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Heater Motor Will Not Run

Started by amgba, May 12, 2019, 12:18:20 PM

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It has been a while since we have corresponded but now I have a new problem. The heater motor won't run. Here's what I've done so far:

Removed the motor and bench tested it. Ran fine.
Checked the switch. OK (I did this with a multimeter on the wires coming through the fire wall. Switch off no voltage, switch on 11.8V

Ran jumper wires from the wires through the fire wall to corresponding wires on the motor. Nothing.
Checked jumpers with OHM Meter. OK. However I'm not getting voltage thru the jumpers! However I tried testing with the leads from the fire wall directly touching the leads on the motor. Nothing.

The only thing I haven't tried is to ground the motor but since it bench tests OK I've kind of ruled that out.

Any ideas on what I might have missed or something else I might try?

Sorry I forgot to mention this is a '67 MGB roadster.

Jim Kraft
Morehead City, North Carolina

American MGB Association
phone/text: 773-769-7084

North America's oldest and largest club for all MGBs, MGB-GTs & Midgets.  A nonprofit organization founded in 1975 and offers color magazines, emagazines, technical advice, member recommendations on service shops, registration of your MG, a tradition of service & FREE member classified ads that appear in the magazine & on the website.


Good to hear from you again, but sorry for your troubles.

Based on what you're seeing with the trouble shooting you've already done, I'd say the obvious fault is a break or short somewhere along the wires from the switch to the fan motor. 
That you are getting no power to the fan lead makes a grounding issue less likely to my thinking, but you should check that once you've got power to the fan lead.

You can check this with the fan motor reinstalled.  You can take power on a lead directly from the fuse box and see if the motor runs. 

If it does, that eliminates any grounding concern.

Clearly you're getting power from the fuse box to the switch and both the switch and motor appear to work properly.  On some cars, there is a resistor in-line before the fan. 

Make sure you've checked if you have power before that to the fan.  You do, the resistor has some issue.  You can also bypass it with power direct to the fan motor to check.

A car this old, I'd suspect and check for any bullet-&-barrel connectors to make sure they are tight, not corroded or broken.  I've been chasing these kinds of issues on occasion over the 30 years I've owned my B. 

In the past, I've had to replace them with newer crimp type bullet connectors, making for some unoriginal looking repairs that work well, but are not as the car was built. 

Original style replica barrel and bullet connectors are now available individually or as kits from most of the catalogs.  Generally it's the thin metal barrels that fail. 

They're hidden within plastic tubes as either single (1 bullet connection at end) or double (2 at each end) so other than having fallen out, faults are hard to see or detect without getting hands-on.

The excerpt from the 67 MGB wiring diagram I've attached shows at least one in-line (circled).  I know on my 74, there's at least (1) at the motor, but another elsewhere in the harness itself.

I hope this helps you.  Keep in touch and let me know how you make out.
Safety Fast!
Art Isaacs
Art Isaacs
AMGBA Tech Staff

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