Cessna Aircraft Electrical System - Page 1
Forum section: electronics / repair

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 06:19:07 am
content: I figured I would check with you guys on this...Been having some problems with the electrical system on a Cessna 172M (1974)The mechanic doesn't seem to be very good in this area, and I have not had much time to look into it myself.The voltage regulator and the alternator have both been replaced and are brand new.The new voltage regulator does not (sadly) have the built in overvoltage protection. So that is handled by the original module.However, the system is only putting out 13.7V where I would expect more like 14.3VI am tired of paying for shotgun parts replacement rather than troubleshooting. So I dug around as much as I could myself (I am not allowed to actually make changes myself as I am not an FAA certified mechanic, though I can certainly troubleshoot.)Low and behold, I found that the capacitor highlighted in yellow in the schematic is a Sprague Atom 500uF 50V 85C cap with a 1973 date code!!! I'm not allowed to take it out of circuit to test it, but I can't imagine it is much good.I am hoping somebody who understands alternators can tell me what effect a high ESR cap right there would have on the system voltage. It is difficult for me to test things with the engine running by myself.I did test the Master Switch Alt side with an Ohm meter, and it stayed right at < 0.2 Ohms no matter how many times I flipped it.The capacitor highlighted in blue reads as 4.3 uF. I don't know what it is supposed to be. I need to find the description someplace. It's not in this old "Owner's Manual."Looking for any thoughts on why the voltage would be 13.7V vs. 14.3V or so.Any help would be appreciated. Thanks.

author: Emo
author_rank: Regular Contributor
timestamp: May 20, 2019, 06:42:54 am
content: Hi Flyinghacker,Not sure why you are expecting 14.3 volts. Depending on the temperature 13.7 might very well be the correct maximum floating voltage for the battery type used.I suggest checking the battery specs(probably sealed lead) especially the voltage versus temperature curve.The electrolytics will filter out the generator noise. If these have failed the first thing you will notice is the distinct whining noise on the radio if you turn down the squelch.Eric

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 07:08:47 am
content: Thanks for the reply.Keep in mind this is a 1974 system. So there are no electronics to charge the battery with a fancy curve. It just gives the battery what it will take.Or are you saying just the changing in internal resistance in the battery would have influence? That last flight was about 90F.In the past, before replacing the regulator or alternator, it was always at 14.2V-14.4V.I think I will do a full manual charge on the battery.

author: Emo
author_rank: Regular Contributor
timestamp: May 20, 2019, 07:33:07 am
content: Even very old regulators in DC alternators had temperature compensation. Take a look at the graph in following web pagehttps://batteryuniversity.com/learn/article/charging_at_high_and_low_temperaturesSo knowing the temperature at which the measurements were taken is important. The regulator should have specifications on what exact Pb chemistry and battery type it expectsEric

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 07:40:54 am
content: Interesting. I see your point. Perhaps the old regulator wasn't as sophisticated. Maybe this new one will make the batteries last more than 2-3 years rather than cooking them into oblivion.I am unable to find any info on the regulator. It's a Lamar DGR6-1.All I can find is that it is approved for the Cessna and should be mounted on a metal surface as a heat sink (the firewall).

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2019, 08:12:26 am
content: The diode in series with the charging system across the battery contactor will account for 0.6-0.7 V and maybe the old reg was adjusted to account for this.Maybe newer Cessna's omitted it and your new reg doesn't expect to have it in circuit.You need check on this. (I think it's to manage back-EMF)Nah scratch that.....the battery charge current is via the ammeter but it's unclear what the component ALT is in series with B+ from the alternator.

author: Emo
author_rank: Regular Contributor
timestamp: May 20, 2019, 09:36:08 am
content: The "alt" component is probably a circuit breaker. The Lamar data sheet doesn't specify parameters unfortunately which is not surprising for a 1970 type.Most regulators on their site are mentioned as 14(or28) volts.Eric

author: sokoloff
author_rank: Super Contributor
timestamp: May 20, 2019, 10:44:31 am
content: Quote from: FlyingHacker on May 20, 2019, 06:19:07 amYou are allowed to do nearly anything that an A&P can do, so long as you're working under the direct supervision of one.14 CFR 43.3.das the citation.Of course, this requires a willing supervisor, though in my experience, when an owner is cordial and clearly seeking the right solution (not just 'cheaping out') as you seem to be, A&Ps are quite willing to supervise owner-performed work, especially where electrons [or fuel tank bladders] are involved. In this case, where the schematic is published and the device in question is not "an instrument", I believe that your A&P can do component-level troubleshooting and repairs and therefore you can do the same, provided you are working under his supervision (generally meaning "in the same hangar", though doesn't need to mean "at the same workbench", and I would think that removal of a component to be tested somewhere off-site and re-installed on-site would also be completely acceptable to the administrator and therefore the supervising A&P.)(Note that there is another set ofpreventative maintenance tasksthat you are also allowed to do as a Part 91 owner-operator. That doesn't apply here, but figured I'd mention it since you say you can't do anything because you aren't a licensed A&P.)

