1986 F-150 Electrical Issues

[fords4life]

Just bought the truck on Saturday and spent sometime yesterday trying to get the rear taillights working. While driving, they work sometimes, other times not or very faint. I started with a test light and unplugged the harness near the firewall. Everything looked good so I cleaned the terminals and added dielectic grease and reconnected. Did the same thing with the plug at the back bumper. Everything was working great in the driveway. Went for a drive and am having issues again. Wondering if anyone has any ideas.

Hazards work front and rear.

Turn Signals do not work front or rear, or if they work, both blink very faintly at the same time.

Brake lights work fine.

No taillights.

I need to check reverse again.....

I'm really surprised that everything was working good after cleaning the connectors, and I've lost it all again. That being said, I might end up having to replace the whole run from the firewall to the rear because one of the POs has a bunch of taps on those wires for a camper he use to have on it.

I checked the ground to the taillights all the way to the firewall and I believe it is good. The weirdest issue I found in testing was key off, transmission in reverse step on the brake and the brake lights would light up, but the blower motor for the heater would also come on at a very slow speed.

[ArdWrknTrk]

There is definitely something weird going on if the blower (which always has power) is using the taillights for ground.

Is the blower functional on all speeds?

Do you have taillights if the blower is on high?

Do the wipers work? (checking another circuit that always has power in 'run')

Scotchloc connectors are an abomination. .

And camper/trailer wiring is the cause of many nightmares.

But I think the ground issue is somewhere up front, where the blower ground is.

Check the lighting ground for power, with the lights off and the blower on.

Potential on both sides of the bulb will cause it not to light up. (this is how the charge indicator works)

 

[Gary Lewis]

There is definitely something weird going on if the blower (which always has power) is using the taillights for ground.

Is the blower functional on all speeds?

Do you have taillights if the blower is on high?

Do the wipers work? (checking another circuit that always has power in 'run')

Scotchloc connectors are an abomination. .

And camper/trailer wiring is the cause of many nightmares.

But I think the ground issue is somewhere up front, where the blower ground is.

Check the lighting ground for power, with the lights off and the blower on.

Potential on both sides of the bulb will cause it not to light up. (this is how the charge indicator works)

I agree with Jim - something weird going on.

You've surely found it by now, but the grounds are shown on Page 6 here: Electrical/EVTM/1986 EVTM/Grounds.

Anyway, Jim has you on a good detective hunt, and I'll follow along.

[fords4life]

I agree with Jim - something weird going on.

You've surely found it by now, but the grounds are shown on Page 6 here: Electrical/EVTM/1986 EVTM/Grounds.

Anyway, Jim has you on a good detective hunt, and I'll follow along.

I did see that Gary. Thank you. It looks like the ground for the blower motor tires back into the same ground as the rear lights which is G701. I checked that from the lights back to the firewall and it seemed ok. I think I was reading 6 ohms from the plug at the bumper to the frame at the bumper, not exactly sure what is acceptable.

Just for fun this morning I unplugged the blower motor and no change in the lights. Plugged it back in and it does work at all speeds.

Also noted that if I use the blinker (either direction) with the headlights on, the rear tails and license plate lamps light up solid on both sides.

Any ideas on someone that makes a replacement harness for these years? I've had zero luck finding one.

[Gary Lewis]

I did see that Gary. Thank you. It looks like the ground for the blower motor tires back into the same ground as the rear lights which is G701. I checked that from the lights back to the firewall and it seemed ok. I think I was reading 6 ohms from the plug at the bumper to the frame at the bumper, not exactly sure what is acceptable.

Just for fun this morning I unplugged the blower motor and no change in the lights. Plugged it back in and it does work at all speeds.

Also noted that if I use the blinker (either direction) with the headlights on, the rear tails and license plate lamps light up solid on both sides.

Any ideas on someone that makes a replacement harness for these years? I've had zero luck finding one.

6 ohms is a pretty poor ground when it is found with such low current as a DVM has. How 'bout checking the voltage drop from the battery's negative post to the ground? And maybe from the battery's negative post to the ground on one of the lights that is malfunctioning.

No one makes a replacement harness that I'm aware of, and if they did it would be expensive. I think you have a ground problem, so I'd look at the negative battery cable, both ends, and do it via the Voltage Drop Test procedures: Electrical/Voltage Drop Test. Also check the engine to cab ground, (Electrical/Grounds/Ground Wires/Engine-To-Firewall Ground) and all other grounds. The best way to test them is as the voltage-drop test procedure shows - under load with the DVM checking the voltage across the ground or cable.

[85lebaront2]

I did see that Gary. Thank you. It looks like the ground for the blower motor tires back into the same ground as the rear lights which is G701. I checked that from the lights back to the firewall and it seemed ok. I think I was reading 6 ohms from the plug at the bumper to the frame at the bumper, not exactly sure what is acceptable.

Just for fun this morning I unplugged the blower motor and no change in the lights. Plugged it back in and it does work at all speeds.

Also noted that if I use the blinker (either direction) with the headlights on, the rear tails and license plate lamps light up solid on both sides.

Any ideas on someone that makes a replacement harness for these years? I've had zero luck finding one.

FWIW, the later trucks have a frame ground back by the left side bumper mount, the 1986 DRW trucks with the clearance lights on the fenders have this ground. If you look, damn near everything behind the firewall grounds at G701 on the back side of the firewall including the fuel pumps for your EFI system, any resistance in the ground system will be real bad once those pumps are running.

