Odd starter relay issue

[Landon427]

I have a '99 Wrangler SE that I just got last fall. It has a knock/tick that I am trying to figure out (topic for a separate thread), so I have only driven it when trying to diagnose the noise. It cranked over a little slow when starting. Last week it would barely crank over at all. The battery is good (new 5/24 and I keep a trickle charger on it; also Autozone proclaimed it 'good' for whatever that is worth). The connections are tight and resistance in the cables (battery to starter and starter to ground) is undetectably low with my multimeter (reads 0.0 Ohms), so I ordered a starter motor (haven't received it yet). I had never heard of a starter relay causing this problem, but I replaced it anyway since it's probably 26 years old.. finding one was a chore (Oreilly's didn't have one, Autozone didn't have one; fortunately Napa did - Proformer AR272SB) and now the motor turns over at a rate that seems normal. I don't know if this helps anyone, but now at least it's documented here... I'm curious if anyone else has had this happen?

 

[Chris]

Never experienced it personally, but at the age that most of the TJs are I'm not surprised to see little things like this fail. I'm glad it was an easy fix and thanks for sharing the fix with us!

 

[B1Toad]

I'm curious what is different about the NAPA relay compared to any other 5 pin ISO relay, or heavy duty 5 pin relay? Is the NAPA one even heavy duty? Does it show an amp rating?

 

[KennethS]

The material the contacts are made of have a direct relationship to the relay’s longevity. Better materials and higher quality manufacturing equals a longer life relay.

The contacts in the old relay likely have high resistance due to arcing/pitting/corrosion from years of use. We see this a lot at my workplace with relays that have been in use for a long time. If you look at the spec sheet for a given relay, there is stated number of cycles the relay is expected to perform before failing.

 

[B1Toad]

I'm kind of skeptical about how a relay can make the starter turn faster. All the relay does is engage the solenoid, when the solenoid is engaged the starter turns by drawing current from the fat cable directly off the battery, not from the relay circuit. The solenoid is either engaged by the relay or it isn't, there's no in between, if the relay is bad the solenoid would drop out and nothing would happen. What am I missing?

 

[Landon427]

I don't see anything showing an amp rating on the Napa relay. I'm not very familiar with different types of relays, I only know that I had to go to three different auto parts stores to find one. O'Reilly's first sold me the wrong relay (1/2 the size and wrong pin configuration), then when I returned and showed them the old one they couldn't find one that matched based on the part number stamped on it. They said a different O'Reilly's store might have one though. Autozone said they didn't have anything that would cross over based on the part number. At that point I started to wonder where to find one. Fortunately the Napa parts man seemed familiar with it, but the Napa relay he sold me shows up as an A/C automatic temperature control relay; it could be that Autozone and O'Reilly's carry it but under that kind of search term (rather than knowing it as a starter relay, which is what I was asking for).

 

[KennethS]

I'm kind of skeptical about how a relay can make the starter turn faster. All the relay does is engage the solenoid, when the solenoid is engaged the starter turns by drawing current from the fat cable directly off the battery, not from the relay circuit. The solenoid is either engaged by the relay or it isn't, there's no in between, if the relay is bad the solenoid would drop out and nothing would happen. What am I missing?

That all sounds reasonable. Higher resistance in the relay, if high enough, would cause problems with the solenoid engaging. The relay resistance shouldn’t affect the starter motor speed.

 

[Landon427]

I'm kind of skeptical about how a relay can make the starter turn faster. All the relay does is engage the solenoid, when the solenoid is engaged the starter turns by drawing current from the fat cable directly off the battery, not from the relay circuit. The solenoid is either engaged by the relay or it isn't, there's no in between, if the relay is bad the solenoid would drop out and nothing would happen. What am I missing?

I was surprised as well, it didn't make sense. I just don't know how else to explain why the motor turned over so much faster with the new one. Maybe I should do a head to head comparison to rule out some other variable? I could throw in the old one and see if it still turns over super slowly.

 

[macleanflood]

Why speculate when you can trigger the relay and measure the ohms...

-Mac

 

[Landon427]

I checked the old relay. Resistance between the two normally closed positions (85 and 86) is supposed to be 50-200 Ohms (or 50-150, depending on the Youtube video...). Mine measures 70 Ohms. Energizing 85/86 with a 9V battery gives a resistance between the normally open positions (30 & 87) of 0.1 Ohms, so the relay seems to be fine. Since the battery and cables checked out, I'm guessing that the variation in engine cranking speed was from the (presumably failing) starter motor? I don't know what else it could be. Thanks for the input, I'm not an experienced mechanic.