Alternator wiring...tach issue

[Greg Kruse]

I've recently completed a wiring harness replacement on my 1990 E32-200/Universal M25XP. I was originally inspired to do this due to intermittent tachometer function as well as intermittent starter button failure...(ie. push button, no start). I also bought new gauges and re-fabricated my cockpit control panel. New tach, temp guage, volt meter.

After landing all wires in engine compartment and at the cockpit controls, I was psyched to hear the engine turn over and start immediately! Partial Success.

However, my tachometer is still not working. I took volt readings between AC tap and ground and was seeing readings of around 300 mV at slow idle and around 0.8 V at max rpm. So I took the alternator into the shop and explained what I was seeing. They put it on their bench and showed me voltages closer to 6 to 7 volts, between the same AC Tap and ground. So, I decided to have them go through and work their magic with brush and diode replacement, and we decided the AC voltage issue I was seeing was possibly a grounding issue.

Short story: Alternator back place, same problem with low voltage between AC tap and ground...and tach still not registering any rpm. I've freshened up all the grounded connections with wire brush/dremel and have verified good conductivity between alternator ground--engine block--battery.

So, maybe I'm just wiring something incorrectly. I've attempted to follow wiring diagrams provided by Catalina Direct with the wiring harness replacement...and re-routing Positive Output directly to the starter post.

I've attached a simple diagram showing the business end of my prestolite alternator and how I've got things wired. Any quick glances at this for obvious errors would be much appreciated. In Lieu of some wiring misstep, are there any ideas out there as to why I'd be getting the low voltage readings at the AC Tap??

Thanks
G.

 

[K2MSmith]

Interesting you mention this. I just had the local diesel mechanic work on my boat to reduce dripping/leaking of the stuffing box. While he was at it, I had told him my tach stopped working a few months ago. In order to fix it, they took the alternator out and took it back to the shop to replace "a broken wire". I'll have to ask for more details once I get the invoice but they told me it is now fixed (along with the leaky stuffing box). It sounds like most of the work is going to be in the alternator fix. The stuffing box was just a matter of tightening...

 

[Jerry VB]

Your diagram indicates you don't have anything connected to the ground terminal. Looking on the Prestolite "Universal Alternator Wiring Instructions", they state "Our alternators have an isolated ground terminal so a ground wire will need to be added from alternator ground to battery ground." If you are missing this wire, nothing will be happening.

The wiring instructions also indicate the SEN (sense) terminal needs to be connected to the battery "+" terminal (connecting it to the alternator output connection is good enough). If the SEN is not connected, the alternator might not be putting out any voltage because it thinks the battery is disconnected.

No tach output might be a symptom of no alternator output. When you run your engine in its current configuration, measure the voltage at your batteries. If it is less than 14v, your alternator is not putting out power. Connect the ground (if not connected) and check again. Connect the "SEN" connection and check again.

If the alternator is working, the fault may be in your wiring or your tachometer on the panel. A faulty tach is less likely because it is new, but sometimes new stuff is broken too. It is also possible the your new tach is incompatible with the alternator tach output.

For completeness, note that you must use the "AC" setting on your multimeter since the tach is an AC signal (DC will read ~0v when fed an AC signal).

Suggested debugging steps:

1. Disconnect the tach wire at the alternator, start your engine, and see if you are getting good voltage on the tach output.
2. Connect the tach wire at the alternator and disconnect it from the tachometer meter on the panel. Check the alternator output again for valid voltage.
3. If (2) passes, connect everything and check the voltage at the tach, measuring the AC voltage from tach terminal - to - terminal and also from tach terminal to a known good ground (ideally a temporary wire you run from the engine block ground). Also, with everything powered off, use the ohmmeter function to measure the resistance between the "ground/negative" connection of the tach to a known good ground - it should be very low (under 1 ohm). You can also measure the resistance across the two tach terminals. I would expect it to be quite high - above 1K ohm, most likely above 10K ohm.

If (1) is good, the fault is in the wiring or the tach itself. If (2) is bad you have a wiring problem (short to ground most likely) somewhere in the wiring harness. If (2) is good, it implies the tach is the problem.

If you need to diagnose the problem further, you may want to remove your wiring harness and panel and then plug them in at the engine so you can get to both sides of the harness and panel more conveniently. This would be especially useful for drilling down on step (3).

