Testing/Experimenting with Fuel Pump Relays

[abscate]

You can test for an open relay, but evaluation the coil and pull in isn’t really possible with a VOM

You can find a really dead one, but not one that is failing on hot/ cold

Ok. Assuming my fail to start issues are caused by freezing temps affecting an iffy relay, if I removed the suspect relay and kept it cold, what would be the tell-tale sign, a certain meter reading in one of the caps?

You might see a change in coil resistance, or a change in the number of amps it draws under load( more difficult test)

The best way would be just freeze it then quickly apply 12 volts to the relay and see if it pulls in

Make sure you get polarity right on the pull in or you will fry it

**** new project for 2020****

Get our hands on some relays that have hot failed and try to characterize and document a diagnostic , which will be named the Sonic Tonic , of course.

[precopster]

When we first purchased our 1995 850 we had the same issue, non start in hot sunbaked weather, even if car was unused.

Pulled the pink relay out and found some dry solder joints under the cover which were dealt with quicker than a western style gun-draw. No more fuel problems for 5 years.....

[SonicAdventure]

Update: the car would not start this AM, 21 F outside temp. Pulled the 103 (this one happened to be made in Hungary) and replaced it with the German-made 103 from my '98 n/a V70. Car immediately started and ran fine.

Took the Hungarian 103 inside and tried to test it asap. No solder joints issues or other physical damage seems evident. The resistance when posts 15 and 31 were probed was the same as the other units I tested last night, ~1.76 on the 2k ohms setting. However, the silver and brown caps were noticeably higher, coming in at 1.19 and 1.79 respectively. Using a laser thermometer, the caps were reading 50 F as the relay had been inside for a few mins. I plan to check them again when they have warmed up to the same temp as the other units as tested last night.

The next step will be to put both of my Hungarian 103s in the freezer for a few hours and check all the readings again and see if there are differences. I would like to test them under load when cold, but I'm not quite sure the best way to do that. If it does come up that freezing (~30 F or lower in my case) temps will prevent an otherwise functional relay from working, perhaps we could theorize that in cold weather non starts where 103 issues are suspect and no spare is on hand, a pull and quick shot with a heat gun (or keep it in your pockets for a few mins?) would be an acceptable emergency protocol?

[volvolugnut]

Please continue your testing. This is getting interesting. Last summer I did some hot tests on relays using a heat gun. I did not find any failures by testing resistance or coil movement.
volvolugnut

[volvolugnut]

You may try to make all your test wire connections and place the relay in an insulating container before placing in freezer. Than get the container assembly out of freezer and quickly test.
volvolugnut

[abscate]

Update: the car would not start this AM, 21 F outside temp. Pulled the 103 (this one happened to be made in Hungary) and replaced it with the German-made 103 from my '98 n/a V70. Car immediately started and ran fine.

Awesome. They fail hot, they fail cold, they fail at room temperature.

Definitely a relay to harvest from a collision-wrecked car in a PnP