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A DRAFT - CAUTION using this

Harness Primer


 Universal Diesel Engine Harness Primer or, "Harness 101" 

-K Kloeber 07/14/2022



For the OEM harness:

First, to be clear we're talking about M25 and XP engines/harnesses and not harnesses on B-series engines.  The harnesses and wiring on the XPA and B series engines (e.g., M-25XPB) are different. 


The original harness had two in-line plugs  (aka "Trailer" :RV" or "Gummy Bear" plugs.)



The Gummy Bear inside the engine compartment (red arrow on right)

Below is NOT on a Catalina 30 (an Erikson)



The harness has these bi-directional functions

[Insert a flow diagram to illustrate this]:

To deliver TO THE PANEL:


1) "Full" battery voltage/current from the positive battery post, via the battery selector switch, battery cables, and the starter solenoid "B" post, to the cockpit panel (red, 10 awg HW [harness wire] #5.)

2) A (solid, clean, interrupted) alternating current pulse signal from the alternator to the tachometer.  
This signal DOES NOT go thru the gummy bear plugs for obvious reasons (HW #10, color varies - orange, gray, brown.)

At the cockpit panel, power is DISTRIBUTED TO the:


3) Blower, when its panel switch is on.

4) Gauges, tach, alarms, and gauge lights when the key switch is on.

IN THE OPPOSITE DIRECTION (from the panel), the harness provides:

5) (Via HW #3.)

Voltage and Current to the starter solenoid "S" terminal when you operate the key switch and/or starter button (depending on panel model) (color varies - yellow w/red stripe, white, solid yellow.)

6) (Via usually gray, HW #2)

Voltage and Current to the glow plugs, when you operate the preheat switch.  
If you have the glow plug solenoid/relay modification, it delivers voltage/current to that relay or solenoid that was added in the engine compartment.

7) (Via black HW #1)

The ground path to the negative battery post, via the grounded engine block and neg battery cable, to operate the blower, gauges, gauge lamps, and alarms.  That's all -- it has nothing to do with starting, your alternator, glow plugs, etc.

8 )  
(via light blue HW #7; tan HW #9)

The path (HW #7) from the panel alarm to the low-oil-pressure switch and (if the option is installed) coolant high-temperature switch (HW #9, tan.)
These both are "normally-open" switches that activate those alarms when the switches close (oil pressure goes low or temp goes high) to complete the alarm circuit negative path all the way back to the neg battery post (via the grounded switch / to the engine block / to the negative battery cable / to the battery / back to the panel via the #1 HW.).

9) (brown HW #8.) The path from the panel temperature gauge to the coolant temperature gauge sender (HW #8, brown.)


10) (pink HW # 11) The path from the panel fuel gauge to the fuel tank level sender.  This DOES NOT go thru the gummy bear plugs.


11) (black, not on the engine harness schematic) Ground wire for the fuel tank and fuel level sender.  sometimes this runs directly to the fuel gauge, and sometimes the gauge picks up the ground from the panel negative wire. This DOES NOT go thru the gummy bear plugs.


Off on a tangent here; NOTE the misnomer.  The industry calls it a temperature "sender" but it sends nothing (no signal or anything else) TO the panel gauge – it RECEIVES a tiny bit of 12v current BACK FROM the gauge.  The temp "sender" is a varistor (variable resistor) that lets a tiny amount of current flow from the gauge / to the sender / to the engine block / to the neg battery cable / back to the neg battery post / back to the gauge via the negative wire to the gauge panel.)  The Temp Gauge reacts to the resistance (or more properly "Impedance," the opposite of resistance) at the sender as engine temperature rises and falls and the "Fuel" needle inside the gauge deflects greater/less as the impedance increases/decreases.

So, a "sender" (e.g., temp sender) is typically a variable resistor and a "switch" (e.g., high-temp switch) is simply ON or OFF.

