Alternator out warning board

bking · #1 04-25-21, 09:53 AM

Hello all, does anyone have a current part number for the(2)REED RELAYs as shown on the "Schematic Alternator Warning" wiring diagram, drawing# 1571000 in the service manual? The part numbers (PN300-884-9944) on my relays are now obsolete.

I have called Don Nieser and several other sources without success so any help/ideas would be a greatly appricated.

Thank you,

BK

hayesjaj · #2 04-27-21, 12:03 PM

I think the unit you are looking for is 1570058-1 (Figure 134 of the IPC, 73-80). Cessna has 5 of them, $1423 ea.

bking · #3 04-27-21, 12:08 PM

Thank you for the info.

Learjetter · #4 04-27-21, 02:33 PM

Quote:

Originally Posted by bking

Hello all, does anyone have a current part number for the(2)REED RELAYs as shown on the "Schematic Alternator Warning" wiring diagram, drawing# 1571000 in the service manual? The part numbers (PN300-884-9944) on my relays are now obsolete.

I have called Don Nieser and several other sources without success so any help/ideas would be a greatly appricated.

Thank you,

BK

I rebuilt the entire board on my '77 NA G model, and replaced the reed relays with new from the manufacturer, Computer Components Inc.
1-800-653-9909 Part#300-884 (x2). ($75 each, plus $17 shipping).
www.relays-unlimited.com

If you're going that far, recommend replacing the diodes and resistors too.
There are a couple posts in the forum discussing the modern equivalent part numbers and sources.

bking · #5 04-27-21, 04:43 PM

Learjetter,

I placed an order with Computer Components but they only had 1 in stock with a 3 month wait for the second relay. I searched the web and found another for $225.00 along the 4 resisters at Wilco.

Long story, but I also found a new surplus PC board for $175.00. After finding the board, I decided to make up a new unit and ordered all new diodes and the MS terminal boards.

The only 2 items that I could not get in the correct part numbers were the fuse blocks and 7 position bottom terminal strip and, according to Cessna, they are no longer offered. That said, I found similar components with the same ratings and also have them on the way.

I should have less than $700.00 total in parts for a new board. New surplus and "as removed boards" were running from $2,600.00 to $4,100.00 so, I'll have some fun and save a few coins.

If you could post a photo of your board it would be great and thank you,

BK

hayesjaj · #6 05-01-21, 01:00 AM

This is incredibly useful info, thanks all. I'll likely need to do this on my 73G restoration. Would anyone be interested in an aftermarket cover for this unit? My cover is missing and I'll probably need to get some metal bent for it...may as well make a few.

MD83driver · #7 07-01-21, 06:31 PM

Quote:

Originally Posted by bking

Learjetter,
Long story, but I also found a new surplus PC board for $175.00.
BK

When you found this board, was there a part number associated with it? I am in the process of rebuilding mine. Thanks

bking · #8 07-01-21, 08:04 PM

I have attached a list of the components, part numbers and vendors I used to replace my assembly and so far, the Alt Out warning light hasn't been flashing.

MD83driver · #9 07-01-21, 08:41 PM

Thanks you for posting that information. I bought the 300-884 Relays a couple of months ago and they actually arrived yesterday. I bought the rectifiers and some diodes. I will use your list to collect the rest.

I have been thinking about why there would be so much trouble with these alternator lights if both regulators are set to the same voltage, either 27.7 or 28.8 volts. One possible area for this is since these are not paralleled in real time by the regulators, there is potential voltage drop from the rear to the front, thereby the incoming voltage from the rear would then be less that from the front. Just spitballing.

mshac · #10 07-02-21, 10:29 AM

Quote:

Originally Posted by MD83driver

I have been thinking about why there would be so much trouble with these alternator lights if both regulators are set to the same voltage, either 27.7 or 28.8 volts. One possible area for this is since these are not paralleled in real time by the regulators, there is potential voltage drop from the rear to the front, thereby the incoming voltage from the rear would then be less that from the front. Just spitballing.

You hit the nail on the head. You can install my 400-Series voltage regulator upgrade, and in all cases so far, the flashing alternator lights stay dark. Adding true paralleling alternators equalizes the load between front and rear.

I'm not clear what Cessna engineers were thinking when they designed the Skymaster's voltage regulation system, but it may have to do with the fact that the engineers at Pawnee (Cessna's single engine factory) had never built a twin before (and would never build another).