1985 F150 302 EFI

I wanted to write this down as soon as I got home before I forget all the details.

I went out today, and then came home this afternoon. It was getting late so I put the lights on.

A few minutes later I glanced down at the instrument panel, and to my horror I noticed the temperature gauge passing the "L" of the "NORMAL", quickly approaching the do not cross line. I took the next right, pulled off the road, and shut the truck off.

I got out, felt the hoses. Top hose was hot, bottom hose was warm (I honestly expected it to be hotter). With a rag I cracked the radiator cap and some coolant came out, nothing crazy. The coolant reservoir had a healthy amount of coolant in it. So it seemed like coolant system was basically working... it wasn't empty or anything crazy.

I waited about 10 minutes and started it back up. I put the lights back on instinctively, because I had them on before. The temperature gauge started it's creep back up past the "M", to the "A".

When the gauge got up to the "L" again I shut the lights off. The needle stopped, and then slowly fell back to the "A". I was nervous because it's never been up this high before so ... not really thinking about it I turned the fan speed on the heat up a bit hoping to pull more heat off the heater core. I had been running the heat on low the entire time.

With no lights on I got the truck home. Most of the time the gauge was hovering between the "M" and the "A" which is higher than it normally runs.

When I got home I experimented a little.

Lights on, needle goes up. Lights off needle comes down. I then turned off the heat, and put the lights on instead. The needle hovered around the high side of the "M". Leaving the lights on, Heat on, needle goes up. Heat off, needle goes down. You get the picture.

OK I was running the lights on and the heat on full blast and the needle skyrockets up fast. I then shut everything off and something really weird happened.

The temp needle fell, down, down past everything. When the needle got to the "N" the truck started to run rough, the lights above the dash started flashing. A few seconds later the truck sputtered and died. I swore to myself and turned the key. Nothing, no starter. It was dead; totally dead. I sat back in the seat with my hand on my forehead trying to figure out what the hell just happened when the dash lights came back on and the buzzer was ringing like I opened the door. I turned the key and the thing started up like a champ.

As ridiculous as that sounds that bit above about having all the electronics on, making the temp needle skyrocket, shutting everything off to have the truck sputter and die is repeatable. I did it twice. The truck didn't die the second time but it smelled so rich I didn't want it to let it run like that so I shut it off.

Does anyone know what the heck just happened??

 

First off, the gauges in these old trucks can be screwy. If you're.concerned about your temperature get a no contact infrared thermometer, the point and shoot type, and take readings at your thermostat housing.

I've had the exact same issue with mine, for years. Lights on, and or fan blower running, and up goes the temp gauge, and to a lesser extent the oil pressure and the gas gauge too.

I did install a fancy solid state instrument cluster voltage regulator (built according to instructions found here) and that helped for a while but it's starting to do it again now...

Curious to know if you've recently installed a radio. The instrument cluster, blower and lights share a common ground in that area and I've been meaning to dig in and clean mine to see if I can finally fix it.

Come to think of it, you should probably check all of your grounds; battery, battery to block, firewall to block, and your frame ground.

Is your choke electric? If your choke isn't getting power it could close and cause a rich idle.

 

I agree, check out your electrical system; the gauge is lying.

Did the gas gauge and oil pressure gauge wig out at the same time? When the ICVR acts up, it affects all three of those gauges.

I also agree to check not only your grounds at the battery and starter but all your primary battery cables as a whole; and use a voltmeter to take some measurements and experiment... what's the voltage at the battery with the engine not running, and with it running - both at idle and at 2,000 RPM.

 

After my experimenting last night I guess I'm not too concerned the truck is actually overheating, but I do have one of those laser heat guns so that should be an easy check.

The truck has no radio at all, actually.

I don't think that the other gauges were getting screwy when the temp gauge was going weird, but I can check that again too.

I'll check the choke.

If it was just the temp gauge then I would be much less concerned, but it seems to affect the vehicle running which is unfortunate. Wish I could chock it up to the full moon last night. I hate troubleshooting electronics.

As always, thanks for the suggestions.

