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Replacing the glow plug solenoid


M3-20B Glow plugs glow now, but….

Testing of old preheat solenoid and replacement with the new.

I performed various voltmeter tests on the old solenoid, as per email from Mark Peters. While not exactly following his expected outcomes, the tests seemed to generally indicate a fault with the solenoid, not with other wiring.

I replaced the preheat solenoid. There were some positive outcomes:

  • The engine warning buzzer turned off as soon as the glow plug switch was depressed. This hasn't happened since a couple of years ago when it began remaining on until the start button was depressed. (I never did learn why.)
  • The fuel pump could be heard to click rapidly with the depressing of glow plug switch. I dimly remember this happening sometime soon after buying the boat, but not again in the years since, even after a replacement (electric) fuel pump was installed two years ago.
  • The engine started immediately after 15 sec. of glow plug-- as was normal before this no-start stuff began a couple weeks ago. Problem solved, but….

… A new problem showed itself with the new solenoid

All the time the engine is running, it drops about 200 rpms every 9 seconds or so for a period of 2-3 seconds. With the engine running and the power switch at BOTH, the #1 (starting) batt. drops from 14.3 v. to 13.9 v. (as measured directly off the batt and at the panel voltmeter) The #2 house batt. drops from 14.3 v to 13. 8v. Throttled at a moderate idle it sounds at first as if the engine might actually die when this happens. Instead it revs back up again to the normal idle speed after a couple of seconds.

This occurs in the same way when the batt switch is on #1, BOTH, or #2.

The same effect (drop of a couple 100 rpms, voltage drop) takes place with the engine pegged at a higher speed (1500 rpms) as well.

Why is this happening? Solenoid flaw? Alternator problem (diode, voltage regulator)? I figure it must be something to do with the electrical system as the engine runs normally and strong except for this.

There have been no other changes made except the solenoid replacement.


Old solenoid from Universal M3-20B diesel. Something loose inside could be heard upon shaking the unit.


New preheat solenoid in place at back of a Universal M3-20B diesel engine, view facing forward. Note the separate ground wire running from the top left mounting bolt. According to the wiring diagram, that's a 10 amp circuit breaker in the small box, lower right side.

Replacing this with a new one ($25) solved a no-start problem when glow plugs tested fine and batteries and fuel system were good. The old solenoid clicked as usual when activated, but was still no good.

The starting problem was solved but a new one arose—engine speed dropping by a couple 100 rpms for 2-3 seconds every 9-10 seconds. This mystery remains to be remedied.

Well, that mystery didn't last very long. While seeming an outside chance that the new solenoid was faulty, I decided to buy a second new one. Voila! Upon installation of the second new solenoid the fuel pump clicked merrily on cue, the warning buzzer turned off when it should with the pressing of the glow plug button, and the engine ran fine with no more dipping RPMs,-- all was as it should be. And the engine's started and run fine ever since. Presumably this is a rare occurrence when a replacement part straight from the factory is faulty-- but it can happen.





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