Minimum voltage for reliable 60g heating?

[georgia mike]

This past fall I installed 8 new AC Delco 60g glow plugs in my '86 M1008. Everything has been fine up until a couple of weeks ago when the truck began to get harder to start. I checked the glow plugs with mixed results. Most checked fine; a couple checked no good until I pulled them from the heads. Weird for sure. Anyway, I checked the voltage coming off the GP relay, as well as at the plugs and I'm only getting 11 volts. Batteries measure ~12.5v each static. It should also be stated that the PO installed deep cycle batteries in this truck (well, the rear one says marine deep cycle, the front says marine starting). I know these types of batteries aren't meant for automotive use, so these will be going to my boat soon and will be replaced with a pair of group 31's.

So I guess the meat of my question is: Is 11 volts too low to reliably heat the 60g glow plugs?

 

[MakersTeleMark]

Have you done the resister bypass?

 

[georgia mike]

Yes it was done before I bought the truck.

 

[MakersTeleMark]

Charge your batteries. How are your alts? How long is the card cycling the plugs?

 

[georgia mike]

As far as I can tell (without actually timing it), I'd say at least 30 seconds. I just rebuilt the passenger side alt a few months ago. Both seem to be charging well. I'm just suspicious of those marine batts. I'll throw them on the charger tomorrow and see if it helps.

 

[doghead]

With your GPs back in,(GP system complete)

Measure the voltage at the top(input) of the GP relay with it activated.(key on)

Measure the voltage of the output(bottom) with it activated.(key on)

If you see a different(lower) voltage on the output side, your relay is bad. That means you have resistance/bad contact in the relay.

A bad relay will result in longer wait times and hard starting.

 

[Recovry4x4]

A bad relay will result in longer wait times and hard starting.

Right up to the point that if fails. Just changed one out. 11.5V is quite low for the task.

 

[georgia mike]

Yes, IIRC I checked the voltage to/from the relay and there was a slight drop going through. I'll double check when I get home this afternoon and report back.

 

[Skinny]

Let's not forget a good voltage drop test across the front battery terminals and the cable going to the 12v post above the GP relay. Could have a sneaky pete problem there too.

Recovry4x4...dig the avatar photo

 

[georgia mike]

Just got home and checked the relay.

Key off, I have 12.5 volts on the input, 0 on the output.
Key on, engine off I have 11.1 volts on input and output.
Key on, engine running:
Input has 14.2 volts
Driver's alternator: 14.2
Passenger's alternator: 28.6 (am I doing something wrong with my meter there?)
Batteries are the same (14.2 & 28.6 depending on which terminal I probe)

I should also mention that it was fairly cold yesterday and the batteries sounded weaker than I've ever heard them. I honestly thought they were going to give out, but the truck finally started. I guess that's where having a 24v system pays off. I'm going to check into some new batteries sooner rather than later, I think. If nothing else I'll have the correct type batteries in it and it will at the very least eliminate my concerns about the deep cycles.

 

[cpf240]

My guess would be that one, or both, of the batteries are weak and should be load tested.

In the first post it was mentioned that some of the glow plugs didn't pass the test while still mounted in the heads, but tested fine on the bench. Perhaps some of them aren't making a good ground connection to the heads, and so are not functioning properly? Or maybe they have an internal connection that is intermittent, and the process of removing tweaked them into a better internal connection?

 

[scottladdy]

As far as starting is concerned. What were the temps when you tried to start it? 30 seconds sounds a little long for GA temps. In zero degrees F up here I was seeing wait light cycle times from the stock GP controller of about 18 seconds. Most of the time these days (low to mid 40's) it is about 12 seconds or so.

The card changes the wait time based on several parameters. It can increase the wait time to compensate for what it thinks are GP's missing from the circuit and will cycle the relay on and off to reduce the chances of cooking what plugs you may have left. Check the stickies regarding the GP circuit.

Was the GP relay changed? I ask because a lot of folks, myself included, had the relay apparently fail after the bypass was performed.

Have you ruled out fuel delivery issues as a possible cause for your starting problems? Is there an air leak somewhere?

Regarding your batteries:

Deep cycle batteries generally aren't designed to handle the high discharge rate loads starters can put on them. They are designed to recover well from a deep discharge that occurs over time.

AGM's tolerate it a good deal better.

On the other hand, starting batteries don't like to be discharged deeply as a general rule.

Again, AGM's tend to deal with it better. Optima has specific charging instructions for recovering a very deeply discharged starting battery.

See if you can get the batteries load tested.

 

[georgia mike]

I called the local Interstate Battery place today and I'm going to go pick up a pair of G31's tomorrow afternoon. I have a boat that needs batteries so the ones in the truck should fit the bill quite nicely.

As for the temperatures, I think the high yesterday was in the high 30's or low 40's when I paid attention to the relay click off. It did start, but not without a fight, and it smokes (white at first, then black as the engine warms a bit) a bit more than normal until it's warmed up. mind you this situation didn't just pop up all of a sudden; it was more like a couple of weeks ago when it had gotten very cold and it seemed to struggle a bit to start, and has just gotten progressively worse since then. It got into the 20's last night I think, but rebounded quite nicely today to the low 50's. It was still a bit more of a struggle to start than when I first installed the GP's, but it did start much easier than it did yesterday. After it's been warmed up, it never fails to fire right up.

 

[georgia mike]

Or maybe they have an internal connection that is intermittent, and the process of removing tweaked them into a better internal connection?

That was my thoughts. I work in a machine shop so I may whip up a little fixture to thread the GP's into so I can test for an internal failure of some sort. It's really strange; the meter showed no change at all when I tested them in the heads, when I pulled them they measured just fine, so I reinstalled them, only to have them show no change at the meter again.

 

[Skinny]

Even though your engine running test of 14.2 volts probably is a non-load test (GP's have turned off by then) on both the battery and alternator, I would guess new batts...

 

[dependable]

I have a boat that needs batteries so the ones in the truck should fit the bill quite nicely. .

Suggest put good batteries in the boat too, but maybe you are only on a lake.

 

[MarcusOReallyus]

In the first post it was mentioned that some of the glow plugs didn't pass the test while still mounted in the heads, but tested fine on the bench. Perhaps some of them aren't making a good ground connection to the heads, and so are not functioning properly? Or maybe they have an internal connection that is intermittent, and the process of removing tweaked them into a better internal connection?

I'm wondering if the block ground strap is getting flaky. Long shot, but worth checking.

 

[georgia mike]

I'm wondering if the block ground strap is getting flaky. Long shot, but worth checking.

I wouldn't rule it out. The PO did replace the engine so it's worth a look.

 

[georgia mike]

Bought a pair of "refurbished" group 31's at my local Interstate Battery store and installed them. I can say that the engine spun over much more quickly than with the deep cycles. Since it was on the warm side this afternoon I couldn't really gauge whether or not it took care of the cold start issue. I'll try it tomorrow AM and see what it's like. Hopefully this will take care of it; if not I'll dig deeper.

Thanks for all the help and advice (as usual)

 

[MarcusOReallyus]

How much did those refurbs set you back? I need to replace mine.