Brake light fuse blows - at my diagnostic wit's end - SOLVED

TheHardWay

Ain't as easy as it looks
SILVER Star
Joined
Dec 29, 2013
Messages
1,555
Location
Las Cruces, NM
Here's what I know: my Olive 77 had Saginaw power steering when I bought it:

IMG_0813.JPG


By the way, it works great. It also came with this clown horn button screwed onto the dash (since removed):

IMG_0804.JPG


I've had this rig over 7 years and can't remember if the horn has ever worked, but I suspect there's a connection between these two 'additions.' After I bought my white 77 WITH a working OEM horn I decided to ditch the clown button and restore the OEM, using the white's working horn as a reference. I've verified with multimeter the following:

Hot wiring the + battery terminal to each hot connector on the horns results in solid, loud Beep
Have continuity in green/white horn wires from driver side terminal spade to passenger side terminal spade
Have continuity in green/red horn relay wire from relay terminal to brake light connector under dash
Have continuity in green/yellow horn relay grounding wire from relay terminal to turn signal assembly steering wheel connector spring button:

IMG_0816.JPG


Plugging the horn relay connector with the 3 wires attached to it into the horn relay switch immediately blows the brake light fuse (20 amp)
Removing the green/yellow horn grounding wire from the connector and plugging it back into the switch does NOT blow the brake light fuse
The green/red horn relay wire terminal shows 12 volts on multimeter
No visible grounding wire on the steering wheel shaft (power steering)
I know the three horn buttons on steering wheel, and horn relay are functional and clean, verified by swapping them with the other rig's working steering wheel buttons and horn relay
What are the usual suspects for blowing the brake light fuse? By the way, with the horn relay disconnected, the brake lights work when brake pedal pushed, and emergency brake light works when emergency brake is pulled
What am I missing here?
 

TheHardWay

Ain't as easy as it looks
SILVER Star
Joined
Dec 29, 2013
Messages
1,555
Location
Las Cruces, NM
This is the relay that was installed when I bought the rig:

IMG_0806.JPG


Definitely not OEM, so I bought a replacement from @ToyotaMatt . During the diagnostics I swapped it with the white 77 relay - works fine! Small haystack for sure, which is why I'm wondering what I'm overlooking here.

IMG_0807.JPG



ToyotaMatt's relay
 
Joined
Nov 29, 2020
Messages
504
Location
Wausau WI
I'm assuming you get the constant 12v at the relay connector on the green/red wire until it's plugged in and the fuse pops? Green/yellow should give you a ground, but only when the horn is pressed.

If you had a short downstream from the relay, you *should* only blow the fuse when the horn button is pressed. And if you had a short upstream on that grn/red, it'd blow the fuse all the time, regardless of being plugged in or not.

Curious what you see at that connector now...but it seems like the 12v is getting a route to the housing somehow, hence my ask on "bad relay?".
 

TheHardWay

Ain't as easy as it looks
SILVER Star
Joined
Dec 29, 2013
Messages
1,555
Location
Las Cruces, NM
I'm assuming you get the constant 12v at the relay connector on the green/red wire until it's plugged in and the fuse pops? Green/yellow should give you a ground, but only when the horn is pressed.

If you had a short downstream from the relay, you *should* only blow the fuse when the horn button is pressed. And if you had a short upstream on that grn/red, it'd blow the fuse all the time, regardless of being plugged in or not.

Curious what you see at that connector now...but it seems like the 12v is getting a route to the housing somehow, hence my ask on "bad relay?".
Correct on the constant 12v.

Agree on the downstream/upstream analysis.

What do you mean by 'the housing?' The relay casing?
 