 

By choke do you mean idle air control?

I also found https://www.ford-trucks.com/forums/1...-the-icvr.html which may help.

 

Pull the codes from the computer to help diagnose the engine-running stuff, but I wouldn't do that until the electrical was first figured out and fixed.

 

My hunch is excessive AC ripple from the alternator. A bad diode can cause all sorts of seeming unexplainable electrical havoc. In a nutshell, excessive AC voltage gets superimposed over the normal DC output and all sorts of wacky things can happen in circuits designed for relatively clean DC voltage. It won't cost anything to check but a few minutes of your time. You will need a digital voltmeter, not an analog (needle style) meter.

These two links explain it well:

https://www.google.com/url?sa=t&rct=...3GaqqKvHivT4xQ


https://www.google.com/url?sa=t&rct=...Qio0QA&cad=rja

If you run a Google search for "Alternator AC ripple", you will see some videos, too.

Although it may not make sense, make sure you set the meter to AC volts, not DC. Any reading more than 0.5VAC indicates a bad alternator diode. The alternator terminals can be a pain to reach, but take your readings directly at the alternator, not the battery.

Another thought is to make sure the engine block is well grounded to the frame, and then to the battery. It's very common for a ground strap to go missing when an engine is pulled for a rebuild, etc.

 

kr98664,

Excellent idea. I will try to check that thanks.

I did check the engine temp and it's is fine, and the fuel gauge also goes up with the temp gauge. The other 2 gauges (charging and oil) don't seem effected.

 

It's curious the temp and fuel gauges are affected, but not the oil pressure. I chase electrons for a living, and have seen lots of strange faults, but this one is baffling.

Please keep in mind the AC ripple test is not 100% conclusive if using a voltmeter (as opposed to an oscilloscope). A voltmeter is expecting to see nice smooth cyclic waves (a normal sine wave pattern) and displays a value based on that. Throw in some spikes and some meters don't really "see" them because they aren't looking. So even if the measured AC ripple is under 0.5 VAC, you could still have bad diodes in the alternator. Don't rush out and get a new alternator, but do keep that possibility in the back of your mind if nothing else fixes it.

How about this quick test to help rule out the alternator. Can you remove the alternator belt without disabling the water pump, power steering, etc.? If so, charge the battery overnight so it's fully charged. Then remove the belt to the alternator so it isn't turning. Start up the engine and see if you can duplicate the fault now. Don't drive very far, as you're draining the battery, but it should give you enough time for troubleshooting.

 

I can put a scope on it.

So .5 Vpp ripple is enough to cause problems? Is it the dash regulator's job to help iron out these ripples from the alternator?

 

Oh sure, make me dig out my old textbooks to remember the terms used when measuring AC voltage. Vpp (Voltage Peak to Peak) is not what you need. Use RMS (Root Mean Squared), which is a computed average value. Some meters do it the hard way, taking multiple samples and then calculating the RMS value from there. Other meters take a simpler path and display 70.7% of the peak value, which is perfectly adequate for most applications and would be fine here. I have both types and don't really notice a difference unless I'm making uberfine adjustments on stuff like autopilots. Just so we are talking about the same thing, I'm using the definitions at this link:

Electronics Measurements: How to Measure Alternating Current - For Dummies

I'd suggest not getting too bogged down with my latest crackpot theory about AC ripple causing your problems. I wouldn't even bother dragging out your scope yet. With a digital meter, just measure the AC voltage at the alternator and see if it's over 0.5VAC. If over the limit, your alternator is bad. If not over the limit, you then get to decide if you want to troubleshoot some more down that path, looking for spikes that a voltmeter won't display. That would be the time to either hook up a scope or remove the alternator drive belt as I previously suggested.

Not really. I think the Instrument Voltage Regulator (IVR) is a holdover from the pre-transistor days, when the battery charging voltage was all over the place. Thermal gauges (bimetallic element) are very sensitive to voltage swings, so somebody just picked a number (5VDC) and designed the instrument system for that. I think the 5VDC value is a holdover from the days when all vehicles had a 6V battery, so it was easy to regulate to 5 as the supply would rarely dip below that. Even with the industry-wide switch to 12V in the late 50s, it was probably easier to just keep the existing 5V instrument systems. Inertia is a big factor in commercial designs.

