Strange 1986 Bronco electrical issue.

[Nickelplate]

So, I am having a really strange electrical issue (or a couple of related issues) that seem to be centered on the Air Conditioning. There are two weird things that happen intermittently:

1. The 30A AC/heater/fan fuse blows out. This happens only sometimes and is a more recent issue. It only started happening after I got a new compressor. I put 1994 5.0 serpentine accessories on the front instead of the old V-belt stuff. I bought a brand new compressor, which is why I switched it out.

2. Whenever the AC selector on the dashboard is on MAX or NORM, the tachometer works differently (seems to just get stuck around 1000 and doesn't move), and the fuel, oil pressure, and engine temperature gauges all slowly climb. I replaced the gauge cluster voltage regulator with a new solid state one. Before I did that, the gauges would all climb to as high as they could go. After the replacement, they still climb but only partway up before they stop. I have experimented around with this and it seems like the gauges do not climb if the AC clutch is not engaged. As soon as I disengage the AC, the gauges will go back down to their real level.

Despite both of these things, when it works, the AC does work great, all fan speeds work, and everything is good.

Problem #1 only started a few weeks ago. Problem #2, I have noticed on and off since 2017. Problem #2 has persisted even though I have replaced: AC clutch, AC clutch wiring (from pressure switch to compressor), AC pressure switch, blower fan, blower fan resistor, blower fan speed selector switch in the dashboard.

My best guesses:

1. that something between the AC switch in the dash and the engine compartment has something messed up. In this case it is probably not the wiring itself, but the AC switch in the dash.

2. it could be that the diode in the engine compartment AC compressor harness is bad or does not need to be there now that I am no longer using the OEM ford ECU that it used to connect to. I am now using an Edelbrock Pro-flo 4 EFI system in it. I'm guessing it's not bad since it's brand new like the rest of the wiring, and maybe just doesn't need to be there.

3. the diode-containing lead that comes off the AC compressor and used to go to the ford ECU needs to be grounded? Right now, it's just hanging and doesn't connect to anything.

Does anyone know anything about what this could be? Anyone experienced it before? What are your thoughts?

[Gary Lewis]

My guess is that you have a bad ground. From the 1986 EVTM, the schematic below shows what circuits use G701, which is behind the left side of the radio on the firewall. And everything you are talking about, and then some, use that ground. So I think that when you bring the blower motor on, meaning not just A/C but Vent or Heater, the voltage on the ground comes up and that is messing up the gauges.

[Nickelplate]

My guess is that you have a bad ground. From the 1986 EVTM, the schematic below shows what circuits use G701, which is behind the left side of the radio on the firewall. And everything you are talking about, and then some, use that ground. So I think that when you bring the blower motor on, meaning not just A/C but Vent or Heater, the voltage on the ground comes up and that is messing up the gauges.

Oh, wow! A lot of stuff uses that ground! I think I am familiar with that ground on the little rail that sticks out of the firewall there. Thanks a lot, I'll check it out and let you know!

[Nickelplate]

My guess is that you have a bad ground. From the 1986 EVTM, the schematic below shows what circuits use G701, which is behind the left side of the radio on the firewall. And everything you are talking about, and then some, use that ground. So I think that when you bring the blower motor on, meaning not just A/C but Vent or Heater, the voltage on the ground comes up and that is messing up the gauges.

I checked this morning, and it seems that the ground there is cranked down tight. When I installed everything, I scraped the paint off of the metal that it screws into. Might the problem be a bad connection between the body and the battery ground? I have the battery grounded to the engine block, the engine block also tied to the frame. But nothing but body bolts from the frame to the cab.

[Gary Lewis]

I checked this morning, and it seems that the ground there is cranked down tight. When I installed everything, I scraped the paint off of the metal that it screws into. Might the problem be a bad connection between the body and the battery ground? I have the battery grounded to the engine block, the engine block also tied to the frame. But nothing but body bolts from the frame to the cab.

There is supposed to be a ground from the engine to the firewall, as shown on our page at Documentation/Electrical/Grounds/Engine To Firewall Ground. Without that there's very little to ground the cab and you will get all sorts of strange symptoms.

