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Wedge 1980 TR7 PI cutting out


Luke Skywalker
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7 thousand miles ago when I bought the car it did not run. I traced it to the fuel injection relay. Purchased a new Bosch one from Rock auto and never had a problem. 2 weeks ago coming back from the TRF summer party a 2 1/2 hour drive and my son driving the 7 and after we got gas 1 mile from home he said it was starting to cut out. I drove it from the station and it seems like the fuel pump is cutting out. It would be hard to believe that it is the relay again but who knows. It is reminiscent of the Volvos I've driven as the fuel injection relays are a problem with them too. Always carry a spare.
To day I decided to drive the 7 for work as it was a cool night and a 78 degree day to see if the problem would show up. Sure enough it did just once and I took the car home. I make 12 to 15 stops a day as I'm a sales rep. The last stop I was in the account for an hour and the problem showed up about 3 miles on the highway. Not enough time for the relay to over heat. However after a 20 mile drive home no problems.
Where should I start looking? Not real familiar with this car yet and wondering if there is a ignition control module somewhere that could be a problem?


Great Pumpkin - R.I.P
I'm confused. How do you know the relay hasn't failed again? Burned and dirty contacts can overheat in a matter of seconds, so I don't understand your comment about not long enough, either.

Your experiences sound very similar to the ones I had with the overdrive relay on my TR3A. Original lasted practically forever, replacements failed (generally becoming intermittent) in a matter of months. My theory is that the original had oversize precious metal contacts, in order to live with the constant arcing from the inductive load; while the replacements (even the expensive reproduction) were not. At any rate, adding a suppression diode to suppress the kickback solved the problem.


Luke Skywalker
Country flag
Well my assumption was that the Bosch relay is still relatively new and of good quality so the problem may be else ware. However I understand they are made by humans too. I was at a Wedge club get to gather at TRF several years ago and I did not even own one yet. Listening to a conversation with a new wedge owner and a experienced one who told him that there is a Delco module that would intermittently go in and out and he should purchase a spare at his local AZ store and keep it in the glove box. Seems to me he pointed it out around the coil. Hope that clears it up Randal. What would this suppression diode suppress?

I do have a spare relay around here some ware. I'll have to dig it out and plug it in and see what happens unless anyone else has an idea.


Great Pumpkin - R.I.P
What would this suppression diode suppress?
The fuel pump motor has a certain amount of inductance. An inductor tries to resist any change in the current flowing through it, so when the relay contacts start to open and interrupt the current, the inductance generates a big voltage spike. Since the contacts have just started to move, they are still very close together and the voltage spike jumps the gap, causing a spark which over time, damages the contacts. Much like the way ignition points wear out. A suppression diode helps suppress the spark, by shorting the voltage spike to ground.

The "old time" solution was to use contacts made of special, expensive materials (silver alloy was common) that would resist the arc damage for the lifetime of the relay (which was still determined by the contact damage). But now that solid state diodes are cheap and reliable, the tendency is to use them instead of the fancy contacts. Hence modern relays, even "high quality" ones, tend to use the less expensive contacts because the expensive precious metal contacts are no longer required.

At least that is my theory. It's possible that it was just "luck of the draw"; but without the diode, even the expensive reproduction relay only lasted a few months. With the diode, the cheap "Made in China" brand relay has been doing fine for some 15 years now.


Luke Skywalker
Country flag
I'll dig out my spare this weekend and see how it works. If ok guess I'll order a new one and get a diode. Although I don't know what I'm buying.
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