11) (via purple HW #6.) Voltage and (very low) current BACK TO the alternator to excite the alternator field so it begins to produce a charging current to the battery.
12) (via HW #9, usually red, sometimes black) Voltage and (very low) current BACK TO the fuel lift pump.  This DOES NOT go thru the gummy bear plugs.

The Gummy Bear Plug Fire Hazard

We all should agree that the gummy bear plugs are a problem (the hard white plug or sometimes black on the engine side is OEM Universal and is only marginally better than the mating amber/brown Catalina/Seaward gummy bear plug.)  
Universal supplied M-25 and XP/XPA engines with that white or black, 8-position, "trailer/RV" plug on its engine harness installed on the Kubota engines.  But if they're so bad why are they there? (Westerbeke/Universal still installs similar plugs on new engines.)

Because the engines came with a plug from the factory, CTY had to decide whether to:

a) Buy the mating harness extension from 
Universal.  NOT! ($$S)


(Universal's harness didn't exactly match the configuration of CTY's/Seaward's panels/instrumentation.)
b) Buy both the Universal harness AND Universal's cockpit panel.  NOT!! (due to co$t)

c) Cut off Universal's plug, and hard wire the harness extension directly to the engine pigtails 
(this might have added about 30 minutes of labor per install and reduced material cost (no special harness with fabricated gummy bear plugs would be necessary.)

d) Remove Universal's wiring and fabricate/connect its own harness to the engine (very time-consuming, $$$) 


e) (unfortunately, what CTY/Seaward did is) Fabricate a harness with a plug to mate to Universal's plug. 

Having a softer, amber color. gummy bear plug molded onto the harness wires.  (Reportedly, these were fabricated out of hot glue.)  Additionally, instead of hard-wiring the cockpit panel CTY, unfortunately, put another hidden gummy bear plug behind the panel.  But if CTY didn't do this, its techs would have had to connect the harness to the back of the panel, gauges, etc. (chance for errors) that came from Seaward. or would have needed to pull the harness down to the engine and connect it there.-- this way the techs just plugged in the panels and screwed them home.  The two plugs were just a time-saving and convenient way of eating the engine panel.

These two sets of plugs were obviously installed without realizing the initial downside of having plugs, or the potential implications for years to come thru who knows how many owners.  It's a prime example of needing to, when it comes to marine installations, totally over-bullet-proof products/materials, instead of doing only what is adequate for ideal "factory floor" conditions.

When I was young/dumb (before these great forums) I thought, "What the hell - clean up those plugs, squeeze the connectors tight to make good contact, tape them up to keep out moisture/corrosion -- and I'm good to go."  That was 1994, 10 years on the OEM harness.  Fast forward to 2006 -- two decades – and here's the engine gummy bear plug after unwrapping the tape during a wiring upgrade.  


So, engine heat destroys the plugs, but that's not the whole story. 

There shouldn't be high current going thru that type of bullet connector TO the cockpit panel and then BACK to the starter solenoid, glow plugs, and BOTH ways ESPECIALLY for the alternator charging circuit (which could theoretically be up to 50 amps.  


Couple that with potential corrosion on the plugs, 





and you have a real problem with (at best) poor or no starting, incomplete battery charging, gauge problems -- and at worst, potentially both plugs overheating, melting, and a fire hazard because there was no over-current protection provided on the original harness.

Thankfully, I had tape-wrapped my engine plug, or I surely would have had a short and maybe a fire in the harness at some point in time.  
For those who don't care to ELIMINATE the gummy bear plugs - Your boat, Your choice, but unfortunately, they could take out your dock neighbors' boats along with yours. 

Harness Issues

For the wiring harness itself - there were different versions but the basic issues are:

1. No overcurrent protection on the power feed to the panel.  Per ABYC there must be a fuse within 7" of the solenoid "B" post. (FIRE HAZARD)

2. The barrel fuse holder on the S wire (near the "S" terminal on the starter solenoid) is useless, problematic (corrosion), and should be REMOVED -- it may be replaced with a better fuse holder. (no-start issue

What does the fuse protect?  In case the quick-connect terminal falls off the solenoid or the starter solenoid locks up. 

