MGB & GT Forum

All,

I am stumped on making the proper connections to a Denso(clone, via E-Bay) Mini Alternator. In the past I converted the car (‘71 B-GT) from a Lucas 5-wire alternator to a Bosch 3-wire. I due to hood clearance issues with the alternator being mounted on top of the A/C compressor is the reason I am going with the Denso alternator as it is physically 2/3’s the size of the previous Bosch alternator, but still rated a 55Amps output, and clears the hood when shut.

So far, the only connections that provide good power output (13.7VDC at idle RPM) and power the “Ignition (alternator) Lamp” are in the photos attached. However, with the engine running (idle and higher RPM’s), the Ignition Light stays, illuminated, despite the alternator is showing 13.7 VDC output. I found on Denso’s website that the alternator needs to reach 1200RPM to initially produce current, then will continue to produce current at lower RPM’s.

My quandary is why is the Ignition Lamp still illuminated while the alternator appears to be producing current? Is this a wiring issue or possibly a defective alternator?

Thanks in advance!

John Morrison

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'71 B-GT; “Petey The Petulant”
'05 Honda S-2000 “Saburo”
‘99 BMW M3 (Track Toy)

is there a ground wire to engine from motor mount to engine block?

I have that same frame 40 amp ND alternator on both my Morris and CBB GT. I ended up installing a diode in one of the small wires. I'll have to go out and have a look in the morning and see which wire (I'll get pics of both). Attached is a wiring diag I used when I wired up my cars. The Morris only used one small wire to get it to work. The GT required both small wires to get it all to work. BTW the electrical connector on mine are different but the connections should be labeled the same.

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CAUTION!!
Life's uncertain, eat dessert first
Know the rules well so you can break them effectively. Dalai Lama
Unapologetic proponent of vented front and 4 wheel disc brakes

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John,

I just got to thinking. Some of the eBay alternators are sold as a 1 wire self energizing alternator. In that case there may not be a provision for the ignition light even-though there are connections.

Does the alternator still work if you disconnect the small wire? If it does it's most likely is a 1 wire alt. An alternator shop should be able to help in getting the light to work using some of their voodoo.

Here's a better image

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CAUTION!!
Life's uncertain, eat dessert first
Know the rules well so you can break them effectively. Dalai Lama
Unapologetic proponent of vented front and 4 wheel disc brakes

Journal index
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Edited 1 time(s). Last edit at 2021-11-28 04:17 PM by geezer.

Yes, left engine mount to engine block.

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'71 B-GT; “Petey The Petulant”
'05 Honda S-2000 “Saburo”
‘99 BMW M3 (Track Toy)

Charles,

The alternator was advertised as a “3-wire”, but FWIW, its producing a charge (13.4-13.7 DCV) with just the brown “sensor” wire connected to the to the “IG” port on the alternator. In fact I drove the car about 10 miles today with the Alternator (Ignition) Light wire disconnected.

Looking forward to which diode you have installed and photos of how you wired it.

Safety Fast!

John

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'71 B-GT; “Petey The Petulant”
'05 Honda S-2000 “Saburo”
‘99 BMW M3 (Track Toy)

John,

I went back through my notes. I used the diode on my Magnette when I installed a bigger frame ND alternator. For some reason the engine would not turn off due to a back-feed from the alt. The diode solved that problem.

I did not use the diode with the mini alternator on my CBBGT or Morris. On the GT I hooked the Brown/Yellow to the L (light) terminal. I also ran a white wire from the white wire (switched) connection on the fuse block to the IG (ignition) terminal on the alt. That made everything work.

I found a couple of wiring diags for your alternator that you can try.

On yours I'm thinking that the brown is hot all of the time. Perhaps these images will help. My brown wires are attached to the BAT terminal.

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CAUTION!!
Life's uncertain, eat dessert first
Know the rules well so you can break them effectively. Dalai Lama
Unapologetic proponent of vented front and 4 wheel disc brakes

Journal index
https://www.mgexp.com/journal/Charles-Durning.2966/toc



Edited 1 time(s). Last edit at 2021-11-29 07:06 AM by geezer.

Charles,

It appears we are both using the “IG” and the “L” connections. I currently have the original “Sense” (brown) wire from the original Lucas alternator installation (used again as “sense” on he Bosch alternator) hooked to the IG port on the Denso alternator. That wire is “hot” with the ignition on. The only connection so far that will cause the Ignition Lamp to illuminate is its Brown/Yellow (N/Y) original lead to be connected to the “L” port on the Denso.

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'71 B-GT; “Petey The Petulant”
'05 Honda S-2000 “Saburo”
‘99 BMW M3 (Track Toy)

What John posted.

I just spent way too long fault finding a Denso alternator on my wife's car.

L (lamp) goes to the brown yellow warning lamp wire. When the alternator is charging, this wire is not a ground, so the lamp is not on. When it is not charging, this wire is grounded turning the lamp on.
Ign (ignition switched control). Needs to be hooked to a switched ignition source, such as the white wire at the fuse box. This is different than the Lucas design. This is the feed that tells the alternator to turn on (not the brown/yellow wire)
S (sense)- some alternators need this connected to a battery fed lead. It's often just jumped to the main output stud. Technically, it should route to the battery in case there is any voltage drop between the alternator and the battery, but in our cars it's not necessary. Some of the alternators (Suzuki Samurai) have this as a dummy and it doesn't need to be connected to anything


Personally, after working on this system (in a 1959 Morgan), I think the 16ACR Lucas alternators are a piece of cake.

Kelvin.

Kelvin nailed it! Just needed to run a jumper wire from the Battery (B+) post to the “S” connector. Ignition Lamp now works as advertised, and seeing 14VDC output from the alternator at idle RPM. Photos attached for reference.

Thanks All!

John M.

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'71 B-GT; “Petey The Petulant”
'05 Honda S-2000 “Saburo”
‘99 BMW M3 (Track Toy)