• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

1500 Alternator Idiot Light

Share this page

B

Blueghost

Senior Member
Offline
'78 1500 Midget. Idiot light question. After searching this site, Teglerizer.com alternator conversion from Lucas to Chevy may well be the answer.

Condition. Engine off; battery voltage 12.5. Idle & up to 2000 plus RPM, headlights on, holds at 14.8 volts, idiot light is NOT on at all. This should be highest single item amp draw. Now turn on any single accessory; blower motor or turn signals or brake lights, voltage stays about 14.8 but idiot light comes on VERY faint (can't see in daylight). First thought, I have just exceeded alternator's ability to keep up. However, turn headlights OFF. Turn on any single accessory; blower motor or turn signals or brake lights, voltage stayes about 14.8 and idiot light again is ON very faint.

Question; how can alternator hold 14.8 V with idiot light off during heavy amp draw of headlights, yet with brake lights ONLY or turn signals ONLY idiot light is on faint? In fact, w/ turn signals ONLY, you can see idiot light pulsate w/ turn signal.

Alternator bought from local LBC parts supplier new as part number 14029N (manufacturer UNK) 3 yrs ago, 8000 miles ago. Has Lucas frame and looks like aftermarket regualator and rectifier assy. Brushes very good. Local alternator shop tested it and agrees it is regulated at about 14.8 V but will only put out 25 Amps. Sounds like a diode out, but under his heavy load, it will not activte his idiot light. He is puzzled.

We all take pride in actually fixing something, but looks like Chevy Camero alternator puts out 2 times amps at half the cost as in Teglerizer.com conversion. Car has been electrical gremlin free for 6 years. What am I missing?

Thanks.
 
I did the conversion and it fit with no mods. (Though I had the bracket from the air pump) I am very pleased.
 
Not all alternators are alike, so it can be a little hard to guess at things like this. I took a look at the '78 wiring diagram in my Haynes book, and it appears that the ignition lamp goes to a separate terminal on the alternator. (I did a Fiesta alternator conversion, and it just went to the output terminal.) Probably, in yours, the lamp goes to a point in the voltage regulator, and it is turned on if the regulator decides that it is not charging correctly. Lots of things could upset that functionality.

If the alternator isn't putting out the rated current, something is wrong. As you suggest, a blown diode is likely. That could bollix up (pardon the technical terminology) the regulation pretty well, and all kinds of strange behavior might result.

Since you know that the alternator is not up to snuff, the simplest thing is to put in the GM one. It's not expensive, and it's nice to have that extra current capability in your hip pocket in case it's needed for something else. And, of course, it's likely that the existing one may fail completely, probably at an inconvenient time, like 2 AM in the rain 150 miles from home. (Yeah I know, but these cars teach you not to be an optimist!)
 
Mine does it to after three alt changes. No problems in 6k but light still on dim. Imma keep on truckin'.
 
Sarastro, I agree 100% w/ your response. Thanks. HOWEVER, I have an extra Lucas alternator P/N on tag, 23796A, 17ACR, 12V237, from a parts car I bought a couple of years ago, condition UNK. Looks like genuine OEM Lucas. Installed minutes ago w/ identical results. Not totally valid test because of UNK condition of second alternator. However, what are the odds?

Also beginning to question validity of test at shop; didn't actually see test performed; "can't go into the shop for insurance reasons." That lame excuse kills me. Will take to another parts store tomorrow for retest. Some shops actaully closed for Thanksgiving. Go figure.

Kellysguy, I'm with you also. If it ain't broke don't fix it. I know how to ignore a false indication. Don't want to spend $60 for nothing.

Whats chance of a high restance in wiriing on either side of idiot bulb? Still seems like if it holds headlights OK (15 amp), it should easily hold blower motor (about 5 amps) or wipers (less than 5 amps). Total load everything on, about (27 amps). What about high restance on accessory circuit? Seems like if that were true I should have have high readings for each individual accessory.
 
My guess, based upon a lot of doing this stuff, is a problem on the ignition side. I don't have a diagram handy for that vehicle, but the lamp gets one voltage feed from alternator "IND" terminal, that is balanced from the ignition side of the switch, I think. If you have corroded, poor, or loose connections feeding that side of the lamp, it's a voltage differential and the lamp will glow.
When running, the IND terminal should be battery voltage (12.5 idle, 14.8 running, as your results show).
Now, what is the voltage at the ignition side?
Quick check would be voltage at fuseblock, compared to battery.
My suspicion is loose connection, corroded wires, poor ignition switch.
But, a battery voltage reading at the fuseblock should tell us.
 
Ahhh...you're suggesting that the voltage at the opposite side of the bulb from the alternator is low, right? Yes, that very well could be the case. It also would explain the light going on when a small additional load is turned on.
 
Thanks TOC, good check info. Engine off. Voltage at Battery and terminal 7 (heavy brown wire) identical. Therefore no resistance from Batt to Term 7.