 

[Greg Kruse]

Your diagram indicates you don't have anything connected to the ground terminal. Looking on the Prestolite "Universal Alternator Wiring Instructions", they state "Our alternators have an isolated ground terminal so a ground wire will need to be added from alternator ground to battery ground." If you are missing this wire, nothing will be happening.

The wiring instructions also indicate the SEN (sense) terminal needs to be connected to the battery "+" terminal (connecting it to the alternator output connection is good enough). If the SEN is not connected, the alternator might not be putting out any voltage because it thinks the battery is disconnected.

No tach output might be a symptom of no alternator output. When you run your engine in its current configuration, measure the voltage at your batteries. If it is less than 14v, your alternator is not putting out power. Connect the ground (if not connected) and check again. Connect the "SEN" connection and check again.

If the alternator is working, the fault may be in your wiring or your tachometer on the panel. A faulty tach is less likely because it is new, but sometimes new stuff is broken too. It is also possible the your new tach is incompatible with the alternator tach output.

For completeness, note that you must use the "AC" setting on your multimeter since the tach is an AC signal (DC will read ~0v when fed an AC signal).

Suggested debugging steps:

1. Disconnect the tach wire at the alternator, start your engine, and see if you are getting good voltage on the tach output.
2. Connect the tach wire at the alternator and disconnect it from the tachometer meter on the panel. Check the alternator output again for valid voltage.
3. If (2) passes, connect everything and check the voltage at the tach, measuring the AC voltage from tach terminal - to - terminal and also from tach terminal to a known good ground (ideally a temporary wire you run from the engine block ground). Also, with everything powered off, use the ohmmeter function to measure the resistance between the "ground/negative" connection of the tach to a known good ground - it should be very low (under 1 ohm). You can also measure the resistance across the two tach terminals. I would expect it to be quite high - above 1K ohm, most likely above 10K ohm.

If (1) is good, the fault is in the wiring or the tach itself. If (2) is bad you have a wiring problem (short to ground most likely) somewhere in the wiring harness. If (2) is good, it implies the tach is the problem.

If you need to diagnose the problem further, you may want to remove your wiring harness and panel and then plug them in at the engine so you can get to both sides of the harness and panel more conveniently. This would be especially useful for drilling down on step (3).

Hi Jerry,

Thank you for your comprehensive reply. This is super helpful!

So the first thing I'll do is check the voltage at the batteries, engine running. I did check the output voltage at the alternator (between (+) output and engine ground) and I was getting 13.03 Volts (DC) consistently regardless of engine rpm. Then I'll add this ground wire to the ground terminal on the alternator. This ground wire existed between ground terminal and engine before I started messing with anything, but when I picked up the alternator from the repair shop, he explained that the grounding would be sufficient through the alternator housing/mounting bracket connections. But this is easy to do and I'll start there. And check voltage at batteries again.

Step 2 will be landing a wire to the SEN terminal. It sounds like just installing a short jumper wire between the SEN and the (+) on the alternator will do the trick. This makes sense...wire from alternator (+) output goes directly to (+) post on the starter motor which also terminates large ga. wire from battery. And check voltage at batteries again.

Okay...here's the embarrassing part...So I was checking output DC voltage on the "AC" Tap...despite the "AC" in the name of the tap! I suppose this likely explains the issue from the very beginning...throughout the process I've had meter set to DC. In some ways, I hope this is the issue, although it does not explain why the tachometer is not responding to engine rpm. I did purchase from Catalina Direct and they claim the tach is set up for the Universal M25xp. And I have a meter to calibrate if necessary if/when I get it to operate at all.

I will go through these initial reality check steps above and then go through the debugging steps you describe...all make sense. My hunch at this point is that maybe the Tach is bad...but debugging steps will establish this. But the SEN-to-battery connection sounds like it could be a likely culprit too.

Again, thanks for your time and help on this. I really appreciate it.

I'll report back.

Greg

 

[Jerry VB]

Hi Jerry,

Thank you for your comprehensive reply. This is super helpful!

So the first thing I'll do is check the voltage at the batteries, engine running. I did check the output voltage at the alternator (between (+) output and engine ground) and I was getting 13.03 Volts (DC) consistently regardless of engine rpm.

That is lower than I would expect, possibly due to the missing sense wire, missing ground, or possibly because that is where the regulator is set. Cars typically charge at 14.7v (+/-) but that is hard on batteries. Boats typically charge at a lower voltage than cars (typically 13.5-14.0v?) to prolong the battery life.

