unplug gauge cluster
3) plug in new gauge cluster
4) install gauge cluster in dash

hope that helped

jvluntzel said:

I know my car is a 91, the one at the junkyard had 1990 written on the side.
My 91 has the older style gauge cluster plugs.

1) Remove gauge cluster from dash
2) unplug gauge cluster
3) plug in new gauge cluster
4) install gauge cluster in dash

hope that helped

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It wasn't the plugs I was curious about. It was how you managed to cram the rectangular 90 cluster into the 91's slope-sided hole.



Either yours isn't a 91, or the yard mislabeled the "90".









Oh, almost forgot...

mine is a 91, and the one in the yard was sloped. Maybe its the super rare "late" 1990

Hmmm... any chance the butt-end of the 90 said "760"?

That fancy-pants version of the 7-series started using the sloped cluster in 88, along with an aluminum hood, "aero" headlights, tilt steering, independent rear suspension, and a crapload of other bling the lowly 740 would have to wait a few years to get (if at all).

Maybe head back to the yard and see what else you can snag?

Just saw this: a 1990 760 from the For Sale forum here:

http://forums.turbobricks.com/showpost.php?p=5812128&postcount=31213


<Img src="https://images.craigslist.org/00E0E_1Xtdyi7AJrm_1200x900.jpg" width=800></a>

You just made me realllllllllllllllllllyy want a 760. It ticks all the boxes. IRS, Aluminum, turbo