Running w/ headlights on, Batt voltage about .1 to .2 higher; no dim idiot light. Running w/ all accessories on (headlights off), Batt is .5 higher, dim light. Running w/ all accesories and headlights, Batt is .6 to .7 higher. I believe this shows .2 differential is not enough to excite light. And .5 is enough to show light.

Now the hard part. Where is resistance? and does it matter if battery stayes charged?

The ignition switch is in parallel with the wire from Alternator "IGN" terminal. That means all accessories are in parallel w/ that circuit. Headlights are NOT in parallel, as they work off full time hot headlight switch, and therefore explains why headlights do not excite light, but accessories do. I would be greatful if someone with more experience than me (wouldn't take a lot) would confirm this is correct.

Which takes us back to where is resistance? and does it matter if battery is staying charged?

Happy Thanksgiving Day to all.
 
For what ever it is worth, you guys got me convinced.
 
How important is it? Voltage drop is due to resistance, resistance and current generate heat, heat melts insulation and causes shorts.
I would be looking at every connection downstream of the main alternator feed wires (brown, if I recall). I'll see if I can find my diagrams.
 
TOC, Brown is correct. And your correct about heat/shorts. I won't be able to get back to this until Saturday afternoon. Another issue is that this thing has been so electrical trouble free that all the wire looms are still wrapped in OEM blue tape. I'm a helicopter mechanic by trade an clearly understand the value of clean connections. Always clean and nearly always coat connections w/ di-electric grease. Hate to open looms to get to connectors. I'll go after easily exposed items 1st.
 
Don't open looms.....I think your problem is external connections at end of looms. Where the body power feeds off the start solenoid, remove and clean those eyelets. Then, (battery disconnected) remove wires into screw posts on fuseblock, noting how loose they were. Clean the wires off, re-insert and re-tighten. Then, if you have screw posts on the ignition switch, same there. I think you're going to find that's where your issue is. I'm in the middle of a couple of discussions on several forums, can't find it here.....are you the one the brake warning light just started working before this happened?
Brown wire from starter solenoid to brown wire on fuseblock, brown wire on fuseblock to brown wire on ignition switch,
White wire out of ignition switch to white wire into fuseblock, back to IND lamp.
Those are what I'd check first, after cleaning the solenoid eyelets.
 
In addition. I think you will find that there are no wrapped wire connections. All connections are made from Loom to Loom with those brown clip into things.

This is really going to be quite easy. to find. IMHO
 
Problem solved. TOC advised great specifics on where to look. Alternator cable and battery cable on starter solenoid had VERY minor corrosion. Cleaned w/ scotchbrite & contact cleaner and applied di-electric grease. Not the problem. Fuse box spade connectors all visually clean. Re-cleaned and greased as above. Not fixed. Ignition switch on ’78 is sealed unit and not cleanable; no external screw posts to come loose. Still able to flood w/ contact cleaner and cycle many times. Didn’t fix problem. Behind instrument pannel on left side are 3 large loom connectors; ignition switch / turn signal & horn assy / wiper & high beam assy. All visually clean. Re-cleaned and greased as above. Problem solved.

Suspect ignition switch loom. Recall that in early part of this string alternator tested low on amps (26 A) which should still carry load. Re-tested original and spare alternator at Auto Zone. Both checked out OK at 36 amps, 14.8 volts, and no idiot light. Moral; when a shop takes your unit out of sight and returns saying it isn’t acceptable, that may or may not be correct. You have to trust somebody, but be suspicious when you can’t see it.

Thanks All!
 
Don't know what happened to my post but it didn't print. I'll try again. Fortunately I'm a better mechanic that blogger.

2nd attempt:
Problem solved. TOC advised great specifics on where to look. Alternator cable and battery cable on starter solenoid had VERY minor corrosion. Cleaned w/ scotchbrite & contact cleaner and applied di-electric grease. Not the problem. Fuse box spade connectors all visually clean. Re-cleaned and greased as above. Ignition switch on ’78 is sealed unit and not cleanable; no external screw posts. Still able to flood w/ contact cleaner and cycle many times. Didn’t fix problem. Behind dash on left side are 3 large loom connectors; ignition switch / turn signal & horn assy / wiper & high beam assy. All visually clean. Re-cleaned and greased as above. Problem solved.

Suspect ignition switch loom. Recall that in early part of string alternator tested low on amps (26 A) which should still carry load. Re-tested original and spare alternator at Auto Zone. Both checked out OK at 36 amps, 14.8 volts, and no idiot light. Moral; when a shop takes your unit out of sight and returns saying it isn’t acceptable, that may or may not be correct. You have to trust somebody, but be suspicious when you can’t see it.

Thanks All!
 
kellysguy said:
Mine does it to after three alt changes. No problems in 6k but light still on dim. Imma keep on truckin'.
Click to expand...
same here put a Ford alternator (whatever I forgot) more amps but did the same faint glow when I turned on windshield wipers. Plus hradlights I think its just the "nature of the beast"
 
Back
Top