Another voltage problem thread
 

Hello all!

I am new to the forum but have been following the electrical threads in hopes that someone would offer a solution to the problem I’m having with my 1st gen pickup. The search function has helped much but, as of yet not gotten down to “brass tacks”.

The truck: 1991 W250LE purchased new by me in early ’91. After identifying minor factory replacement parts, I will call it a 1990.5. 120,000 miles. Recalled and replaced injection pump. Bone stock after that.

The problem: Voltage gauge shows overcharging for 3 to 10 minutes after heating and starting.

Background: Thevoltage gauge showed overcharge up to 17 volts almost continuously. The battery tested bad and was replaced. After replacement when the truck is started initially and driven the gauge will show 17 volts for 3 to 10 minutes thenquickly go to just a hair over the 14V mark and remain there for the rest of the day. It doesn’t matter how many times the truck is stopped and restarted that day. Indicated voltage will remain at 14+.

After a cold start the gauge will read a bit over 14 then increase with engine RPM to the 17 and remain there. When the heater cycles it will drop it to around 15. If the engine RPM is not increased manually the volts will stay at the 14 mark, decrease when the heater kicks in then return to 14 when the heater kicks out. The gauge reacts with the audible click of the heating solenoid. However there is a second set of clicks, which I assume are heater clicks, that affect the gauge differently. When these occur the gauge increases slowly and doesn’t reduce.

So to summarize, and I hope you can follow my ramblings. Starting on a cool morning and not adjusting engine RPM. Turn key, 9V, wait to start light turns off, start truck, 14 V, heater on click, 9V, heater off click, 14V, heater on click, 14 V slowly increase to 15.5, heater off click, 15.5V, heater on click, 12v, heater off click, 15.5V, heater on click, 15.5 V slowly increasing to 17V, heater off click, 17V, heater on click, 15.5V, heater off click, 17V. When the heater finishes and remains off the gauge will stay at 17V, then, as described above, drop to 14+ and life is good for the rest of the day.

After reviewing some of the applicable posts on the forum I’ve checked the battery and found the volts shown on the gauge are accurate. I’ve cleaned all the grounds I can find under the hood and verified with a meter that there is only an ohm or so between the negative terminal and any metal thing I can touch under the hood. I’ve also verified a couple of other things should one try to measure battery voltage with a volt meter when it’s set to measure ohms, 1) smoke comes out of the meter and 2) fuses are your friend. Also, it’s best not to do this in front of your wife.

Voltage regulator? I would appreciate any input.

Thank you,
BlueW250

Call me crazy, but that appears to be roughly correct.

Are you reading this with a digital VOM or are you going off the factory gauge?

I want to say mine runs around 14.35V at normal charge. Voltage will increase slightly the more the battery is drained, while it is charging back up.

I agree when you had a bad battery, it was overcharging trying to keep it up.

Do you have any issues with battery boiling, outgassing, etc?

I used an older analog type volt meter to verify the factory gauge as correct then followed it.

No battery problems that I have seen but the headlights are noticebly brighter when the guage shows 17V. While the gauge is showing high I try not to use any electrical I have control of (radio, fans, mirror adjusts, etc.)

Thank you,
BlueW250

One thing to look over carefully would be your battery cable and cable ends condition, if there is corrosion either between the connector and cable, or under the covering of the cable, you could see increased resistance and this problem.

The Key On voltage needs to be the same as the battery voltage. Test it with the engine not running, Key On voltage, is the smaller terminal on the VR plug in.

Ifthe key on voltage reads less than the battery voltage, the regulator will "ask" for more voltage, even if it is not needed, thus creating the over charge.

What some people have done is use the key on voltage to pull a relay that then directs the battey voltage to the key on terminal, resulting in proper operation.

Thanks for the suggestions!

Neither the positive or negative battery cables or ends show signs of corrosion, and the cable insulation adjacent to the clamps and ends is not swollen.

The key on voltage at the smaller terminal of the voltage regulator plug is 11.5 after the heater cycles off. The battery is 12.5 after the heater cycles off. Both seem to increase very, very slowly. I have read in other threads that contacts in the ignition module can become worn in trucks of this era, increasing resistance. Could this be a cause of the voltage difference and does the added relay mentioned bypass this?

From the voltage gauge readings during warmup it appears that one of the heaters isn't drawing voltage. Could this also be a contributing factor?

Thank you,
BlueW250

Yes, a worn ignition switch can cause this. Since the regulator is seeing less voltage, it is allowing the alternator to keep charging.

And yes, doing the relay bypass fixes the charging issue.

No a non working heater will not cause this, unless the heater is stuck on.

Sounds like I have a project. Time to drop by NAPA and look for the relay/solenoid. Will let you all know how it works out.

Thank you,
BlueW250

Excellent! What you want to do is run a hot from the battery that will pass through the relay to the Key On terminal on the regulator. The Key on wire controls the relay.

There may be a sticky on this.

"After a cold start the gauge will read a bit over 14 then increase with engine RPM to the 17 and remain there."
This comment really sticks out in my mind as in bad alternator.

After the VR mod, if this still happens, the alt does need to be checked, on the bench. It has been my experience that over charging is VR related, while under charging is a bad alternator, unless the regulator is built into the alternator.

After reading the post concerning a possible bad alternator I decided to try the VR mod without the relay. I placed a jumper wire into the small VR pin connector, plugged the connector back into the VR and had my wife start the truck. As the engine idled and heaters were cycling I touched to other end of the jumper to the positive battery terminal. Wife said that the gauge didn't rise above the 16 mark as the heaters cycled. It was warm enough that the heaters only cycled a couple of times. Following that the voltage stablized at idle to 14+ whether the jumper was energized or not. All of this took no longer than a minute I think I'll repeat this when the engine is cooler so the heaters will draw down the battery a little further and use the VOM so I won't have to depend on the factory gauge and a second set of eyes.

While the engine was running I briefly removed the VR connector and replaced it. Wife said the gauge dropped to about 12.5 when it was not connected.

After the above I did recheck the key on voltage using a VOM. 12.5 at the battery terminals and 12.25 at the small pin. I'm going to proceed with the VR/relay mod as it seems evident that there is a voltage drop along the existing path to the small pin and the relay mod would correct this.

It's not a job, it's an adventure!

Thank you,
BlueW250

What guage was your wife reading? My factory guage only hs the hash marks.

Using the multimeter, what reading did you get at the battery with the engine running?

My factory gauge is the same. I told her the hash marks were 8, 10, 12, 14, 16 and 18 so she would have a way to tell me where the needle was as I was outside the truck fiddling with the wires.

Went back out in the cool of the morning and did it all again with the VOM. The factory gauge is about a volt low from the VOM readings. On the VOM the heaters cycled below 17. Heaters finished and engine running was 15 plus a RCH, no difference whether the jumper was energized or not. All readings from the VOM are between the pos. and neg. battery terminals.

I'm beginning to suspect the VR. But also believe the relay mod has value due to the voltage drop, retested prior to initial start at about 12.8, 12.25, battery, pin.

Unplugged the VR plug with engine running, VOM on battery showed 14 and very slowly dropped to 13 over about a minute.

Off to find a relay this afternoon. NAPA has energize to open but an energize to close threw 'em for a loop. Going to try to get a part # off the picture in the sticky.

Thank you,
BlueW250

Standard Fog Light relays are energize to close.

I am guessing that since the VR is unplugged, the truck is running on battery only, hence the drop in voltage.