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 03:40:11 pm
content: Quote from: Emo on May 20, 2019, 09:36:08 amYes, that is the main 60A breaker.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 03:50:23 pm
content: Quote from: sokoloff on May 20, 2019, 10:44:31 amYes, both true statements. As a pilot you have to know this stuff to get a pilot certificate (license). I was just trying to simplify things for the non-pilots so that they knew I could just mess with it willy-nilly like a car.That was another thought was to assist. First I need to determine if this 13.7V is actually a problem.

author: Magnum
author_rank: Regular Contributor
timestamp: May 20, 2019, 07:21:15 pm
content: Quote from: FlyingHacker on May 20, 2019, 06:19:07 amWhere are you measuring the voltage? What do you measure directly at the alternator / battery? At what RPM are you checking? Do you have all electrical systems (lights, pitot heat, avionics, etc.) switched off? IMHO you can take out and bridge the overvoltage protection for troubleshooting, just make sure to keep the avionics switched off (just in case...). But think that won't change anything.What does the maintenance manual say about troubleshooting? Why did you replace the regulator, same problem? Have you checked your voltmeter?

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 07:48:21 pm
content: Quote from: Magnum on May 20, 2019, 07:21:15 pmThe clock has a built in voltmeter (compared with my Fluke, and it is accurate). It reads the voltage from where you see the clock in the schematic.I have tried turning everything on and everything off. The voltage remains at 13.7V regardless.There is no maintenance manual regarding troubleshooting.The initial problem was that the system stopped charging in flight. Evetually the battery ran down low enough that nothing really worked except the transponder (it's a periodic load) (had everything else turned off... radio transmit would reset the radio, etc.). This was around 10.8V or so. Turning Alt portion of master switch off and on had no effect. The mechanic said he found a bad main cable from the alternator. This cable was replaced. Everything seemed to work OK.I went on another flight. The voltmeter read 14.3V at the start of the flight. About 20 or 30 mins in the system stopped putting out enough voltage again. I saw around 11.7V and dropping... Toggling Alt switch did cause the voltage to jump up to around 12.8V for about 30 sec and then drop again. I did this twice with the same result. I turned off all that I could and landed safely, even with the radio used sparingly (towered airport).Mechanic said replace the voltage regulator. He charged the battery here. He replaced the regulator.I go on another flight. Voltage starts at 13.7V, which is lower than my previous 14.3V. I get about ten mins out and voltage starts dropping again. It's kinda holding around 12V. I found toggling Alt switch had little if any effect.Mechanic says replace alternator. So I pay for it...With new alternator, but not charging battery I get where I am now. At last flight (only flight post alternator) I see about 11.7V or so at startup. Then she makes 13.7V when running. I flew for an hour. It mainly stayed at 13.7V. I saw it dip to 13.6V for a few minutes, and even to 13.5V for less than a minute. I tried turning everything I could ON (including pitot heat, but my lights are LED; so lower draw). Through turning things on and off, or transmitting on the radio I was not able to get the voltage to change. So maybe everything is fine??Two things make me question this. Other mechanics in a Cessna 172 forum insist it should be between 13.8V-14.4V. OK, take free info with a grain of salt, but they have been doing this a while, and actually seem to understand electrical systems, at least more than my mechanics. Also my Garmin G5 (mini primary flight display) indicated a charging icon for its internal battery, however I watched the charge percentage go from 76% to 74% in about 20-30 mins. This could be due to it getting hotter as I flew longer, since the ground temp was about 92F and I was under 3000Ft AGL.So that is where I am now, and the whole story as I remember it.

