Test data: starter and glow amps

[MR.T]

Thought this data might be interesting -- No problems, just checking some test equipment. It's starter amperage, cranking voltage, and glow plug amperage. Test data is from a 1988 7.3L with Mitsubishi starter, Ford glow plugs, and NAPA glow controller.

Initial conditions, battery 12.54 V (SoC), 41 F ambient, block temp 39 F, not started for 12 hours.

650+ starter Amps @ 10.41 Volts minimum (no Glow amps).

Cranking speed was good. Peak amps during cold cranking went as high as 1200 amps, but this is probably for only a fraction of a second during the initial engagement (and the meter is only spec'd to 1000 amps). After testing, it was started normally and fired within 1-2 seconds. Batteries (2) are rated @ 930 CCA, and tested @ 1020 and 1000 CCA ** (Large Pulse method-temperature corrected, Argus Analyzer).
​

Initial 370+ Glow plug Amps tapering to ~150 (18.75/plug) @ 14 seconds (WTS light out). A second meter was on an individual plug as a cross check.

The battery dipped to 11.4 volts *. Note that this glow amperage measurement was made at the 2 wires feeding the glow system below the starter solenoid on the RH fender. Troubleshooting Idea: The reading on one plug should equal the total reading divided by eight if all the plugs are good (and have equal resistance). The amperage is a moving target, so take the total amp reading just as the relay clicks off. Wait a few minutes, move the meter to a plug and repeat. If making a measurement using the larger wire from the battery terminal, subtract the non-glow amperage (about 10 amps) from the reading. It's not precise enough for comparing with a single plug, but access is easier.
​

Warm test, block temp 180 F:

410 starter Amps @ 10.73 Volts minimum (no Glow amps).​

Initial 310+ Glow plug Amps tapering to ~150 @ 7 seconds (WTS light out).

Even though the engine was warm, the hood was up and ambient temp back at the glow controller was probably still in the 40's. The glow controller may be designed to sense ambient temperature and change the amperage setpoint when glow heat turns off, and/or eliminate glow heat at higher ambient temps. Different controllers may use different methods, but in any case the plugs increase resistance and decrease current as temperature increases.​

General Notes:
* The voltage drop between Bat + and the glow controller (2 yellow wires) was roughly 0.9 v drop at 150 A (~0.6 V drop for every 100 A). Significantly higher probably indicates a bad connection (there is a multi-pin connecter C-129 on the RH fender for these wires).

** The cold cranking amp (CCA) rating refers to the number of amps a battery can provide for 30 seconds at a temperature of 0°F until the battery voltage drops to 7.20 volts for a 12V battery. When batteries are wired in parallel (like these trucks), their ampacity doubles.

 

[Ataylor]

Mr. T,

I was going to modify my glow plug system to go with a manual control (push button). I was going to use a 250 amp fuse to supply power to the GP controler. But after looking at your numbers, I don't think that fuse will be sufficent. I read some other threads about using a 250 amp fuse. The fuseable links can handle over 300 amps?

I also have new battery cables with 3/0 from battery #1 (drivers side) to battery #2, then 4/0 from battery #2 to starter. So I think I should be o.k. for the starter load.

Also, what brand of camper shell do you have on your truck.

Thanks,
Archie

 

[nyteshades]

Mr. T if you're going to do some more testing, I would be interesting in seeing the numbers with the motor plugged in. Great info btw.

 

[MR.T]

I've revised the amperage in the first post for cold cranking based on a re-test this morning. I'm not sure why the previous cold starter amperage was higher -- It had sat longer and was a bit colder, but I may not have cranked long enough to let the auto ranging work properly. Will probably do another test when it's colder.

Mr. T if you're going to do some more testing, I would be interesting in seeing the numbers with the motor plugged in. Great info btw.

If I do, I'll post. Funny thing is that in 22 years I've never plugged in the block heater.

Mr. T,

I was going to modify my glow plug system to go with a manual control (push button). I was going to use a 250 amp fuse to supply power to the GP controler. But after looking at your numbers, I don't think that fuse will be sufficent. I read some other threads about using a 250 amp fuse. The fuseable links can handle over 300 amps?

I also have new battery cables with 3/0 from battery #1 (drivers side) to battery #2, then 4/0 from battery #2 to starter. So I think I should be o.k. for the starter load.

Also, what brand of camper shell do you have on your truck.

Thanks,
Archie

The truck shell on mine is a Gem Top, which is apparently BrandFXbody now. Had it about 5 years, and like it.

Fuses have different time delays, and I think fusible links have quite long time delays. There is an inverse time relationship, higher current means less time to blow. I'd lean toward a fusible link for reliability with a long inverse time delay. It's amazing how much amperage wire can take for short intermittent duty. Glad these numbers were of use.