3. The starter solenoid should be one with a threaded post "S" terminal, not a quick connect (I have replacements or you can fix your own connection there.) That 
type of terminal is just plain stupid -- it should be a threaded post, not a push-on/fall-off that can loosen from heat expansion and corrode, or pull off from tension on the wire.)  But remember that the starter solenoid was installed for a tractor and farmer who's on terra firma and accustomed to repairing his equipment -- not for an engine that's in one of the worst possible environments (high heat, corrosion, vibration, plus out of sight/mind, impossible to see/access to service, etc.)

(no-start issues.)

4. The original harnesses had the battery charging current/circuit traveling through the engine gummy bear plug, all the way to the cockpit, through the panel gummy bear plug, to the panel (ammeter,) and back through the panel gummy bear plug, to the engine space, through the engine gummy bear plug again, to the solenoid "B" post.  That's a recipe for at best incomplete charging of the battery due to the voltage loss each step of the way -- and at worst, plug overheating, total failure (corrosion, loose barrel connectors - FOUR of them in the plugs.)   (FIRE HAZARD and no-start issues.)

5. Older harnesses had a lighter gauge "S" wire (mine is 16 AWG.)  That's stupid for a critical circuit that needs full voltage/current to energize the critical starter solenoid.  I haven't measured it, but I suspect the solenoid's pull-in amps may be too high for 16 AWG, and fo
r consistent starts, it's certainly critical to maintaining MINIMAL voltage loss in that circuit.

(no-start issues.)

6. Universal put the harness negative wire on an exhaust manifold stud.  Stupid - where is constant expansion/contraction and corrosion more prevalent than at that location?
(panel/gauge issues.)

7. The connections to the engine components were non-marine-grade, non-heat shrink adhesive terminals.  A recipe for corrosion and failure.  (no-charge or poor-
charge or no-start or panel gauge/switch issues.)

8. The pigtail wire itself on the engine side was non-tinned copper.  That would have been acceptable if Universal used adhesive-heat-shrink terminals that seal the wire ends, but it did not.  A recipe for corrosion and failure. 

9. The terminals that Seaward used at the panel (switches, alarms, gauges, etc) were non-heat shrink adhesive Stak-on type terminals.  A recipe for corrosion and failure. (no-start or no-charge or panel gauge/switch issues.)

10. The power lead to the panel was 10 AWG - good for 50 amps or so.  But the voltage loss is considerable if you're running the blower, glow plugs, and trying to crank the starter.  Say it's 25 amps total - that can put you back at the starter solenoid with a little over 11 volts on a good day.  The situation is even worse if you have a 16-awg "S" wire between the start switch and the solenoid.


Quick Incremental Mods (some quicker than others) that can Improve the Electrical/Starting/Charging System



"AHS" = adhesive heat shrink (terminals, tubing, etc.) FTZ brand terminals are superior to many retail commonly available brands.  (IMO) the well-known brand "A...: terminals and lugs (@ West Marine) are inferior based on their gauge, size, and construction.

"AWG", "awg", "ga" = American Wire Gauge

"S" wire = yellow/red stripe (starter solenoid) wire in the engine harness, energized by the panel start button or key switch.

"S" terminal = 1/4" quick-disconnect terminal on the starter solenoid for the "S" wire.

"B" post = M8 terminal post on the solenoid for the positive battery cable.

"Heavy cable" = sized for your individual situation (alternator charging amps, charging and battery set up, energy budget; how you use the boat, future upgrades planned, etc.)




#1 (eliminate the fire hazard):


Fuse the (10 awg red) power supply wire from the solenoid "B" post to the cockpit panel, within 7-inches of the "B" post.