ToyotaMatt

Factory Focus-Point Mind-Set Approach
Supporting Vendor
GOLD Star
Joined
Aug 12, 2007
Messages
16,592
Location
EAST COAST
Website
vintageteqparts.com
Here's what I know: my Olive 77 had Saginaw power steering when I bought it:

View attachment 2788999

By the way, it works great. It also came with this clown horn button screwed onto the dash (since removed):

View attachment 2789002

I've had this rig over 7 years and can't remember if the horn has ever worked, but I suspect there's a connection between these two 'additions.' After I bought my white 77 WITH a working OEM horn I decided to ditch the clown button and restore the OEM, using the white's working horn as a reference. I've verified with multimeter the following:

Hot wiring the + battery terminal to each hot connector on the horns results in solid, loud Beep
Have continuity in green/white horn wires from driver side terminal spade to passenger side terminal spade
Have continuity in green/red horn relay wire from relay terminal to brake light connector under dash
Have continuity in green/yellow horn relay grounding wire from relay terminal to turn signal assembly steering wheel connector spring button:

View attachment 2789021

Plugging the horn relay connector with the 3 wires attached to it into the horn relay switch immediately blows the brake light fuse (20 amp)
Removing the green/yellow horn grounding wire from the connector and plugging it back into the switch does NOT blow the brake light fuse
The green/red horn relay wire terminal shows 12 volts on multimeter
No visible grounding wire on the steering wheel shaft (power steering)
I know the three horn buttons on steering wheel, and horn relay are functional and clean, verified by swapping them with the other rig's working steering wheel buttons and horn relay
What are the usual suspects for blowing the brake light fuse? By the way, with the horn relay disconnected, the brake lights work when brake pedal pushed, and emergency brake light works when emergency brake is pulled
What am I missing here?


this is a area to inspect closly like even pull back the original harness tape a tad to see if fried together up the line either way ?

i have run into this more then several times in my life.......


it's worth crossing off list , cost no money exept new black harness tape few spins back on .........




this is a 79, UV damage to all YAZAKI Pre-UV rated NYLON housings turned to CHALK DUST TORTURE !


OMG i said , this was thr wort one ever i saw and still have seen till this day ......



dash 1.JPG
dash 2.jpg
dash 3.JPG
dash 5.JPG
dash 6.JPG
dash 8.JPG
dash 9.JPG
dash 10.JPG
dash 11.JPG
dash 12.JPG
 
Joined
Nov 29, 2020
Messages
504
Location
Wausau WI
Correct on the constant 12v.

Agree on the downstream/upstream analysis.

What do you mean by 'the housing?' The relay casing?
Interesting on the 12v...that helps, but doesn't (if you know what I mean.)

Yeah, I just meant the metal casing on the relay...that 12v feed is getting grounded, but doesn't seem like it's in the harness as it's only when you plug it in. I wonder what a multimeter reads if you check that pin to the casing with it unplugged.

(And I'd debated cleaning up my OEM relay, but I stashed it in the parts pile and ordered a new Standard RY308 from Rock Auto instead... 🙃 )
 

TheHardWay

Ain't as easy as it looks
SILVER Star
Joined
Dec 29, 2013
Messages
1,555
Location
Las Cruces, NM
Interesting on the 12v...that helps, but doesn't (if you know what I mean.)

Yeah, I just meant the metal casing on the relay...that 12v feed is getting grounded, but doesn't seem like it's in the harness as it's only when you plug it in. I wonder what a multimeter reads if you check that pin to the casing with it unplugged.

(And I'd debated cleaning up my OEM relay, but I stashed it in the parts pile and ordered a new Standard RY308 from Rock Auto instead... 🙃 )
Well, and with the green/yellow wire removed from the connector it can be plugged into the relay with no fuse blow. Insert the green/yellow back into the connector, plug into relay again and POP.
 

ToyotaMatt

Factory Focus-Point Mind-Set Approach
Supporting Vendor
GOLD Star
Joined
Aug 12, 2007
Messages
16,592
Location
EAST COAST
Website
vintageteqparts.com
Have you ever seen a 1977 4x8 sheet of plywood old skool paper print media before ?


Let’s study this and see ?