First, make sure the "flag" in the middle of the battery negative cable is solidly grounded to the frame, Second check all the connectors in that ground section as shown on page 6 of the EVTM, connector C126 is back by the bumper, C127 is the one up front. C127 if I remember is not a weatherproof connector, C126 should be. I would look at possibly running a ground from the back of the rear harness to the frame. If your truck has the trailer option, there will be a white wire to the frame from the trailer harness.

With the parking lights on and the engine running, check from the rear harness side of C127 to the dash side for any kind of voltage reading, should be extremely low, if it is over 1.0 volt, check to engine and body, FWIW, the only engine to body ground I remember is from the wiper motor mount to the back of the left head and there really isn't a body to frame ground per se, just a little one mainly for the EFI system and that is more to tie the engine to the cab so the EFI works.

[fords4life]

FWIW, the later trucks have a frame ground back by the left side bumper mount, the 1986 DRW trucks with the clearance lights on the fenders have this ground. If you look, damn near everything behind the firewall grounds at G701 on the back side of the firewall including the fuel pumps for your EFI system, any resistance in the ground system will be real bad once those pumps are running.

First, make sure the "flag" in the middle of the battery negative cable is solidly grounded to the frame, Second check all the connectors in that ground section as shown on page 6 of the EVTM, connector C126 is back by the bumper, C127 is the one up front. C127 if I remember is not a weatherproof connector, C126 should be. I would look at possibly running a ground from the back of the rear harness to the frame. If your truck has the trailer option, there will be a white wire to the frame from the trailer harness.

With the parking lights on and the engine running, check from the rear harness side of C127 to the dash side for any kind of voltage reading, should be extremely low, if it is over 1.0 volt, check to engine and body, FWIW, the only engine to body ground I remember is from the wiper motor mount to the back of the left head and there really isn't a body to frame ground per se, just a little one mainly for the EFI system and that is more to tie the engine to the cab so the EFI works.

I didn't have much time to play with it last night, but I did give the ground from the firewall to the block a slight tug and pulled it right out. I grabbed a new ring terminal and got it reinstalled, but no change on the lights. Depending on how much snow clears today, I'll be doing some more digging again tonight.

[Gary Lewis]

I didn't have much time to play with it last night, but I did give the ground from the firewall to the block a slight tug and pulled it right out. I grabbed a new ring terminal and got it reinstalled, but no change on the lights. Depending on how much snow clears today, I'll be doing some more digging again tonight.

One down, but probably several to go. The grounds are really the only thing connecting the devices that are acting up, so it has to be a ground issue somewhere.

Having said that, if the only symptoms are of turn signals or brake/tail lights, then the turn signal switch is a common denominator.

[fords4life]

One down, but probably several to go. The grounds are really the only thing connecting the devices that are acting up, so it has to be a ground issue somewhere.

Having said that, if the only symptoms are of turn signals or brake/tail lights, then the turn signal switch is a common denominator.

Still fighting this darn thing. Replaced the hot side batter cable and starter relay because both about fell apart when I touched them. The ground on the drivers side front fender was a little loose, tightened it up and seemed to help make the headlights a bit brighter.

Check the ground from the battery to the engine I'm getting less than 1ohm.

From passenger side engine to drivers side back of engine I'm at 1.1ohms.

From Drivers side back of engine to firewall I was right around 1ohm IIRC.

From Firewall to fender was 1.8ohms.

Haven't checked anything from the engine bay to the back of the truck yet. Had lights working one time the other night and then I started it, lights are not working again. Also found out the rear tank fuel pump/sending unit or the switch on the dash are not working. Haven't had a chance to start diagnosing this one yet. Will try to post a couple pictures tonight of an interesting power(assuming) connection under the dash. There are 5 or 6 red wires all connected together via a bolt under the instrument cluster.

Finally broke down and ordered a Power Probe so I can start isolation and testing individual components and separate sections of harness to test. Hopefully it will help to narrow down where the problem is.

Anyone have a good place for purchasing OEM style electrical connectors? I'd like to start replacing sections of the harness as I can but would like to use OEM style plugs without paying Ford's price.

 

[Gary Lewis]

Still fighting this darn thing. Replaced the hot side batter cable and starter relay because both about fell apart when I touched them. The ground on the drivers side front fender was a little loose, tightened it up and seemed to help make the headlights a bit brighter.

Check the ground from the battery to the engine I'm getting less than 1ohm.

From passenger side engine to drivers side back of engine I'm at 1.1ohms.

From Drivers side back of engine to firewall I was right around 1ohm IIRC.

From Firewall to fender was 1.8ohms.

Haven't checked anything from the engine bay to the back of the truck yet. Had lights working one time the other night and then I started it, lights are not working again. Also found out the rear tank fuel pump/sending unit or the switch on the dash are not working. Haven't had a chance to start diagnosing this one yet. Will try to post a couple pictures tonight of an interesting power(assuming) connection under the dash. There are 5 or 6 red wires all connected together via a bolt under the instrument cluster.

Finally broke down and ordered a Power Probe so I can start isolation and testing individual components and separate sections of harness to test. Hopefully it will help to narrow down where the problem is.

Anyone have a good place for purchasing OEM style electrical connectors? I'd like to start replacing sections of the harness as I can but would like to use OEM style plugs without paying Ford's price.

You simply cannot use a DVM on the resistance scale to check grounds on a vehicle. It will not work. At all. The current supplied by the DVM isn't even the same league as the current pulled by the components in your truck, much less that of the starter. And the resistance of the connection will change dramatically as the current flow goes up.

The only way to test grounds is the voltage drop test.