Terminal 15 supply fault

[bald eagle]

Hi,

Does anyone know what the resistance of the starter solenoid should be. Mine is measuring approx 0.2 ohms on a calibrated meter. I would have expected this to be a little higher as the Fuse supplying it is 40A.

The reason I ask is that I am having intermittent failure of fuse 28 which then obviously stops the car running. Examination of the fuses show them to have melted rather than blown.

thanks in advance

Mike

[rwbaldwin]

Fuse 28 supplies the terminal 15 relay which feeds the terminal 50 relay which feeds the starter solenoid. Most of the items that hang off the terminal 15 relay have their own fuses except the terminal 50 relay.

Starter motors are different on different cars and you don't specify any details (fuel/engine/year/model). I would expect most solenoids to draw about 30A.

A faulty terminal 50 relay will given the symptoms you describe. Does the fuse blow at times other than when you are starting the car?

[bald eagle]

Hi,

Thanks for your reply. The first time the fuse blew I was driving at approx 30 mph, 30 mins into a journey. The second time a few seconds after starting and the third time on starting. The fuse has been replaced and the car starts and runs. The wiring has been checked to the relay box and the starter solenoid with no faults found.

The condition of the fuses suggest to me an overload condition or stressing due to them melting (leaving a small air gap) rather than an outright short.

Can you please expand on the terminal 50 relay fault, eg description of possible events.

Digging a bit deeper, I hear some cars have a relay that disconnects some circuits during starting, do you know if this is relevant here.

thanks

Mike

[rwbaldwin]

The relief relay doesn't switch terminal 15, so couldn't affect fuse 28. The feeds from the terminal 15 relay are protected by 5 or 10A fuses, so wouldn't cause the problem with the fuse 28 overheating.

When the solenoid is energised it initially draws a high current. Under normal circumstances you can keep cranking without fuse blowing because the holding current is far less than any initial current. An intermittant supply to the solenoid due to a faulty relay will cause a constantly high current. A faulty relay would explain why the fuse blows when you're not cranking.

If it were just shorted coils in the solenoid, it would only blow when you were cranking. After cranking the fuse would cool very quickly. It takes time to overheat a fuse until blows.

[bald eagle]

Hi Again,

Have managed a little more investigation tonight. I borrowed a DC current clamp. I checked the current to the starter solenoid during cranking several times and this was between 8.0 and 8.6A peak (set current clamp on max hold).

I then connected the current clamp around the battery feed to the engine bay fuse box. On switching the ignition on it was reading 24 - 26A and fluctuating, the glow plug warning light was out . Heated rear screen, lights etc were not on. This seemed a bit high. I checked the inrush peak which was 91A a couple of times. Also noted interior light dimming. NB did not attempt to crank at this point.

The only thing I switched was the ignition key, now inrush is 13A and steady state 7A. Have been unable to repeat above figures even after leaving car for an hour. Interior light is no longer dimming.

Car starts and runs, something is lurking, I was hoping to be able to pull fuses and relays to eliminate where the current was going. Obviously something else not connected may have been switching in. Current with ignition switch in off position is 0.2A approx

The only other thing I noticed was a high pitch whistle from a power supply, this may or may not be normal, I don't usually stand with the bonnet open and the ignition switched on.

Aren't intermittent faults fun.

[rwbaldwin]

Most clamp multimeters have a low sample rate. The 8/8.6A is about right for the holding current on a good solenoid but it will draw more on initial energisation.

The current from the battery is terminal 30 which feeds the terminal 15 relay, but also feed the glowplug control unit/relay, ECU power relay, etc. It's possible the glow plugs come on very briefly so you might expect a very high current for a tenth of a second or so.

Fuse 28 feeds all the terminal 15 items via their own individual fuses (5A/10A) with the exception of the terminal 50 relay contacts (most of the other relays have contacts fed from terminal 30).

The noise you heard could have been from the terminal 50 relay contacts. I've heard them buzz but I haven't heard a high pitch whistle.

If you can repeat the high current draw, pull the terminal 50 relay. It's worth taking it out to have a look anyway.