88/89 MC18 with PGM II, MC24 engine. Had it running on the stand for the first time a while ago but the RC Valve circuit was fried. Sent the PGM II to 2TRacelabs for rebuild of the RC Valve circuit and total refurbishment. Nick says he tested it and it was working perfectly. Was unable to start the bike yesterday. There was spark at the front cylinder but not the rear. Today i can get intermittent, or more accurately, occasional spark at the rear cylinder. I switched the front and rear coils and same result. The coil hooked to the front LT wires gives spark as intended, the one hooked to the rear HT wires only sparks occasionally. All connectors and terminals were cleaned and look in good shape. The black wire going to the rear coil is quite a bit shorter than the green and white wire suggesting a possible issue there? Battery fully charged.

HOping to hear this is a known problem with known solution, before working my way back through the whole wiring harness. Any suggestions welcome.

Have you checked all the grounding points are good? If it’s sparking occasionally then that sounds like it could be a damaged wire that feeds the rear coil, I’d check those first.

Thanks Zeus. I think that's exactly what it is. I was just hoping for a shortcut where to look. I am suspicious of the connector itself but it's hard to pull the cover off because it is a right angle connector.

Are you getting a current from ignition pickups?
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Why do things simply when you can complicate them

Thanks Rob. That is the kind of thing I'll be checking. how/where should i check? Electrical is a mystery to me!

In my manual it says

Remove the side cover
Remove the pulse generator coupler and measure the resistance between the W/Y - Y wires (rear pulse) and the W/BU - BU wires (front pulse generator)
Standard : 180 - 220 ohms (cap off at 20 degrees)
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Why do things simply when you can complicate them

tleighbell wrote:
Thanks Rob. That is the kind of thing I'll be checking. how/where should i check? Electrical is a mystery to me!

You'll find the Electrex Fault Finding Guide in the NSR-WORLD Workshop section.

As a member of the our forum, you also have access to our Honda Approved Engish Service Manuals* via the Members Content.

*Please note NSR-WORLD/nsr250.net is the ONLY officially approved resource for the NSR250 English Service Manuals. We kindly ask all members to please not support plagiarism by any other sources/sites, or buy [often incomplete] copies of our manuals if offered.

This is the part RobH is referring to:



And the MC18 R5/6K section:



Can I just ask... what's an MC24 motor? An MC21/MC28 hybrid?! 🤣🤣🤣
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NSR-WORLD.COM

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Thanks Andy. this should be very helpful. This applies to a PGM II, correct?

MC24?? LOL i'm not so good with math either. It' an MC28. Maybe i was thinking of the year, 1994.

The first table is the "general NSR250R" service information, and the second table contains the values specific to the R5/6K from the 1989 Supplement.
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Andy.
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Update for anyone still with me on this.

checked the pulse generators resistance as per manual and they are within spec. 208 ohm for front, 211 for rear.

Checked coils: resistance across the two terminals is 0.4 ohm front coil, 0.3 ohm rear coil. Slightly above manual specs but they both will send spark when hooked up to front cylinder wiring. Green terminal to plug cap resistance is 7.98 kohms rear, 9.24 kohms front, again within spec.

checked continuity of all wires from the 4 pin and 16 pin plugs from ECU to the female terminals that attach to the coil terminals. Zero ohms resistance for the green/white, black/blue and black/yellow wires. All pins in the plugs are in place and wires look secure.

One thing that seems strange is that i get zero or very low voltage when 1 probe in the green/white wire terminals and other to ground with ignition on. I get 0.18 amps at the green/white terminals and 0.25 at the black/blue and black/yellow wire terminals when connecting those terminals to ground with the probes.

|I have not done the max voltage tests of the coils and pulse generators as per the manual supplement for the R5K because i don't have a peak voltage adaptor. Is is worthwhile?

To recap: bike started and seemed to run fine on stand, except that RC circuit was burned out. Removed ECU and sent to 2TRacelab for repair of RC circuit and total refurbishment. After repair was tested by Nick and said to be in perfect working order. Reinstalled and bike won't start. Have good spark at front cylinder, rear sparks but only on rare occasions. Same result when front and rear coils are switched so coils must be OK.

I could do voltage in and out tests on ECU but i know there is voltage in and out because i get spark at front cylinder and can see the temp needle move slightly when ignition turned on.

Is it possible there is a problem with the ECU? wish i had a known good PGM II to swap just to see. Anyone got one to sell or loan ?

I feel this has got to be a problem with the circuit related to the green/white or black/blue wires to the rear coil. The wires themselves seem to be fine, so is there a way to test that circuit at the ECU?

Turned out to be as suspected: frayed wire at the connector on the black/blue wire on rear coil. Zero Ohm resistance reading with multimeter was misleading.

Glad you got it sorted.