author: bdunham7
author_rank: Super Contributor
timestamp: May 20, 2019, 08:00:30 pm
content: First, at 90F the 13.7 volts is not a problem for conventional chemistry batteries--like you'd find in 1974. So you would need specs for your particular battery to see if that is OK or not, but I suspect it is fine. Some batteries might need more than that to recharge fully from a discharged condition, but they won't discharge if they are presented anything over about 12.6 volts. You should always float or trickle charge your aircraft battery and it should be fully charged before every takeoff. Flying the airplane to charge the battery after it just barely got it started is not a good idea for many reasons--it unnecessarily strains your alternator and electrical system, it leaves you with less reserve in case of alternator failure, etc. If you do it this way, 13.7 volts is enough and your battery will last much longer.As far as doing repairs, yes there are legal limitations on what you can do. However, there are crummy A&Ps and more than once I've had to decide whether to live with their hack work or fix something myself and when it comes to flying home legally or safely, I choose the latter.As to your question, the capacitors are just noise/surge filters to prevent your overvoltage sensor from triggering and interrupting the stator (S) feed to the voltage regulator. The stator (S) is simply an enable input on the VR. Neither would change your VR setpoint. And on that subject, if you did want to change the VR setpoint, they may be adjustable. The original was likely a mechanical regulator, although I hope the replacement is not--but it still may have internal adjustments. Which you can't touch, of course...EDIT: It sounds like the new VR changed the voltage, but the replacement alternator actually fixed the intermittent problem...I hope!

author: Emo
author_rank: Regular Contributor
timestamp: May 20, 2019, 08:09:32 pm
content: There is some parts that might need checking. The circuit breakers themselves. If one of them has a lousy contact it would explain the story above

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2019, 09:02:09 pm
content: Quote from: Emo on May 20, 2019, 08:09:32 pm+1Thank's for the full story FH and IMO 13.7V is a little too low however if the battery is drawing the full alternator output it's not unreasonable to measure this low until the battery becomes close to fully charged.Low charge V has the disadvantage of exposing the battery to large starting currents where the possibility of battery damage can occur. Typically you want max charge voltage in the range 13.8-14.2V on older batteries and the new chemistry's 14.4V.However as small planes are subjected to lots of vibration I'd want to see each and every terminal in the current path for alternator to battery and not excluding the grounds properly inspected for tightness and continuity.One bad cable....whatever that meansdoesn't exclude more un-found issues.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 09:06:29 pm
content: Quote from: bdunham7 on May 20, 2019, 08:00:30 pmGood points. I need to wire in the harness to attach my charger externally. This has been on my list for a while. Of course such an installation (even if performed by an A&P) is of questionable legality, though many, many planes have such a setup.QuoteAnd yes, that was my take away as well.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 09:10:48 pm
content: I think ideally I would measure the voltage coming of the the alternator, and then everywhere along the cables to see where there is a drop. OF course finding all the points and actually getting a meter to attach to the terminals is often easier said than done. Many terminals are shrouded (I wish they all were for safety's sake, but we are talking 1974).I'll have to try it with my Fluke 27 and that way we can see if it can survive a direct prop strike (of course my pro and engine would hate that and instantly become un-airworthy).

author: jmelson
author_rank: Super Contributor
timestamp: May 20, 2019, 09:19:15 pm
content: Quote from: tautech on May 20, 2019, 09:02:09 pmAn EXCELLENT point! If the alternator is grounded back to the minus side of the battery through the engine, then there will have to be a hefty ground cable from engine to the ground block, somewhere. This could have become fatigued from vibration.Otherwise, there would need to be a hefty ground cable from alternator to the ground block.But, a really good test would be when you think the voltage is dropping, turn on the landing light. if the voltage immediately drops below 12.6, the charging system ISN'T working. If it holds 13.7 with the landing light on, then it is absolutely working, although at a slightly lower voltage than might be expected. This test ought to work fine on the ground, no need to get up in the air to debug the charging system.Jon

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: May 20, 2019, 09:29:11 pm
content: Quote from: jmelson on May 20, 2019, 09:19:15 pmTurning on the landing light or the pitot heat (landing light is LED) has no effect on the voltage reading in flight.

author: bdunham7
author_rank: Super Contributor
timestamp: May 20, 2019, 10:09:40 pm
content: Quote from: FlyingHacker on May 20, 2019, 09:10:48 pmNo need for a prop strike--just take it off for testing. You have a safety wire pliers, right? No one need know...What you can do is use a car and some jumper cables, or a battery charger/power supply and just hook up to the alternator and then measure your current and voltage drop. Carefully, of course!But first, where are you? Does it cool off sometime? I'd sure want to know what it did with a fully charged battery at 72F before I got too worried.