Use an FTZ-brand M8 starter lug, (not ring terminal) and a 10 ga, tinned wire, ATC weathertight fuse holder, butt-crimped to the harness power wire with heavy (mil-spec) AHS tubing on the ends.

Start w/ a 30 amp fuse - theoretically the glow plugs are 6amp each but they have run as high as 10a.  You don't want to fuse at the capacity of the 10 awg because there are lighter wires beyond that point and so you need to keep the fuse as low as possible.  You cannot size the fuse for the lightest wire (typically 16 ga on older harnesses) because the glow plugs will blow the fuse.  Unfortunately, it's not a perfect solution, but any fusing on the panel feed is better than no fusing at all.  To fuse this situation correctly you would need to put a second lower amperage fuse at the panel to protect the lighter gauge wires.


#2 (eliminate the fire hazard):


Eliminate the gummy bear plugs on the engine harness (engine end and behind the cockpit panel (butt crimping the ends.)  Extend the harness if necessary.  (also see *** below.)


#3 For better starting and/or charging and/or safety:


  • If you have an ammeter on the cockpit panel, do the voltmeter modification.
  • Move the battery negative cable from the bell housing to a starter bolt (both are M8 bolts.)
  • Replace the harness "S" wire with a 10 AWG wire (if yours is lighter than 10 gauge) to minimize voltage loss to the solenoid switch.  Eliminate the barrel fuse holder at the solenoid end or substitute an ATC fuse holder (use a 10 AWG holder w/ tinned wire, and a weather-tight cap.)


***Depending on the condition of the harness, wire gauges used, and condition of the terminals (no OEM ones were "marine grade" AHS terminals,) replace wire ends and terminals at the engine as needed.  Same for the panel, if a mess, rewire using AHS terminals. 

Or, replace the old harness w/ a new plug 'n play, drop-in replacement w/ "right sized" conductors.

  • Replace your Alt 10 awg charge wire with a heavy cable (min. 8 awg) from the Alt out charge post to the solenoid "B" post.
  • Add a heavy alternator ground cable (same gauge as the charge cable.)
  • Fix the quick-disconnect terminal on the starter solenoid (for the harness "S" wire.) 

(See also


  • Either:

* Replace w/ a solenoid w/ a threaded "S" post (change the "S" wire terminal to an AHS 10 awg (yellow) #6 ring terminal.

~ OR ~

* FIRST crimp an uninsulated FM quick disconnect, onto a 6" long, 10 awg, yellow wire.

* THEN solder the FM QD onto the solenoid tab.  Crimping the terminal first prevents solder from wicking up and affecting the S wire.

* Cover with mil spec AHS tubing (form a wire strain/bend relief so the wire doesn’t fail at the terminal.)


*  On the end of your pigtail you have 3 options:

 a.  Butt crimp an ATC fuse holder (w/ 10 awg, tinned wire and a weather cap, 20a fuse) between your pigtail and harness "S" wire.

*** The OEM fuse on the "S" wire is no longer needed if you fuse the harness red wire feed to the panel (Mod #1,) but it doesn’t hurt and it's a convenient location to troubleshoot voltage if you have a starting problem.

~ OR ~

b.  Crimp an FTZ fully insulated male QD on the pigtail; crimp a matching female QD on the "S" wire.

*** This QD location can be used to troubleshoot or plug in a remote starter switch to crank the engine instead of at the cockpit panel.

~ OR ~


 c. (a. + b.)


  • Add a power post or buss strip in the engine compartment for the 12v negatives (harness negative, fuel pump negative, Alt negative, bilge pump negative, etc.)  Jumper a heavy cable from that negative post or buss to the starter bolt (ie, battery negative cable.)  


  • Not part of the harness per se but, IF the battery ground cable is bolted to the bell housing it should be moved to the outer starter bolt.  The bolts are the same diameter (M8) but you might need a longer starter bolt if you have installed the alt and/or negative buss to that bolt.