Cold advance/high idle drop in power when switching to low idle

bbjordan

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I'm thinking it might be a timing issue. You said you were travelling at night, at near idle. So, some load on the alternator. Will the alternator keep up when the engine is idling when it's warm?

When the Cold Advance Solenoid kicks off, the engine instantly loses about 3 degrees of timing. I'm thinking this retards the timing to sub optimal, and the governor has to supply more fuel to keep the engine at speed. It take a little bit of time for the governor to kick in and that may be where the hesitation is.

Maybe try advancing the timing a few degrees and see if that helps.

I know with my truck, just before the high idle/CAS kicks off, it rattles pretty good. Then, when it kicks off, it's just right again.
 

chillman88

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So this might be a stupid question but how do I tell if that's the type of alternator I have? Been under the impression since I'm rocking an ambulance that I've got some kind of upgraded/higher output alternator to help manage the increased electrical load from running lights and all the equipment in the back.

I'm not sure if the alternator is any bigger, or if it just has a smaller pulley to spin faster, or both. I put a 3G alternator on mine and used the pulley from an Ambulance. I found out I needed the shorter belt for the ambulance prep package.

@Selahdoor is right, if you can get a couple pictures it would help us ID it.
 

thevanbulance

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Sorry for the delay in response guys! Been trying out different scenarios to dig a bit further. So far, I think bbjordan may be on the button. I've only noticed the power loss when coasting near idle. If I'm revved up there's just a slight dip in headlight brightness when things switch over but no noticeable power loss. It doesn't appear to be an issue at the moment but I'm about to take it to the shop for an unrelated braking problem so I may have them check the timing while it's there.

I'll snag some pictures tomorrow when I'm off work. Thanks a ton for all the help fellas!
 

1992 idi 73

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When I noticed this on my truck I thought that there was a major problem, but I have determined that it is the timing advance solenoid and fast idle solenoid kicking off. Its just switching them off draws some power especially at low RPMs when the alternator is not making much power. Now the loud clicking you hear is something I don't but I like my music loud:rock: so it could be something I have too. I just figured that it was one of the quirks of these old trucks.:Thumbs Up
 

thevanbulance

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Reviving this thread as I've had a good bit of time to diagnose this. Still haven't solved the issue but here's a bit more information about what's happening to me in case someone else is experiencing this:

I feel like it's something to do along the path between my batteries (dual Optima Yellow Top's with 800CCA each) and the cold start timing advance system. Here's why:

Pre-turning her over:

When I first turn the key over partway to allow the glow plug relay to come on and the "wait to start" light is on my voltage is typically in the 10.5-11 range which falls in the "good" starting range on my meter. However, sometimes I read a voltage closer to 12-13 at this point (no idea why sometimes it's different). BUT If I wait long enough with the key in this position the voltage will almost always jump up (guessing the second battery kicks in at some point?) closer to the 12-13v number. Sometimes it takes 20-30sec for this to happen but it almost always does if I wait long enough. Anyone know why there's a delay happening here? What controls the flow of voltage between the batteries and the glow plug relay/cold start timing advance system? It feels to me like I'm initially getting just one of the two batteries involved and then the second one joins the party after waiting long enough but I don't know why I'm getting that second voltage bump after waiting... I also have an amp meter and during this whole process my amps are drawing from the battery which is to be expected at this point.

Turning her over and running:

After I've waited for that second voltage bump I turn the key the rest of the way. As she's turning over and starting my voltage is typically right around 11v maybe 11.5v which is just fine. Once started and idling under the influence of the cold start timing advance things look good at this point. I'm at 14v and everything is peachy (my amp meter is showing positive input into the batteries) until the cold start kicks off... Once that kicks off and the idle drops I get a big reduction in voltage back down to starting numbers (11v or so) and I'm drawing amps vs adding amps. Sometimes things kick right back over but most times it takes anywhere from 5-30 seconds to get my voltage back up to 14v and positive amps back into the battery. This behavior happens regardless of driving, idling or coasting.

My thinking is somewhere along the lines when the cold advance kicks off my truck somehow thinks we're back in a starting condition vs driving condition. Definitely not willing to rule out the alternator but the erratic voltage behavior while starting leads me to believe the alternator is not the most likely candidate here. Anyone have an idea on what might be going on or even good information on wiring diagrams for the starting, glow plug and cold advance timing systems they might be willing to share?

Apologies for the novel too but I feel like precise and specific information helps more vs just saying I'm getting weird voltage behavior :)
 

aggiediesel01

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Pre-turning her over:

When I first turn the key over partway to allow the glow plug relay to come on and the "wait to start" light is on my voltage is typically in the 10.5-11 range which falls in the "good" starting range on my meter. However, sometimes I read a voltage closer to 12-13 at this point (no idea why sometimes it's different). BUT If I wait long enough with the key in this position the voltage will almost always jump up (guessing the second battery kicks in at some point?) closer to the 12-13v number. Sometimes it takes 20-30sec for this to happen but it almost always does if I wait long enough. Anyone know why there's a delay happening here? What controls the flow of voltage between the batteries and the glow plug relay/cold start timing advance system? It feels to me like I'm initially getting just one of the two batteries involved and then the second one joins the party after waiting long enough but I don't know why I'm getting that second voltage bump after waiting... I also have an amp meter and during this whole process my amps are drawing from the battery which is to be expected at this point.