[Nickelplate]

There is supposed to be a ground from the engine to the firewall, as shown on our page at Documentation/Electrical/Grounds/Engine To Firewall Ground. Without that there's very little to ground the cab and you will get all sorts of strange symptoms.

There used to be a ground there, back when I was using the stock EFI and the problem still occurred. However, I looked and saw that the ground from the firewall to the engine was not present. I put in a little extra work and ran a firewall ground from the same screw as G701, then along my custom wiring segment and straight into the battery ground.

I can't tell if the gauges issue still happens yet because I need to run the truck for a bit. But the "fuse blows when AC engaged" issue is still there. All fan speeds will work fine, but when the AC segment is added, it pops the fuse.

Any ideas on that one now that G701 is definitely grounded?

Another small bit of info, it doesn't immediately pop when AC is engaged, but waits 3-5 seconds.

[Nickelplate]

There used to be a ground there, back when I was using the stock EFI and the problem still occurred. However, I looked and saw that the ground from the firewall to the engine was not present. I put in a little extra work and ran a firewall ground from the same screw as G701, then along my custom wiring segment and straight into the battery ground.

I can't tell if the gauges issue still happens yet because I need to run the truck for a bit. But the "fuse blows when AC engaged" issue is still there. All fan speeds will work fine, but when the AC segment is added, it pops the fuse.

Any ideas on that one now that G701 is definitely grounded?

Another small bit of info, it doesn't immediately pop when AC is engaged, but waits 3-5 seconds.

Also, just for search results for others who are looking into a similar problems, here is a link to the solid state voltage reg I got for the gauge cluster: https://www.dennis-carpenter.com/trucks/electrical/battery/d1az-10804-a-instr-cluster-voltage-regulat

[ArdWrknTrk]

There used to be a ground there, back when I was using the stock EFI and the problem still occurred. However, I looked and saw that the ground from the firewall to the engine was not present. I put in a little extra work and ran a firewall ground from the same screw as G701, then along my custom wiring segment and straight into the battery ground.

I can't tell if the gauges issue still happens yet because I need to run the truck for a bit. But the "fuse blows when AC engaged" issue is still there. All fan speeds will work fine, but when the AC segment is added, it pops the fuse.

Any ideas on that one now that G701 is definitely grounded?

Another small bit of info, it doesn't immediately pop when AC is engaged, but waits 3-5 seconds.

I'm looking at these two pages from the '86 EVTM

And thinking that if you have aftermarket engine management you probably don't have a TPS input spliced ahead of the pressure switch on the dryer.

I would definitely get rid of the diode and vacuum solepot shown with EEC.

Have you ohmed out the clutch field coil?

[Gary Lewis]

I'm looking at these two pages from the '86 EVTM

And thinking that if you have aftermarket engine management you probably don't have a TPS input spliced ahead of the pressure switch on the dryer.

I would definitely get rid of the diode and vacuum solepot shown with EEC.

Have you ohmed out the clutch field coil?

Yes, I'd get rid of the diode. But I'd be surprised if it is blowing the fuse several seconds after the clutch comes in. I'd have thought it would blow immediately.

So I'm with Jim that you need to check out that clutch. It may be the culprit.

[Nickelplate]

I'm looking at these two pages from the '86 EVTM

And thinking that if you have aftermarket engine management you probably don't have a TPS input spliced ahead of the pressure switch on the dryer.

I would definitely get rid of the diode and vacuum solepot shown with EEC.

Have you ohmed out the clutch field coil?

Looks like we were all on the same page! I went through and tested in various conditions. It reliably blows fuses when the diode is installed. Also the diode gets wicked hot after the fuse blows. I'm sure that has to be it.

Thanks for the help, guys! Looks like the "fuses blowing" issue is taken care of.

The "gauges rising" issue is still slightly in play. I took these pics to illustrate. First one is without AC on, second one is with AC on. This is WAY less of a change than what used to happen, so I think the ground helped. But having a direct ground straight to the battery should have cured it entirely, right?