Then I'll add this ground wire to the ground terminal on the alternator. This ground wire existed between ground terminal and engine before I started messing with anything, but when I picked up the alternator from the repair shop, he explained that the grounding would be sufficient through the alternator housing/mounting bracket connections. But this is easy to do and I'll start there. And check voltage at batteries again.

Normally that is the case but this alternator (per the data sheet) is made with the ground isolated from the frame (it is a better technique but more expensive to do this).

Step 2 will be landing a wire to the SEN terminal. It sounds like just installing a short jumper wire between the SEN and the (+) on the alternator will do the trick. This makes sense...wire from alternator (+) output goes directly to (+) post on the starter motor which also terminates large ga. wire from battery. And check voltage at batteries again.

It is better to connect to the battery "+" because then it will compensate for the wire losses between the alternator and the battery, e.g. if you have 0.5v drop in the wire, the alternator will adjust its output +0.5v so the battery is at the target voltage. You should not have significant wire losses so a simple jumper to the alternator (+) should be fine. (As the battery is charged, the charging current goes down so the wire loss also goes down. The net result is the same other than a theoretical improvement in battery charging rate/time.)

Update: your tach problem is very likely due to the missing ground connection.

 

[Jerry VB]

For completeness, you want to turn off your shore power battery charger (flip the breaker) while you are testing your alternator -> battery charging. The 13v on your battery was quite possibly your battery charger's "float" voltage.

Don't forget to turn the breaker back on when you are done. ;-)

 

[Jerry VB]

More on the sense (SEN) input connection... if you have an ammeter in your cockpit panel, that is the Old Way and is a Bad Way because it entails a long length of undersized wire and probably an inappropriate connector. You really want to rewire your alternator output to the starter (+) connection which will have a good connection back to the battery. See Universal Diesel Engine – Wiring Harness Upgrade for details on how and why.

If you have the Old Bad ammeter wiring, you will benefit from connecting the SEN input to the battery (safety first: install a fuse at the battery end). With the SEN connected to the battery, it will compensate for the voltage drop in the undersized wire going up to the ammeter and then back down to the battery. Compensating for the wire's voltage drop will (theoretically) shorten the time it takes to recharge your battery.

The reason I contend connecting the SEN line to the battery is normally unnecessary is because of how batteries charge.

• The alternator will charge a very discharged battery at the maximum current it can put out, e.g. 40A. This is "constant current" mode and the battery voltage will be less than ~14v. The voltage drop in the wiring to the battery is at a maximum in "constant current" mode because the voltage drop (E) is the current (I) x the resistance (R) of the wire (E=IR). With the SEN connected to the battery, the alternator will increase its output voltage by +E which compensates for the voltage drop in the wire.
• As the battery charges, the voltage will rise to the alternator regulator setpoint (~14.1..14.5v). The alternator will transition into "constant voltage" mode and the current will drop as the battery approaches being fully charged (and the charging goes slower and slower). As the charging current decreases, the voltage drop in the wire to the battery decreases; "R" (its resistance) stays the same but "I" drops so "E" also drops proportionally. When the battery is fully charged, the voltage drop "E" in the wire from the alternator to the battery will go to 0v because the charging current "I" goes to zero amps.
  
  The result is that your battery will eventually be fully charged even with a Old Bad ammeter wiring but it will take measurably longer and might melt (or worse) the Old Bad wiring and connector (see the pictures in the MarineHowTo.com article).

If you have a good (large gauge) wire from the alternator to the battery, the wire's resistance "R" will be very low and thus the voltage drop will be low and thus the battery will charge better and faster even if the SEN connection is to the alternator output rather than to the battery.

P.S. I'm sure the reason "they" rationalized / justified using an undersized wire is because "normally, the battery is nearly charged and the current is very low." Unfortunately, abnormal situations do happen where e.g. the battery is faulty and pulls excessive current for a long time causing melting and potentially a fire.

 

[Kenneth K]

I've attached a simple diagram showing the business end of my prestolite alternator and how I've got things wired.

Minor comment about your wiring diagram (and this would NOT be related to your Tach problem). In Ericson's wiring for the alternator excitation circuit (the purple wire), power comes from the engine panel and not the starter post itself, thus, the excitation circuit is only energized when the engine panel key switch is "on." In your wiring, the excitation circuit will be hot any time a battery is selected ON.

Ref the Tach: I think most Diesel tachometers are designated either Mechanical Takeoff (alternator signal) or Magnetic Pickup. The two types are not interchangeable.

