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MGB 77 MGB Electrical Problem

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kyreb1862

kyreb1862

Jedi Knight
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I have an odd problem with my electrical system. When I am driving the car and turn on the turn signals they work great except if the cooling fans happen to kick on at the same time. If this happens the signals start to flash very slowly. Before I converted to a 100 amp alternator they simply would not work with the cooling fans on. Any ideas on what might be behind this? I have been doing some research in Rick Astley's book but so far I haven't found an answer to this odd problem.
 
try installing a modern flasher unit if you dont have one already installed and clean the connectors and replace the bulbs.
 
If everything else stays up (headlights, dash lights, etc), what is happening is it appears you are loading down the system.
That does not mean the alternator.

I am guessing you may have the fans powered from the same leg of power the turn signals are powered from (and, without seeing a diagram, not sure if that is "stock" or not).
You either have a small-gauge wire, a poor bullet connector, or a screw-down terminal loose or corroded.

Fist question, what does your ammeter do when this hapens?
Then, does anything else get dim or slow?
 
sounds like you might want to put a relay on the fan
 
I drove the car this morning with the headlights, heater fan and wipers on and there was no slowing of the turn signals. It only seems to happen when the cooling fans kick on.
 
wonder if there is an issue with the draw at that exact moment. It may be that there is a surge protector of some sort in the flasher unit itself. I'm thinking of the opposite of the sort of thing that flashes fast when there is a burned out bulb
 
John - You have a high resistance connection somewhere that is dropping the voltage when there is a high current draw. You will need to use a volt meter and start working back from the cooling fan one connection at a time, to the battery (or you can start from the battery to the cooling fan if that makes more sense to you) until find where the voltage is dropping. Once you find the bad connection (or wire) repair it and the problem should clear up. You may be able to find some information that will help you in the article, Electrical Connection Preparation in the Other Tech Articles section of my web site at: https://homepages.donobi.net/sufuelpumps/
Cheers,
 
kyreb1862 said:
I drove the car this morning with the headlights, heater fan and wipers on and there was no slowing of the turn signals. It only seems to happen when the cooling fans kick on.
Click to expand...

Okay, now, what happens (and I will repeat myself),to headlamp brightness, heater fan speed, etc, when the cooling fan comes on and the T/S are activated.
Does the charge indicator glow?
Do the headlamps get dim?
Does the heater fan slow to less of a breeze than normal?
I see you tried everything else on, but did not see any reports on ammeter.

You may have a higher resistance in the feed to the fan and t/s if they are on the same circuit.
A relay will bypass directly to battery for fan feed.

Now, just for a drill:

Engine off, keys in pocket, bonnet open.......how loose is the fanbelt?
Can you turn the cooling fan by hand without the crank turning?

One of the reason I asked about the ammeter in the first reply was that a bad alternator, poor alternator connections, or loose (slippping) fan belt under high load will also do that, BUT it will also manifest itself in other electrical functions.

The heater fans can be a fairly high draw, or, the fans could be on their way out, like bearings going bad.

Have we spun the cooling fans by hand to see if they are starting to lock up?

Another off-the-wall issue that I have seen before (not often), is rust.
I have seen body/chassis rust issues that adversely affected the ability of the ground path of the power at the load to return to the source.

Fenders (wings) where they bolt down, radiator supports, engine to frame, battery, etc.

Now, theoretically, if the fan ground (common) is connected to the same place electrically as say the front turn signals, and the corroded/rusted grounds will handle the turn signals, but not the fan, and cause the turn sinals to slow when the ground point voltage level increases when the fan is on......

I have no idea the condition of your vehicle/grounds/wiring changes/ etc.


Got into the shop this morning to look up the electrics.


Main power feed off battery terminal of starter solenoid goes to pin 7 of fuse block.
Main power to ignition switch also comes out of 7.
The (usually) green wire direct to the thermostat for cooling fans also is in 7.
BUT:
Fuse in pins 5 and 6 next to 7 and 8, 6 feeds the turn signals (and all sorts of other bits).

IF you or someone (shop or P/O) has connected fan thermostat to the wrong pins (most likely the one next to 7, which is 5), you are no longer loading direct from battery, but now off the feed to accessories.

Something to look at anyway.
 
Headlights don't get dim. Heater fan doesn't seem to slow down. My car doen't have an ammeter. Both cooling fans turn freely. However one cooling fan motor has been replaced while the other is original. The alternator is the new 100 amp conversion.

I think the next step will be to check the grounds and then hook relays into the system. One relay per fan.
 
I'm going to quote myself (was a long post, and this was at the bottom):

Got into the shop this morning to look up the electrics.


Main power feed off battery terminal of starter solenoid goes to pin 7 of fuse block.
Main power to ignition switch also comes out of 7.
The (usually) green wire direct to the thermostat for cooling fans also is in 7.
BUT:
Fuse in pins 5 and 6 next to 7 and 8, 6 feeds the turn signals (and all sorts of other bits).

IF you or someone (shop or P/O) has connected fan thermostat to the wrong pins (most likely the one next to 7, which is 5), you are no longer loading direct from battery, but now off the feed to accessories.

Could also be loose connections at the block.
 
To keep individual post lengths lower, one thing you can do is if the cigar lighter has a functional socket, and you can fit a voltmeter to the pins through the front, when the cooling fan comes on, what does the voltage drop to?
Another question is, do the fans continue to run once the vehicle is shut off?
THAT is an important question.
 
Arrrrrrrrrrgggggggggggghhhhhhhhhhhhh.

I HATE it when a question like this causes one to go back and re-live those thrilling days of yesteryear......

I remember.

30 some odd years ago, folks weren't used to cooling fans going on when the keys were in your pocket.
I remember the warning signs....."Fans may start at any time!"

I remember re-wiring them to NOT come on with ignition off.

Relays were preferred, but I can imagine someone connecting the power feed for the fans to "ign" circuit.......

Fortunately, this happened early in the day, and I will probably be able to sleep tonight.
 
Cooling fans are prolly the culprit. My $.02> disconnect cooling fans and wire your VOM as an ammeter to each one separately, apply power to fan motor with a jumper wire and measure current draw of each one. My guess is one will be much higher than the other. Then before replacing the motor start troubleshooting ground connections. Most problems I run into with electrics and high current draw is bad ground somewhere in circuit.

Have you replaced the flashostat for the turn signals just to rule that out?
 
Looking more at the diagram, the battery feeds the [post on the solenoid....and direct to the fusebox....and direct to the fan t-stat......no fuses.


Electrically, same as connecting fans directly to solenoid.

Keeps the wire out of the firewall, and fusebox.
I'd add an in-line fuse.
You never know.
 
Okay, one more try.
Without getting under the dash, opening the bonnet, tracing wires or using a voltmeter, try this:

Run the vehicle until the fans come on.
Turn the engine off.
You should notice a change in pitch as the higher voltage of the charging system goes away, then the fans should continue to run until the coolant temperature allows the fan thermostatic switch to open.
Does it do this?
 
Electrical problem solved. After carefully reviewing a wiring diagram I found that my DPO had hooked a wire from the number 3 fuse to the white ignition wire effectively bypassing the ignition relay. I put in a new relay and removed the jumper and now everything works as designed. Thanks for the help and encouragement.
 
That's what I was looking at.
If the fan does not continue running when you shut down, there is an issue.
The fan is "hot" off battery all the time.

Funny what PO's do......and don't do.

Dave
 
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