This experience is your glowplug controller timing out. It can run for a while up to ~20sec or so if it's really cold. Then it can go into a quick on-off repetitive cycle for another 10-15 sec. Your main starting batteries are hardwired in parallel so the only thing controlling flow of current from them is the internal resistance of the batteries. There's no staging of the batteries in the normal ford starting/running circuit.

Turning her over and running:

After I've waited for that second voltage bump I turn the key the rest of the way. As she's turning over and starting my voltage is typically right around 11v maybe 11.5v which is just fine. Once started and idling under the influence of the cold start timing advance things look good at this point. I'm at 14v and everything is peachy (my amp meter is showing positive input into the batteries) until the cold start kicks off... Once that kicks off and the idle drops I get a big reduction in voltage back down to starting numbers (11v or so) and I'm drawing amps vs adding amps. Sometimes things kick right back over but most times it takes anywhere from 5-30 seconds to get my voltage back up to 14v and positive amps back into the battery. This behavior happens regardless of driving, idling or coasting.

When amps are leaving your batteries (needle on the neg side of 0 amps) that means an additional load has been connected not removed. If your alternator is good, it will make up the additional load by providing additional current up to it's capacity (it does this by increasing voltage up to it's limit), then if it's maxed out the voltage starts to fall and current is now leaving your charging system. Your description here tells me that some large additional electrical load is being added to the charging circuit and trying to remove more current than the alternator can make up. In your auxiliary ambulance wiring do you have additional batteries or another large load like an inverter? Maybe the charging of these batteries is by a controller that is timed to start it's cycle sometime after the engine is started and the starting batteries have had a chance to recover their charge? Maybe whatever this auxiliary load is has a problem and it's connected through a circuit breaker under your dash and that breaker is opening after it gets hot enough to open up and then it cools off and resets later. I really don't think that the cold idle advance and high idle solenoid behaves in the manner you've described.

My thinking is somewhere along the lines when the cold advance kicks off my truck somehow thinks we're back in a starting condition vs driving condition. Definitely not willing to rule out the alternator but the erratic voltage behavior while starting leads me to believe the alternator is not the most likely candidate here. Anyone have an idea on what might be going on or even good information on wiring diagrams for the starting, glow plug and cold advance timing systems they might be willing to share?

It's possible that your glowplug controller is misbehaving and turning the plugs back on at some point during your drive. That's usual but if there's a problem in your ignition switch maybe it's being triggered when the steering column is flexed like during a turn or.....but if the engine is warm then the glow plug cycle should be less than 8 sec.
 

thevanbulance

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large load like an inverter?

^^This! I do have an inverter under the driver's seat. However, to my knowledge nothing is connected currently. I've got no house batteries and keep the master switch for all the additional electronics (lights, siren, pa, etc) off to prevent them from drawing power when not in use.

The previous owner did overload the 120v in the back of the ambulance running a xBox and some tv's and I've been unable to locate where the overload hit (assuming there's a big ass fuse somewhere that blew but everywhere I've thought to look has come back clean. Plus, I can still get 120v back there when plugged into shore power so it's been a bit confusing to sort out). No idea if that could be at play here in some way.

Thank you for the incredibly thorough reply too btw! I grew up just north of you in Dallas so good to see some TX representation :)
 

Cubey

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3.) About halfway home (usually 10ish minutes into the drive) the high ideal will abruptly switch off with an audible "thunk" noise coming from under what sounds like the drivers side fuse box.

It shouldn't take 10 minutes of driving to shut off the advance. Usually it does within 1-5 minutes, depending how much idling you do before you start driving. The faster it warms up, the faster it'll shut off. 10 minutes is a crazy long amount of time when actively driving. I'm thinking your temp sensor for (near the thermostat) might be bad. That, or your engine is running way too cold, due to a stuck open thermostat.

I hear a weird thunk sometimes too, which is I dunno what, but I know that's different from cold advance shutting off. I hear that shutting off when at a stop sign usually, when city driving.
 

aggiediesel01

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In an ambulance application it's much more likely that circuit breakers were used instead of fuses for all auxiliary circuits. Reason being if a fuse blows then the staff, who is trained to focus on life support, is distracted for a much more significant amount of time than if it's a breaker they can just flip or push to reset. If it blows again then they manage until they get back to the station and put in the maintenance order. Some breakers are auto reset and some are manual reset. If the breaker feeding whatever load has a problem is an auto reset then you are going to be chasing your tail trying to find it unless you can make it happen when you can pull the covers off and start poking around as soon as you catch it happening. If it's been owned by others since the department then who knows what snafu might have been created. My Electrical Vacuum Troubleshooting Manuals only cover F-series but someone recently posted that I think Autozone has decent uploads of many older vehicle wiring diagrams and such. Maybe that's a place to start? There's a few of us Texans floating around here and mostly we try to be a helpful bunch. Good luck and posting some pictures of what type of charging system is on the truck might help some of us narrow down or eliminate some known problems with particular systems.
 

Cubey

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In an ambulance application it's much more likely that circuit breakers were used instead of fuses for all auxiliary circuits.

The conversion part is typically completely separate from the chassis with it's own battery bank. The added bank is just linked to the main batteries, so if the chassis system goes to crap, the added system will still work. At least, that's how they do RVs.

There is a circuit breaker in the Ford fuse panel, but it's for wipers. No idea why it would be clicking randomly once sometimes.
 
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