 

[Black dawg]

Ive seen around 800amps at 32* so that sounds reasonable for almost 40*

 

[jim x 3]

Mr. T,

I was going to modify my glow plug system to go with a manual control (push button). I was going to use a 250 amp fuse to supply power to the GP controler. But after looking at your numbers, I don't think that fuse will be sufficent. I read some other threads about using a 250 amp fuse. The fuseable links can handle over 300 amps?

I also have new battery cables with 3/0 from battery #1 (drivers side) to battery #2, then 4/0 from battery #2 to starter. So I think I should be o.k. for the starter load.

Also, what brand of camper shell do you have on your truck.

Thanks,
Archie

Archie,

I have a #6 AWG connection from my passenger fender relay hot side through a fuse to my GPC. The fuse is 175 A. I haven't had a problem with this configuration.

Regards,

 

[sle2115]

I run my gp's with 4 gauge cable and a 250 amp maxi fuse...never had a problem although I keep a spare in the glove box!

 

[Agnem]

Too bad you didn't include a voltage reading at a glow plug for your draw tests, however your amperage figures pretty much validate that the 7.3 GPC uses current sensing to determine glow time, which is what makes it a better system. Glow plug amps relates directly to glow plug tip temperature, so your 160 amp reading pretty much indicates that a glow plug that is heat saturated will draw 20 amps.

 

[MR.T]

Too bad you didn't include a voltage reading at a glow plug for your draw tests, however your amperage figures pretty much validate that the 7.3 GPC uses current sensing to determine glow time, which is what makes it a better system. Glow plug amps relates directly to glow plug tip temperature, so your 160 amp reading pretty much indicates that a glow plug that is heat saturated will draw 20 amps.

Mel, that's a good idea. Battery voltage dip with glow added to the first post.

Also, the initial test was measuring the glow amps at the large wire from the battery term to the solenoid -- and there's about 10 extra amps from non glow loads. Made another test to confirm, it's 18.75 amps per plug (150 total measured glow amps) when the WTS goes out.

One last observation. I disconnected two plugs, the WTS still goes out at 150 amps, but each plug had ~25 amps instead. Naturally that made for a much shorter WTS time too. A single plug's amperage can indicate whether all of them are working (because any single plug amperage should be 1/8 the total amps). Fun with meters...

 

[rjjp]

In a parallel circuit current remains constant and voltage varies, therefore all 8 plugs are getting 150 amps. Or do I have series and parallel confused, if something reminds me I'll look at this tomorrow when I'm awake.

 

[MR.T]

In a parallel circuit current remains constant and voltage varies, therefore all 8 plugs are getting 150 amps. Or do I have series and parallel confused, if something reminds me I'll look at this tomorrow when I'm awake.

All 8 plugs are in parallel and they (should) have equal resistance, so the current in each plug should be approximately 1/8 the total. With two meters, observation and theory agree. The voltage is approximately the same at each plug.

Using water as current, and voltage as pressure, is a good analogy for this. If you had a big hose (wiring) with eight small holes (glow plug resistance) of equal diameter and then pressurized the hose, all the small holes would flow the same and be 1/8 the total flow in the hose. The pressure in the hose would be constant.

In this analogy, if the holes get larger relative to the hose, the holes furthest downstream will have less pressure and flow less then the upstream holes. Same happens with wiring, but it's call voltage drop instead of pressure drop. Anyway, it's easier too assume it's a pure parallel circuit and ignore the resistance of the wire for an example -- but to some small extent it affects the circuit.

Sorry for such a long answer, didn't have time to make it shorter.

 

[Agnem]

In a parallel circuit current remains constant and voltage varies, therefore all 8 plugs are getting 150 amps....

Even if we didn't know Ohms Law or the formula for parrallel circuits, we could look at a single wire going to a single glow plug, and know that that thing ain't gonna handle 150 amps!

Speaking of that single wire, there is an interesting voltage loss you can measure there, since you are closer to ground at a glow plug, then you are up at the GPC. Take a reading there, and you will see that a cold plug will drop that voltage way down to about 6 or 7 volts, and as it heats the voltage will climb until it almost reaches your battery voltage. This has always been the method I've used to determine total glow plug draw. If you have a bad plug or two, you will see it with a volt meter at glow plug #2.

 

[mechanoman]

Thanks!

Great discussion, thanks for all the info.

 

[MR.T]

Added the approximate voltage drop between bat+ and the glow controller (where the two yellow wires connect) to the first post. Might help with troubleshooting the '87 and up glow system.

 

[rockchuk]

Mr T thank-you for the post. you answeresd my question about how voltage works on the glow plug curcit.