My gut says bad combo switch , but never guess work. Always confirm and verify via fsm and ewd oem




Here's what I know: my Olive 77 had Saginaw power steering when I bought it:

View attachment 2788999

By the way, it works great. It also came with this clown horn button screwed onto the dash (since removed):

View attachment 2789002

I've had this rig over 7 years and can't remember if the horn has ever worked, but I suspect there's a connection between these two 'additions.' After I bought my white 77 WITH a working OEM horn I decided to ditch the clown button and restore the OEM, using the white's working horn as a reference. I've verified with multimeter the following:

Hot wiring the + battery terminal to each hot connector on the horns results in solid, loud Beep
Have continuity in green/white horn wires from driver side terminal spade to passenger side terminal spade
Have continuity in green/red horn relay wire from relay terminal to brake light connector under dash
Have continuity in green/yellow horn relay grounding wire from relay terminal to turn signal assembly steering wheel connector spring button:

View attachment 2789021

Plugging the horn relay connector with the 3 wires attached to it into the horn relay switch immediately blows the brake light fuse (20 amp)
Removing the green/yellow horn grounding wire from the connector and plugging it back into the switch does NOT blow the brake light fuse
The green/red horn relay wire terminal shows 12 volts on multimeter
No visible grounding wire on the steering wheel shaft (power steering)
I know the three horn buttons on steering wheel, and horn relay are functional and clean, verified by swapping them with the other rig's working steering wheel buttons and horn relay
What are the usual suspects for blowing the brake light fuse? By the way, with the horn relay disconnected, the brake lights work when brake pedal pushed, and emergency brake light works when emergency brake is pulled
What am I missing here?






8F38B148-D9C6-4D4B-951D-152E724B654D.jpeg
1AC45FF2-B6B3-4C06-8CFE-47EA49DAA493.jpeg
1C84F7C5-5416-4F78-AD73-66E723F10E89.jpeg
77A54603-A343-4ADF-BEFA-C2731FF8D35F.jpeg
86C4CC4C-BB76-4315-BD21-E25F91B360FB.jpeg
1C5EAE60-DEBB-4EAE-BDC5-ACD6FC22D824.jpeg
9A6CA8A7-2110-473A-9A45-F16FD0174859.jpeg
DBBC959F-74A4-4576-882C-D81C7DE82609.jpeg
FCB3FBB6-6A63-46E3-B7AE-CC2DAAA91A6B.jpeg
 
Joined
Nov 29, 2020
Messages
504
Location
Wausau WI
Well, and with the green/yellow wire removed from the connector it can be plugged into the relay with no fuse blow. Insert the green/yellow back into the connector, plug into relay again and POP.
Gotcha - I had read that the other way before, I thought you plugged the lone grn/yel into the relay out of the plastic clip.

Odd that that making a difference...dorsn't make a ton of sense.
 
Joined
Nov 29, 2020
Messages
504
Location
Wausau WI
Hm, what does it do if you tickle that ground terminal yourself in that configuration when you have it plugged in but with the G/Y unpinned?

Does that G/Y show a ground when the button isn't pushed?
 

TheHardWay

Ain't as easy as it looks
SILVER Star
Joined
Dec 29, 2013
Messages
1,555
Location
Las Cruces, NM
Hm, what does it do if you tickle that ground terminal yourself in that configuration when you have it plugged in but with the G/Y unpinned?

Does that G/Y show a ground when the button isn't pushed?
You mean put one multimeter lead on the GY wire spade outside the relay plug and ground to the fender with the other lead? I'm still learning how to use it - which setting and what am I measuring?
 
Joined
Nov 29, 2020
Messages
504
Location
Wausau WI
You mean put one multimeter lead on the GY wire spade outside the relay plug and ground to the fender with the other lead? I'm still learning how to use it - which setting and what am I measuring?
Yeah - would be the ohms/resistance setting. Should read open (often "O.L"), a short would be just 0.0. Some have a continuity beep option on that setting that can be handy too...if there's a path between the two leads, it beeps.

It should be open, but given it's blowing the fuse, there's a path somewhere, at least with the connector plugged in... checking the relay without the connector on is just another good data point to remove some hay from the haystack. 😆
 

Users who are viewing this thread

Top Bottom