I recently purchased a 1991 E350 5.8 litre van and noticed the temperature gauge reading high(past 1/2) while idling and when I turn on the A/C the gauge goes to HOT. The 1st thing I did was look inside the radiator and see that it was somewhat clogged w/calcium so I replaced the radiator. The gauge still read the same so I replaced the thermostat, then the fan clutch, then the temperature sending unit. Nothing changed but by then I put a thermometer in the radiator and realized the temperature, when reading HOT on the gauge was really only 185* in the radiator. I know these instrument clusters use a instrument voltage regulator to control the fuel/oil pressure & temperature switch but the the fuel & oil gauges appear to be indicating properly. Is it possible I needed to replace the temp gauge all along?

 

Yes.
Having said that the van is old and new parts would be needed soon anyway.

 

I believe your temp gauge is correct. I bet if you check the temp at the hot side (instead of a thermometer in the radiator cap hole) then you'll see temps matching your gauge.

 

This is where things get tricky. My pyrometer shows 185* on the backside of the the thermostat(engine side) & 176* on the radiator hose side, all the while the gauge reads Pegged to HOT. When I say the needle goes to HOT or pegged when I turn on the A/C, I mean within 5-10 seconds. As you know at 185* the thermostat isn't even open yet so I don't think the engine is really overheating & think there must be some problem w/the gauge. I tested the gauge by grounding the sending unit wire and it swung to Hot & back after I removed the ground. The schematic shows power to the gauge via a instrument voltage regulator and the sending unit provides the variable resistance ground. Simple enough but this is causing me quite a bit of confusion.

 

Sounds more like a ground issue. These can be super tough to remedy with in-dash anomalies. Let us know what you find.

 

Yup. Wiring/connections.

What year is the vehicle? 1993?
You can probably get $650 scrap money for it.

 

The van is a 1991 E350 5.8. So just now I disconnected the gang plug @ the A/C compressor & turned the A/C controls to "on" and the temp gauge immediately went to HOT, without the A/C being engaged.

 

Why do you always make the same remarks for anything older that a 2000 model year???

One poster wrote about his '93 and you told him it's too old to fix.
I had a '94 for 10 years, and replace it with a '92 2 years ago, I went backwards and still can fix it, and find parts.

Off soap box.......

supertwin1 - what you just tested might have shown up as a bad ground to the instrument panel and/or cluster area, the switch being in circuit with the dash panels.

Might be worth while to pull the instrument panel out, and inspect it for a back/broken circuit run, to the temp gauge, and the entire circuit board.

At worst, I have a '91 E350 cube van that's going to the scrap yard someday when we're done using it for storage, PM me if you need the cluster.

 

Vehicles over 10 years old are "asking for trouble", and at 15+ years..... Scrap value.

Kelley Blue Book agrees with me, and aside from that it's based on personal experience.

I understand there are odd situations where people seek out dinosaur vans (like Annaleigh on here), but hopefully she is aware that her "results may vary".

 

Those with the least to contribute in helpful or knowledgeable ways revert to smart alec replies, case in point here and other similar remarks not only in this forum but elsewhere in FTE too.

Also notice there's no real advice how to fix something as in hands on how-to's that could lead someone to solving a problem. It seems gaining attention or trying to start conversations is the main goal, not contributing much beyond silly quips.

 

Lash out at me all you want JWA (as your history corroborates) but there is large truth in what I said.
As a matter of fact, the only place where I'm "wrong" is regarding matters of opinion, I.e. The value of time vs money spent, level of necessity of reliability, etc.

Also please apply your own words to your post: "there's no real advice how to fix something as in hands on how-to's that could lead someone to solving a problem."

 

And "old people" have a duty to die, lol

 

You can do a whole lot of repairs for what it cost for a monthly payment, insurance, registration on a new vehicle!! And you can do the repairs yourself. I'd take an old vehicle over a new one any day of the week!!

The factory gauges are anything but accurate. I always prefer installing a good set of aftermarket mechanical gauges for oil pressure & water temp. Then you know what temp it's running at or the actual oil pressure. Not just it's running between the "R" and the "M" in the normal label on the gauge. I also like to add a good voltmeter and a vacuum gauge. The vacuum gauge helps with economy and can help diagnose engine issues that may pop up.

One warning though. If you go with a mechanical oil pressure gauge make sure to toss the plastic tube kit that comes with it. Either run a copper line kit or a braided stainless steel hose. Nothing will ruin your day like one of those plastic lines breaking and dumping oil into your interior!

 

New vehicles are for rich people and businesses. I'm not talking about new.

I'm talking about proper-aged vehicles.

 

Like a fine Cheese... They get kinda crumbly