 

[Greg Kruse]

More on the sense (SEN) input connection... if you have an ammeter in your cockpit panel, that is the Old Way and is a Bad Way because it entails a long length of undersized wire and probably an inappropriate connector. You really want to rewire your alternator output to the starter (+) connection which will have a good connection back to the battery. See Universal Diesel Engine – Wiring Harness Upgrade for details on how and why.

If you have the Old Bad ammeter wiring, you will benefit from connecting the SEN input to the battery (safety first: install a fuse at the battery end). With the SEN connected to the battery, it will compensate for the voltage drop in the undersized wire going up to the ammeter and then back down to the battery. Compensating for the wire's voltage drop will (theoretically) shorten the time it takes to recharge your battery.

The reason I contend connecting the SEN line to the battery is normally unnecessary is because of how batteries charge.

• The alternator will charge a very discharged battery at the maximum current it can put out, e.g. 40A. This is "constant current" mode and the battery voltage will be less than ~14v. The voltage drop in the wiring to the battery is at a maximum in "constant current" mode because the voltage drop (E) is the current (I) x the resistance (R) of the wire (E=IR). With the SEN connected to the battery, the alternator will increase its output voltage by +E which compensates for the voltage drop in the wire.
• As the battery charges, the voltage will rise to the alternator regulator setpoint (~14.1..14.5v). The alternator will transition into "constant voltage" mode and the current will drop as the battery approaches being fully charged (and the charging goes slower and slower). As the charging current decreases, the voltage drop in the wire to the battery decreases; "R" (its resistance) stays the same but "I" drops so "E" also drops proportionally. When the battery is fully charged, the voltage drop "E" in the wire from the alternator to the battery will go to 0v because the charging current "I" goes to zero amps.
  
  The result is that your battery will eventually be fully charged even with a Old Bad ammeter wiring but it will take measurably longer and might melt (or worse) the Old Bad wiring and connector (see the pictures in the MarineHowTo.com article).

If you have a good (large gauge) wire from the alternator to the battery, the wire's resistance "R" will be very low and thus the voltage drop will be low and thus the battery will charge better and faster even if the SEN connection is to the alternator output rather than to the battery.

P.S. I'm sure the reason "they" rationalized / justified using an undersized wire is because "normally, the battery is nearly charged and the current is very low." Unfortunately, abnormal situations do happen where e.g. the battery is faulty and pulls excessive current for a long time causing melting and potentially a fire.

Hi Jerry,

Thanks for the additional information. Fortunately, I have re-worked the original/incorrect wiring with the new wire harness installation. I replaced tach and temperature gauges (still optimistic the tach will work once I get these wiring gremlins worked out) and replaced ammeter with a voltmeter. And I have a 10 gauge wire running the short distance from the (+) output on the alternator right to the (+) post on the starter motor...no long run to the cockpit controls and back. So that's resolved.

I'm a big fan of doing things the "right way" and from our thread here, I'm concluding the right thing to do is a jumper from the SEN terminal right to the (+) output on the alternator. Rather than running this wire from SEN to battery. We are dealing with such short wire runs in the first place...wire feet from SEN to battery might add up to 4 LF, max.

Looking forward to getting down to the boat this weekend to sort through all these recommendations and hopefully see that tach register rpm.

Thanks
Greg

 

[Greg Kruse]

Minor comment about your wiring diagram (and this would NOT be related to your Tach problem). In Ericson's wiring for the alternator excitation circuit (the purple wire), power comes from the engine panel and not the starter post itself, thus, the excitation circuit is only energized when the engine panel key switch is "on." In your wiring, the excitation circuit will be hot any time a battery is selected ON.

Ref the Tach: I think most Diesel tachometers are designated either Mechanical Takeoff (alternator signal) or Magnetic Pickup. The two types are not interchangeable.

Hi Kenneth K,

I realize my wiring diagram was not clear on the point you raised. The purple wire (in reality on my boat) comes from the load side of the starter switch in the cockpit...I see that my drawing makes it look like its going to the starter motor post. amateur hour wiring diagram, to say the least.

Thanks for looking that this, really appreciate the community here!

Greg

 

[Greg Kruse]

For completeness, you want to turn off your shore power battery charger (flip the breaker) while you are testing your alternator -> battery charging. The 13v on your battery was quite possibly your battery charger's "float" voltage.

Don't forget to turn the breaker back on when you are done. ;-)

Thanks for this point as well. I'm sure this was likely playing a part to confuse matters. I did not consider this.

G

 

[dxulander]

Greg, tach issue ever get resolved?