On my '84, the IVR is controlled thermally, not electronically. When the output exceeds 5VDC, the bimetal contacts open and power shuts off until the output drops below 5. The gauges deliberately move slowly and hide the on/off pulses. When ambient temps are cold and the truck is first started, I can see the gauges rise in steps until they reach equilibrium. It's subtle, but if you know to look, you can see it. Oh anyways, when the IVR contacts are closed, the IVR is delivering whatever the alternator is supplying, no matter how dirty the hopefully clean DC should be.

Some replacement/aftermarket IVRs may be electronically controlled. I'm not sure if/when Ford switched over at the factory. I have no idea how much filtering the electronic versions may do. All I know is the one I have is a thermal type.

Sorry if I got off-topic with the all the conehead speak. You sorta asked...

One more thing: As far as smoothing out the normal ripples from the alternator, that's one of the functions of the battery. It acts like a giant capacitor. In rare cases, a marginal battery can be the problem, not the alternator itself. Usually a battery dies of other causes before a loss of capacitance action is noticeable, but it can happen.

 

Here's 11 pages of info on the icvr's function and a simple solid state solution.

https://www.ford-trucks.com/forums/1197148-icvr-thoughts-and-observations.html

 

OK no problem at all I'll start with the meter and go from there. I don't think it's a crackpot theory and I appreciate you spending the time to provide suggestions. It's helps a lot since I usually wouldn't think of these things (don't think much at all, actually).


No worries! More information is good!

ArdWrknTrk thanks for the link!

 

Any progress yet? I got to thinking. Don't worry, it's just social thinking and I can stop anytime I want...

The erratic gauges are one thing, but the repeatable rough running is far more serious. I'm also baffled why only 2 of the 3 gauges are acting up. The ammeter lives in its own little isolated world so let's ignore that for now, but the temp, oil press, and fuel gauges are all on the same circuit, fed through the IVR. Then it hit me. You may actually have 3 faults at play at the same time. The first two have already been discussed, but you'll have to trudge through a tedious recap before reading the third. No skipping ahead, the NSA will know...

Fault #1: Verify the alternator DC output is within specs. If low, and the battery is marginal, this would cause a weak spark and the rough running you noticed right away with the lights on and heater fan on high. A fresh, fully charged battery would mask the low system voltage for a few minutes until the battery ran down. That's why I'm now thinking a tired old battery may be part of the fault, in addition to an alternator issue. How old is the battery? If you can repeat the rough running, monitor the DC voltage while it's happening. The rough running may not have anything to do with AC ripple. Or if the battery is good, it may have just been partially discharged due to low charging voltage from a bad alternator

Fault #2: The 2 erratic gauges could be caused by AC ripple passing through the IVR, or an IVR fault itself. (More on the third gauge next) Investigate the rough running first by monitoring the DC voltage, and then turn your attention to the gauges. Actually, you can do some preliminary troubleshooting of the gauges by checking for AC ripple at the alternator while monitoring the DC output. It's fairly common for an alternator with low DC output to also have high AC ripple, but few people have reason to check for it, so it typically goes unnoticed.

Fault #3: This is what really had me stumped. Why are only 2 of the 3 gauges acting up? I was doing lots of mental aerobics, picturing electrons zipping here and there, and nothing made sense why if you had 3 functioning gauges, only 2 were erratic. Then it hit me and I had to slap myself in the forehead. There may be a third unrelated fault in the circuit to the oil pressure gauge. It could have been reading low, due to a poor connection somewhere in the circuit, or a bad sender, for example. When the supply voltage went wacky, whether due to an IVR fault or AC ripple, the needle moved a much smaller amount and so wasn't as noticeable as the other two.

That's my TheoryDuJour™ and I'm sticking to it, at least until the next thought crosses my mind. Oh, look, a squirrel...

 

Ed Snowden's got the logfiles to prove it.